Environments: specify packages for developer builds (#15256)

* allow environments to specify dev-build packages

* spack develop and spack undevelop commands

* never pull dev-build packges from bincache

* reinstall dev_specs when code has changed; reinstall dependents too

* preserve dev info paths and versions in concretization as special variant

* move install overwrite transaction into installer

* move dev-build argument handling to package.do_install

now that specs are dev-aware, package.do_install can add
necessary args (keep_stage=True, use_cache=False) to dev
builds. This simplifies driving logic in cmd and env._install

* allow 'any' as wildcard for variants

* spec: allow anonymous dependencies

raise an error when constraining by or normalizing an anonymous dep
refactor concretize_develop to remove dev_build variant
refactor tests to check for ^dev_path=any instead of +dev_build

* fix variant class hierarchy

27 files changed, 1123 insertions, 85 deletions

diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index 1bc177a68b..b8d0b4d2f1 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -47,6 +47,7 @@ __all__ = [
     'install_tree',
     'is_exe',
     'join_path',
+    'last_modification_time_recursive',
     'mkdirp',
     'partition_path',
     'prefixes',
@@ -920,6 +921,15 @@ def set_executable(path):
     os.chmod(path, mode)
 
 
+def last_modification_time_recursive(path):
+    path = os.path.abspath(path)
+    times = [os.stat(path).st_mtime]
+    times.extend(os.stat(os.path.join(root, name)).st_mtime
+                 for root, dirs, files in os.walk(path)
+                 for name in dirs + files)
+    return max(times)
+
+
 def remove_empty_directories(root):
     """Ascend up from the leaves accessible from `root` and remove empty
     directories.
diff --git a/lib/spack/spack/cmd/dev_build.py b/lib/spack/spack/cmd/dev_build.py
index d9ad0fb891..55f5d201a9 100644
--- a/lib/spack/spack/cmd/dev_build.py
+++ b/lib/spack/spack/cmd/dev_build.py
@@ -12,7 +12,6 @@ import spack.config
 import spack.cmd
 import spack.cmd.common.arguments as arguments
 import spack.repo
-from spack.stage import DIYStage
 
 description = "developer build: build from code in current working directory"
 section = "build"
@@ -72,6 +71,14 @@ def dev_build(self, args):
             "spack dev-build spec must have a single, concrete version. "
             "Did you forget a package version number?")
 
+    source_path = args.source_path
+    if source_path is None:
+        source_path = os.getcwd()
+    source_path = os.path.abspath(source_path)
+
+    # Forces the build to run out of the source directory.
+    spec.constrain('dev_path=%s' % source_path)
+
     spec.concretize()
     package = spack.repo.get(spec)
 
@@ -80,14 +87,6 @@ def dev_build(self, args):
         tty.msg("Uninstall or try adding a version suffix for this dev build.")
         sys.exit(1)
 
-    source_path = args.source_path
-    if source_path is None:
-        source_path = os.getcwd()
-    source_path = os.path.abspath(source_path)
-
-    # Forces the build to run out of the current directory.
-    package.stage = DIYStage(source_path)
-
     # disable checksumming if requested
     if args.no_checksum:
         spack.config.set('config:checksum', False, scope='command_line')
@@ -97,7 +96,6 @@ def dev_build(self, args):
         keep_prefix=args.keep_prefix,
         install_deps=not args.ignore_deps,
         verbose=not args.quiet,
-        keep_stage=True,   # don't remove source dir for dev build.
         dirty=args.dirty,
         stop_before=args.before,
         stop_at=args.until)
diff --git a/lib/spack/spack/cmd/develop.py b/lib/spack/spack/cmd/develop.py
new file mode 100644
index 0000000000..957060536d
--- /dev/null
+++ b/lib/spack/spack/cmd/develop.py
@@ -0,0 +1,102 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import os
+import shutil
+
+import llnl.util.tty as tty
+
+import spack.cmd
+import spack.cmd.common.arguments as arguments
+import spack.environment as ev
+
+from spack.error import SpackError
+
+description = "add a spec to an environment's dev-build information"
+section = "environments"
+level = "long"
+
+
+def setup_parser(subparser):
+    subparser.add_argument(
+        '-p', '--path', help='Source location of package')
+
+    clone_group = subparser.add_mutually_exclusive_group()
+    clone_group.add_argument(
+        '--no-clone', action='store_false', dest='clone', default=None,
+        help='Do not clone. The package already exists at the source path')
+    clone_group.add_argument(
+        '--clone', action='store_true', dest='clone', default=None,
+        help='Clone the package even if the path already exists')
+
+    subparser.add_argument(
+        '-f', '--force',
+        help='Remove any files or directories that block cloning source code')
+
+    arguments.add_common_arguments(subparser, ['spec'])
+
+
+def develop(parser, args):
+    env = ev.get_env(args, 'develop', required=True)
+
+    if not args.spec:
+        if args.clone is False:
+            raise SpackError("No spec provided to spack develop command")
+
+        # download all dev specs
+        for name, entry in env.dev_specs.items():
+            path = entry.get('path', name)
+            abspath = path if os.path.isabs(path) else os.path.join(
+                env.path, path)
+
+            if os.path.exists(abspath):
+                msg = "Skipping developer download of %s" % entry['spec']
+                msg += " because its path already exists."
+                tty.msg(msg)
+                continue
+
+            stage = spack.spec.Spec(entry['spec']).package.stage
+            stage.steal_source(abspath)
+
+        if not env.dev_specs:
+            tty.warn("No develop specs to download")
+
+        return
+
+    specs = spack.cmd.parse_specs(args.spec)
+    if len(specs) > 1:
+        raise SpackError("spack develop requires at most one named spec")
+
+    spec = specs[0]
+    if not spec.versions.concrete:
+        raise SpackError("Packages to develop must have a concrete version")
+
+    # default path is relative path to spec.name
+    path = args.path or spec.name
+
+    # get absolute path to check
+    abspath = path
+    if not os.path.isabs(abspath):
+        abspath = os.path.join(env.path, path)
+
+    # clone default: only if the path doesn't exist
+    clone = args.clone
+    if clone is None:
+        clone = not os.path.exists(abspath)
+
+    if not clone and not os.path.exists(abspath):
+        raise SpackError("Provided path %s does not exist" % abspath)
+
+    if clone and os.path.exists(abspath):
+        if args.force:
+            shutil.rmtree(abspath)
+        else:
+            msg = "Path %s already exists and cannot be cloned to." % abspath
+            msg += " Use `spack develop -f` to overwrite."
+            raise SpackError(msg)
+
+    with env.write_transaction():
+        changed = env.develop(spec, path, clone)
+        if changed:
+            env.write()
diff --git a/lib/spack/spack/cmd/install.py b/lib/spack/spack/cmd/install.py
index 69158275f6..ef7433c5f7 100644
--- a/lib/spack/spack/cmd/install.py
+++ b/lib/spack/spack/cmd/install.py
@@ -392,13 +392,8 @@ environment variables:
                 if not answer:
                     tty.die('Reinstallation aborted.')
 
-            for abstract, concrete in zip(abstract_specs, specs):
-                if concrete in installed:
-                    with fs.replace_directory_transaction(concrete.prefix):
-                        install_spec(args, kwargs, abstract, concrete)
-                else:
-                    install_spec(args, kwargs, abstract, concrete)
+            # overwrite all concrete explicit specs from this build
+            kwargs['overwrite'] = [spec.dag_hash() for spec in specs]
 
-        else:
-            for abstract, concrete in zip(abstract_specs, specs):
-                install_spec(args, kwargs, abstract, concrete)
+        for abstract, concrete in zip(abstract_specs, specs):
+            install_spec(args, kwargs, abstract, concrete)
diff --git a/lib/spack/spack/cmd/undevelop.py b/lib/spack/spack/cmd/undevelop.py
new file mode 100644
index 0000000000..6319b88e06
--- /dev/null
+++ b/lib/spack/spack/cmd/undevelop.py
@@ -0,0 +1,40 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import llnl.util.tty as tty
+
+import spack.cmd
+import spack.cmd.common.arguments as arguments
+import spack.environment as ev
+
+
+description = 'remove specs from an environment'
+section = "environments"
+level = "long"
+
+
+def setup_parser(subparser):
+    subparser.add_argument(
+        '-a', '--all', action='store_true',
+        help="remove all specs from (clear) the environment")
+    arguments.add_common_arguments(subparser, ['specs'])
+
+
+def undevelop(parser, args):
+    env = ev.get_env(args, 'undevelop', required=True)
+
+    if args.all:
+        specs = env.dev_specs.keys()
+    else:
+        specs = spack.cmd.parse_specs(args.specs)
+
+    with env.write_transaction():
+        changed = False
+        for spec in specs:
+            tty.msg('Removing %s from environment %s development specs'
+                    % (spec, env.name))
+            changed |= env.undevelop(spec)
+        if changed:
+            env.write()
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 7424b8a90d..928d35b956 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -36,6 +36,7 @@ import spack.compilers
 import spack.architecture
 import spack.error
 import spack.tengine
+import spack.variant as vt
 from spack.config import config
 from spack.version import ver, Version, VersionList, VersionRange
 from spack.package_prefs import PackagePrefs, spec_externals, is_spec_buildable
@@ -61,6 +62,29 @@ class Concretizer(object):
         self.abstract_spec = abstract_spec
         self._adjust_target_answer_generator = None
 
+    def concretize_develop(self, spec):
+        """
+        Add ``dev_path=*`` variant to packages built from local source.
+        """
+        env = spack.environment.get_env(None, None)
+        dev_info = env.dev_specs.get(spec.name, {}) if env else {}
+        if not dev_info:
+            return False
+
+        path = dev_info['path']
+        path = path if os.path.isabs(path) else os.path.join(
+            env.path, path)
+
+        if 'dev_path' in spec.variants:
+            assert spec.variants['dev_path'].value == path
+            changed = False
+        else:
+            spec.variants.setdefault(
+                'dev_path', vt.SingleValuedVariant('dev_path', path))
+            changed = True
+        changed |= spec.constrain(dev_info['spec'])
+        return changed
+
     def _valid_virtuals_and_externals(self, spec):
         """Returns a list of candidate virtual dep providers and external
            packages that coiuld be used to concretize a spec.
@@ -328,6 +352,18 @@ class Concretizer(object):
         preferred_variants = PackagePrefs.preferred_variants(spec.name)
         pkg_cls = spec.package_class
         for name, variant in pkg_cls.variants.items():
+            any_set = False
+            var = spec.variants.get(name, None)
+            if var and 'any' in var:
+                # remove 'any' variant before concretizing
+                # 'any' cannot be combined with other variables in a
+                # multivalue variant, a concrete variant cannot have the value
+                # 'any', and 'any' does not constrain a variant except to
+                # preclude the values 'none' and None. We track `any_set` to
+                # avoid replacing 'any' with None, and remove it to continue
+                # concretization.
+                spec.variants.pop(name)
+                any_set = True
             if name not in spec.variants:
                 changed = True
                 if name in preferred_variants:
@@ -335,6 +371,14 @@ class Concretizer(object):
                 else:
                     spec.variants[name] = variant.make_default()
 
+            var = spec.variants[name]
+            if any_set and 'none' in var or None in var:
+                msg = "Attempted non-deterministic setting of variant"
+                msg += " '%s' set to 'any' and preference is." % name
+                msg += "'%s'. Set the variant to a non 'any'" % var.value
+                msg += " value or set a preference for variant '%s'." % name
+                raise NonDeterministicVariantError(msg)
+
         return changed
 
     def concretize_compiler(self, spec):
@@ -761,3 +805,7 @@ class NoBuildError(spack.error.SpackError):
         msg = ("The spec\n    '%s'\n    is configured as not buildable, "
                "and no matching external installs were found")
         super(NoBuildError, self).__init__(msg % spec)
+
+
+class NonDeterministicVariantError(spack.error.SpecError):
+    """Raised when a spec variant is set to 'any' and concretizes to 'none'."""
diff --git a/lib/spack/spack/database.py b/lib/spack/spack/database.py
index 01fb4fc7a5..4cc7ed6406 100644
--- a/lib/spack/spack/database.py
+++ b/lib/spack/spack/database.py
@@ -1114,8 +1114,10 @@ class Database(object):
             new_spec._hash = key
 
         else:
-            # If it is already there, mark it as installed.
+            # If it is already there, mark it as installed and update
+            # installation time
             self._data[key].installed = True
+            self._data[key].installation_time = _now()
 
         self._data[key].explicit = explicit
 
diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index f6496bb62a..7b2084d229 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -50,7 +50,7 @@ from spack.version import Version, VersionChecksumError
 __all__ = []
 
 #: These are variant names used by Spack internally; packages can't use them
-reserved_names = ['patches']
+reserved_names = ['patches', 'dev_path']
 
 _patch_order_index = 0
 
diff --git a/lib/spack/spack/environment.py b/lib/spack/spack/environment.py
index 9b9f8c7d17..e656e5e368 100644
--- a/lib/spack/spack/environment.py
+++ b/lib/spack/spack/environment.py
@@ -26,6 +26,7 @@ import spack.repo
 import spack.schema.env
 import spack.spec
 import spack.store
+import spack.stage
 import spack.util.spack_json as sjson
 import spack.util.spack_yaml as syaml
 import spack.config
@@ -707,6 +708,15 @@ class Environment(object):
         configuration = config_dict(self.yaml)
         self.concretization = configuration.get('concretization')
 
+        # Retrieve dev-build packages:
+        self.dev_specs = configuration['develop']
+        for name, entry in self.dev_specs.items():
+            # spec must include a concrete version
+            assert Spec(entry['spec']).version.concrete
+            # default path is the spec name
+            if 'path' not in entry:
+                self.dev_specs[name]['path'] = name
+
     @property
     def user_specs(self):
         return self.spec_lists[user_speclist_name]
@@ -722,6 +732,7 @@ class Environment(object):
 
     def clear(self):
         self.spec_lists = {user_speclist_name: SpecList()}  # specs from yaml
+        self.dev_specs = {}               # dev-build specs from yaml
         self.concretized_user_specs = []  # user specs from last concretize
         self.concretized_order = []       # roots of last concretize, in order
         self.specs_by_hash = {}           # concretized specs by hash
@@ -976,6 +987,71 @@ class Environment(object):
                 del self.concretized_order[i]
                 del self.specs_by_hash[dag_hash]
 
+    def develop(self, spec, path, clone=False):
+        """Add dev-build info for package
+
+        Args:
+            spec (Spec): Set constraints on development specs. Must include a
+                concrete version.
+            path (string): Path to find code for developer builds. Relative
+                paths will be resolved relative to the environment.
+            clone (bool, default False): Clone the package code to the path.
+                If clone is False Spack will assume the code is already present
+                at ``path``.
+
+        Return:
+            (bool): True iff the environment was changed.
+        """
+        spec = spec.copy()  # defensive copy since we access cached attributes
+
+        if not spec.versions.concrete:
+            raise SpackEnvironmentError(
+                'Cannot develop spec %s without a concrete version' % spec)
+
+        for name, entry in self.dev_specs.items():
+            if name == spec.name:
+                e_spec = Spec(entry['spec'])
+                e_path = entry['path']
+
+                if e_spec == spec:
+                    if path == e_path:
+                        tty.msg("Spec %s already configured for development" %
+                                spec)
+                        return False
+                    else:
+                        tty.msg("Updating development path for spec %s" % spec)
+                        break
+                else:
+                    msg = "Updating development spec for package "
+                    msg += "%s with path %s" % (spec.name, path)
+                    tty.msg(msg)
+                    break
+        else:
+            tty.msg("Configuring spec %s for development at path %s" %
+                    (spec, path))
+
+        if clone:
+            # "steal" the source code via staging API
+            abspath = path if os.path.isabs(path) else os.path.join(
+                self.path, path)
+
+            stage = spec.package.stage
+            stage.steal_source(abspath)
+
+        # If it wasn't already in the list, append it
+        self.dev_specs[spec.name] = {'path': path, 'spec': str(spec)}
+        return True
+
+    def undevelop(self, spec):
+        """Remove develop info for abstract spec ``spec``.
+
+        returns True on success, False if no entry existed."""
+        spec = Spec(spec)  # In case it's a spec object
+        if spec.name in self.dev_specs:
+            del self.dev_specs[spec.name]
+            return True
+        return False
+
     def concretize(self, force=False):
         """Concretize user_specs in this environment.
 
@@ -1248,6 +1324,53 @@ class Environment(object):
         self.concretized_order.append(h)
         self.specs_by_hash[h] = concrete
 
+    def _spec_needs_overwrite(self, spec):
+        # Overwrite the install if it's a dev build (non-transitive)
+        # and the code has been changed since the last install
+        # or one of the dependencies has been reinstalled since
+        # the last install
+
+        # if it's not installed, we don't need to overwrite it
+        if not spec.package.installed:
+            return False
+
+        # if spec and all deps aren't dev builds, we don't need to overwrite it
+        if not any(spec.satisfies(c)
+                   for c in ('dev_path=any', '^dev_path=any')):
+            return False
+
+        # if any dep needs overwrite, or any dep is missing and is a dev build
+        # then overwrite this package
+        if any(
+            self._spec_needs_overwrite(dep) or
+            ((not dep.package.installed) and dep.satisfies('dev_path=any'))
+            for dep in spec.traverse(root=False)
+        ):
+            return True
+
+        # if it's not a direct dev build and its dependencies haven't
+        # changed, it hasn't changed.
+        # We don't merely check satisfaction (spec.satisfies('dev_path=any')
+        # because we need the value of the variant in the next block of code
+        dev_path_var = spec.variants.get('dev_path', None)
+        if not dev_path_var:
+            return False
+
+        # if it is a direct dev build, check whether the code changed
+        # we already know it is installed
+        _, record = spack.store.db.query_by_spec_hash(spec.dag_hash())
+        mtime = fs.last_modification_time_recursive(dev_path_var.value)
+        return mtime > record.installation_time
+
+    def _get_overwrite_specs(self):
+        ret = []
+        for dag_hash in self.concretized_order:
+            spec = self.specs_by_hash[dag_hash]
+            ret.extend([d.dag_hash() for d in spec.traverse(root=True)
+                        if self._spec_needs_overwrite(d)])
+
+        return ret
+
     def install(self, user_spec, concrete_spec=None, **install_args):
         """Install a single spec into an environment.
 
@@ -1260,7 +1383,11 @@ class Environment(object):
 
     def _install(self, spec, **install_args):
         # "spec" must be concrete
-        spec.package.do_install(**install_args)
+        package = spec.package
+
+        install_args['overwrite'] = install_args.get(
+            'overwrite', []) + self._get_overwrite_specs()
+        package.do_install(**install_args)
 
         if not spec.external:
             # Make sure log directory exists
@@ -1288,14 +1415,18 @@ class Environment(object):
         # a large amount of time due to repeatedly acquiring and releasing
         # locks, this does an initial check across all specs within a single
         # DB read transaction to reduce time spent in this case.
-        uninstalled_specs = []
+        specs_to_install = []
         with spack.store.db.read_transaction():
             for concretized_hash in self.concretized_order:
                 spec = self.specs_by_hash[concretized_hash]
-                if not spec.package.installed:
-                    uninstalled_specs.append(spec)
-
-        for spec in uninstalled_specs:
+                if not spec.package.installed or (
+                        spec.satisfies('dev_path=any') or
+                        spec.satisfies('^dev_path=any')
+                ):
+                    # If it's a dev build it could need to be reinstalled
+                    specs_to_install.append(spec)
+
+        for spec in specs_to_install:
             # Parse cli arguments and construct a dictionary
             # that will be passed to Package.do_install API
             kwargs = dict()
@@ -1583,6 +1714,17 @@ class Environment(object):
         else:
             view = False
         yaml_dict['view'] = view
+
+        if self.dev_specs:
+            # Remove entries that are mirroring defaults
+            write_dev_specs = copy.deepcopy(self.dev_specs)
+            for name, entry in write_dev_specs.items():
+                if entry['path'] == name:
+                    del entry['path']
+            yaml_dict['develop'] = write_dev_specs
+        else:
+            yaml_dict.pop('develop', None)
+
         # Remove yaml sections that are shadowing defaults
         # construct garbage path to ensure we don't find a manifest by accident
         with fs.temp_cwd() as env_dir:
diff --git a/lib/spack/spack/installer.py b/lib/spack/spack/installer.py
index 401e2428b3..29fce5617a 100644
--- a/lib/spack/spack/installer.py
+++ b/lib/spack/spack/installer.py
@@ -565,7 +565,6 @@ install_args_docstring = """
                 otherwise, the default is to install as many dependencies as
                 possible (i.e., best effort installation).
             fake (bool): Don't really build; install fake stub files instead.
-            force (bool): Install again, even if already installed.
             install_deps (bool): Install dependencies before installing this
                 package
             install_source (bool): By default, source is not installed, but
@@ -575,6 +574,8 @@ install_args_docstring = """
             keep_stage (bool): By default, stage is destroyed only if there
                 are no exceptions during build. Set to True to keep the stage
                 even with exceptions.
+            overwrite (list): list of hashes for packages to do overwrite
+                installs. Default empty list.
             restage (bool): Force spack to restage the package source.
             skip_patch (bool): Skip patch stage of build if True.
             stop_before (InstallPhase): stop execution before this
@@ -638,6 +639,10 @@ class PackageInstaller(object):
         # Cache of installed packages' unique ids
         self.installed = set()
 
+        # Cache of overwrite information
+        self.overwrite = set()
+        self.overwrite_time = time.time()
+
         # Data store layout
         self.layout = spack.store.layout
 
@@ -727,7 +732,9 @@ class PackageInstaller(object):
             # Check the database to see if the dependency has been installed
             # and flag as such if appropriate
             rec, installed_in_db = self._check_db(dep)
-            if installed_in_db:
+            if installed_in_db and (
+                    dep.dag_hash() not in self.overwrite or
+                    rec.installation_time > self.overwrite_time):
                 tty.debug('Flagging {0} as installed per the database'
                           .format(dep_id))
                 self.installed.add(dep_id)
@@ -778,7 +785,10 @@ class PackageInstaller(object):
         if restage and task.pkg.stage.managed_by_spack:
             task.pkg.stage.destroy()
 
-        if not partial and self.layout.check_installed(task.pkg.spec):
+        if not partial and self.layout.check_installed(task.pkg.spec) and (
+                rec.spec.dag_hash() not in self.overwrite or
+                rec.installation_time > self.overwrite_time
+        ):
             self._update_installed(task)
 
             # Only update the explicit entry once for the explicit package
@@ -1417,6 +1427,12 @@ class PackageInstaller(object):
         # always installed regardless of whether the root was installed
         install_package = kwargs.pop('install_package', True)
 
+        # take a timestamp with the overwrite argument to check whether another
+        # process has already overridden the package.
+        self.overwrite = set(kwargs.get('overwrite', []))
+        if self.overwrite:
+            self.overwrite_time = time.time()
+
         # Ensure not attempting to perform an installation when user didn't
         # want to go that far.
         self._check_last_phase(**kwargs)
@@ -1543,7 +1559,23 @@ class PackageInstaller(object):
             # Proceed with the installation since we have an exclusive write
             # lock on the package.
             try:
-                self._install_task(task, **kwargs)
+                if pkg.spec.dag_hash() in self.overwrite:
+                    rec, _ = self._check_db(pkg.spec)
+                    if rec and rec.installed:
+                        if rec.installation_time < self.overwrite_time:
+                            # If it's actually overwriting, do a fs transaction
+                            if os.path.exists(rec.path):
+                                with fs.replace_directory_transaction(
+                                        rec.path):
+                                    self._install_task(task, **kwargs)
+                            else:
+                                tty.debug("Missing installation to overwrite")
+                                self._install_task(task, **kwargs)
+                    else:
+                        # overwriting nothing
+                        self._install_task(task, **kwargs)
+                else:
+                    self._install_task(task, **kwargs)
                 self._update_installed(task)
 
                 # If we installed then we should keep the prefix
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 3a0c17da88..14b1d1089a 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -916,6 +916,11 @@ class PackageBase(with_metaclass(PackageMeta, PackageViewMixin, object)):
         return stage
 
     def _make_stage(self):
+        # If it's a dev package (not transitively), use a DIY stage object
+        dev_path_var = self.spec.variants.get('dev_path', None)
+        if dev_path_var:
+            return spack.stage.DIYStage(dev_path_var.value)
+
         # Construct a composite stage on top of the composite FetchStrategy
         composite_fetcher = self.fetcher
         composite_stage = StageComposite()
@@ -1230,16 +1235,14 @@ class PackageBase(with_metaclass(PackageMeta, PackageViewMixin, object)):
         Creates a stage directory and downloads the tarball for this package.
         Working directory will be set to the stage directory.
         """
-        if not self.spec.concrete:
-            raise ValueError("Can only fetch concrete packages.")
-
         if not self.has_code:
             tty.debug('No fetch required for {0}: package has no code.'
                       .format(self.name))
 
         start_time = time.time()
         checksum = spack.config.get('config:checksum')
-        if checksum and self.version not in self.versions:
+        fetch = self.stage.managed_by_spack
+        if checksum and fetch and self.version not in self.versions:
             tty.warn("There is no checksum on file to fetch %s safely." %
                      self.spec.cformat('{name}{@version}'))
 
@@ -1275,9 +1278,6 @@ class PackageBase(with_metaclass(PackageMeta, PackageViewMixin, object)):
 
     def do_stage(self, mirror_only=False):
         """Unpacks and expands the fetched tarball."""
-        if not self.spec.concrete:
-            raise ValueError("Can only stage concrete packages.")
-
         # Always create the stage directory at this point.  Why?  A no-code
         # package may want to use the installation process to install metadata.
         self.stage.create()
@@ -1587,6 +1587,12 @@ class PackageBase(with_metaclass(PackageMeta, PackageViewMixin, object)):
         their build process.
 
         Args:"""
+        # Non-transitive dev specs need to keep the dev stage and be built from
+        # source every time. Transitive ones just need to be built from source.
+        dev_path_var = self.spec.variants.get('dev_path', None)
+        if dev_path_var:
+            kwargs['keep_stage'] = True
+
         builder = PackageInstaller(self)
         builder.install(**kwargs)
 
diff --git a/lib/spack/spack/schema/env.py b/lib/spack/spack/schema/env.py
index 18a2048557..a7e3dc608f 100644
--- a/lib/spack/spack/schema/env.py
+++ b/lib/spack/spack/schema/env.py
@@ -73,6 +73,25 @@ schema = {
                             'type': 'string'
                         },
                     },
+                    'develop': {
+                        'type': 'object',
+                        'default': {},
+                        'additionalProperties': False,
+                        'patternProperties': {
+                            r'\w[\w-]*': {
+                                'type': 'object',
+                                'additionalProperties': False,
+                                'properties': {
+                                    'spec': {
+                                        'type': 'string'
+                                    },
+                                    'path': {
+                                        'type': 'string'
+                                    },
+                                },
+                            },
+                        },
+                    },
                     'definitions': {
                         'type': 'array',
                         'default': [],
diff --git a/lib/spack/spack/schema/spec.py b/lib/spack/spack/schema/spec.py
index 8ec17e7e76..c61447ef9b 100644
--- a/lib/spack/spack/schema/spec.py
+++ b/lib/spack/spack/schema/spec.py
@@ -96,6 +96,12 @@ properties = {
                     'version': {'type': 'string'},
                 },
             },
+            'develop': {
+                'anyOf': [
+                    {'type': 'boolean'},
+                    {'type': 'string'},
+                ],
+            },
             'namespace': {'type': 'string'},
             'parameters': {
                 'type': 'object',
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 0b4e7dfacf..47ff0c5d06 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -2063,7 +2063,8 @@ class Spec(object):
             # still need to select a concrete package later.
             if not self.virtual:
                 changed |= any(
-                    (concretizer.concretize_architecture(self),
+                    (concretizer.concretize_develop(self),  # special variant
+                     concretizer.concretize_architecture(self),
                      concretizer.concretize_compiler(self),
                      concretizer.adjust_target(self),
                      # flags must be concretized after compiler
@@ -2698,6 +2699,10 @@ class Spec(object):
 
         if user_spec_deps:
             for name, spec in user_spec_deps.items():
+                if not name:
+                    msg = "Attempted to normalize anonymous dependency spec"
+                    msg += " %s" % spec
+                    raise InvalidSpecDetected(msg)
                 if name not in all_spec_deps:
                     all_spec_deps[name] = spec
                 else:
@@ -2850,6 +2855,9 @@ class Spec(object):
         if not other.satisfies_dependencies(self):
             raise UnsatisfiableDependencySpecError(other, self)
 
+        if any(not d.name for d in other.traverse(root=False)):
+            raise UnconstrainableDependencySpecError(other)
+
         # Handle common first-order constraints directly
         changed = False
         for name in self.common_dependencies(other):
@@ -4123,8 +4131,23 @@ class SpecParser(spack.parse.Parser):
 
                         if not dep:
                             # We're adding a dependency to the last spec
-                            self.expect(ID)
-                            dep = self.spec(self.token.value)
+                            if self.accept(ID):
+                                self.previous = self.token
+                                if self.accept(EQ):
+                                    # This is an anonymous dep with a key=value
+                                    # push tokens to be parsed as part of the
+                                    # dep spec
+                                    self.push_tokens(
+                                        [self.previous, self.token])
+                                    dep_name = None
+                                else:
+                                    # named dep (standard)
+                                    dep_name = self.token.value
+                                self.previous = None
+                            else:
+                                # anonymous dep
+                                dep_name = None
+                            dep = self.spec(dep_name)
 
                         # Raise an error if the previous spec is already
                         # concrete (assigned by hash)
@@ -4509,6 +4532,14 @@ class UnsatisfiableDependencySpecError(spack.error.UnsatisfiableSpecError):
             provided, required, "dependency")
 
 
+class UnconstrainableDependencySpecError(spack.error.SpecError):
+    """Raised when attempting to constrain by an anonymous dependency spec"""
+    def __init__(self, spec):
+        msg = "Cannot constrain by spec '%s'. Cannot constrain by a" % spec
+        msg += " spec containing anonymous dependencies"
+        super(UnconstrainableDependencySpecError, self).__init__(msg)
+
+
 class AmbiguousHashError(spack.error.SpecError):
     def __init__(self, msg, *specs):
         spec_fmt = '{namespace}.{name}{@version}{%compiler}{compiler_flags}'
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index cf32bc92f8..ca3781deba 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -5,13 +5,15 @@
 
 from __future__ import print_function
 
+import errno
+import getpass
+import glob
+import hashlib
 import os
+import shutil
 import stat
 import sys
-import errno
-import hashlib
 import tempfile
-import getpass
 from six import string_types
 from six import iteritems
 
@@ -491,6 +493,41 @@ class Stage(object):
 
         print_errors(errors)
 
+    def steal_source(self, dest):
+        """Copy the source_path directory in its entirety to directory dest
+
+        This operation creates/fetches/expands the stage if it is not already,
+        and destroys the stage when it is done."""
+        if not self.created:
+            self.create()
+        if not self.expanded and not self.archive_file:
+            self.fetch()
+        if not self.expanded:
+            self.expand_archive()
+
+        if not os.path.isdir(dest):
+            mkdirp(dest)
+
+        # glob all files and directories in the source path
+        hidden_entries = glob.glob(os.path.join(self.source_path, '.*'))
+        entries = glob.glob(os.path.join(self.source_path, '*'))
+
+        # Move all files from stage to destination directory
+        # Include hidden files for VCS repo history
+        for entry in hidden_entries + entries:
+            if os.path.isdir(entry):
+                d = os.path.join(dest, os.path.basename(entry))
+                shutil.copytree(entry, d)
+            else:
+                shutil.copy2(entry, dest)
+
+        # copy archive file if we downloaded from url -- replaces for vcs
+        if self.archive_file and os.path.exists(self.archive_file):
+            shutil.copy2(self.archive_file, dest)
+
+        # remove leftover stage
+        self.destroy()
+
     def check(self):
         """Check the downloaded archive against a checksum digest.
            No-op if this stage checks code out of a repository."""
@@ -655,7 +692,8 @@ class ResourceStage(Stage):
 
 @pattern.composite(method_list=[
     'fetch', 'create', 'created', 'check', 'expand_archive', 'restage',
-    'destroy', 'cache_local', 'cache_mirror', 'managed_by_spack'])
+    'destroy', 'cache_local', 'cache_mirror', 'steal_source',
+    'managed_by_spack'])
 class StageComposite:
     """Composite for Stage type objects. The first item in this composite is
     considered to be the root package, and operations that return a value are
diff --git a/lib/spack/spack/test/cmd/dev_build.py b/lib/spack/spack/test/cmd/dev_build.py
index 94c7690de6..2739bf1524 100644
--- a/lib/spack/spack/test/cmd/dev_build.py
+++ b/lib/spack/spack/test/cmd/dev_build.py
@@ -2,16 +2,22 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
 import os
 import pytest
 import spack.spec
+import llnl.util.filesystem as fs
+import spack.environment as ev
 from spack.main import SpackCommand, SpackCommandError
 
 dev_build = SpackCommand('dev-build')
+install = SpackCommand('install')
+env = SpackCommand('env')
 
 
 def test_dev_build_basics(tmpdir, mock_packages, install_mockery):
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -23,9 +29,12 @@ def test_dev_build_basics(tmpdir, mock_packages, install_mockery):
     with open(os.path.join(spec.prefix, spec.package.filename), 'r') as f:
         assert f.read() == spec.package.replacement_string
 
+    assert os.path.exists(str(tmpdir))
+
 
 def test_dev_build_before(tmpdir, mock_packages, install_mockery):
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -41,7 +50,8 @@ def test_dev_build_before(tmpdir, mock_packages, install_mockery):
 
 
 def test_dev_build_until(tmpdir, mock_packages, install_mockery):
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -59,7 +69,8 @@ def test_dev_build_until(tmpdir, mock_packages, install_mockery):
 
 def test_dev_build_until_last_phase(tmpdir, mock_packages, install_mockery):
     # Test that we ignore the last_phase argument if it is already last
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -73,10 +84,12 @@ def test_dev_build_until_last_phase(tmpdir, mock_packages, install_mockery):
 
     assert os.path.exists(spec.prefix)
     assert spack.store.db.query(spec, installed=True)
+    assert os.path.exists(str(tmpdir))
 
 
 def test_dev_build_before_until(tmpdir, mock_packages, install_mockery):
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -118,7 +131,8 @@ def test_dev_build_drop_in(tmpdir, mock_packages, monkeypatch,
 
 def test_dev_build_fails_already_installed(tmpdir, mock_packages,
                                            install_mockery):
-    spec = spack.spec.Spec('dev-build-test-install@0.0.0').concretized()
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
 
     with tmpdir.as_cwd():
         with open(spec.package.filename, 'w') as f:
@@ -147,3 +161,226 @@ def test_dev_build_fails_nonexistent_package_name(mock_packages):
 def test_dev_build_fails_no_version(mock_packages):
     output = dev_build('dev-build-test-install', fail_on_error=False)
     assert 'dev-build spec must have a single, concrete version' in output
+
+
+def test_dev_build_env(tmpdir, mock_packages, install_mockery,
+                       mutable_mock_env_path):
+    """Test Spack does dev builds for packages in develop section of env."""
+    # setup dev-build-test-install package for dev build
+    build_dir = tmpdir.mkdir('build')
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' %
+                           build_dir)
+    spec.concretize()
+
+    with build_dir.as_cwd():
+        with open(spec.package.filename, 'w') as f:
+            f.write(spec.package.original_string)
+
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - dev-build-test-install@0.0.0
+
+  develop:
+    dev-build-test-install:
+      spec: dev-build-test-install@0.0.0
+      path: %s
+""" % build_dir)
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            install()
+
+    assert spec.package.filename in os.listdir(spec.prefix)
+    with open(os.path.join(spec.prefix, spec.package.filename), 'r') as f:
+        assert f.read() == spec.package.replacement_string
+
+
+def test_dev_build_env_version_mismatch(tmpdir, mock_packages, install_mockery,
+                                        mutable_mock_env_path):
+    """Test Spack constraints concretization by develop specs."""
+    # setup dev-build-test-install package for dev build
+    build_dir = tmpdir.mkdir('build')
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' % tmpdir)
+    spec.concretize()
+
+    with build_dir.as_cwd():
+        with open(spec.package.filename, 'w') as f:
+            f.write(spec.package.original_string)
+
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - dev-build-test-install@0.0.0
+
+  develop:
+    dev-build-test-install:
+      spec: dev-build-test-install@1.1.1
+      path: %s
+""" % build_dir)
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            with pytest.raises(spack.spec.UnsatisfiableVersionSpecError):
+                install()
+
+
+def test_dev_build_multiple(tmpdir, mock_packages, install_mockery,
+                            mutable_mock_env_path, mock_fetch):
+    """Test spack install with multiple developer builds"""
+    # setup dev-build-test-install package for dev build
+    # Wait to concretize inside the environment to set dev_path on the specs;
+    # without the environment, the user would need to set dev_path for both the
+    # root and dependency if they wanted a dev build for both.
+    leaf_dir = tmpdir.mkdir('leaf')
+    leaf_spec = spack.spec.Spec('dev-build-test-install@0.0.0')
+    with leaf_dir.as_cwd():
+        with open(leaf_spec.package.filename, 'w') as f:
+            f.write(leaf_spec.package.original_string)
+
+    # setup dev-build-test-dependent package for dev build
+    # don't concretize outside environment -- dev info will be wrong
+    root_dir = tmpdir.mkdir('root')
+    root_spec = spack.spec.Spec('dev-build-test-dependent@0.0.0')
+    with root_dir.as_cwd():
+        with open(root_spec.package.filename, 'w') as f:
+            f.write(root_spec.package.original_string)
+
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - dev-build-test-install@0.0.0
+  - dev-build-test-dependent@0.0.0
+
+  develop:
+    dev-build-test-install:
+      path: %s
+      spec: dev-build-test-install@0.0.0
+    dev-build-test-dependent:
+      spec: dev-build-test-dependent@0.0.0
+      path: %s
+""" % (leaf_dir, root_dir))
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            # Do concretization inside environment for dev info
+            leaf_spec.concretize()
+            root_spec.concretize()
+
+            # Do install
+            install()
+
+    for spec in (leaf_spec, root_spec):
+        assert spec.package.filename in os.listdir(spec.prefix)
+        with open(os.path.join(spec.prefix, spec.package.filename), 'r') as f:
+            assert f.read() == spec.package.replacement_string
+
+
+def test_dev_build_env_dependency(tmpdir, mock_packages, install_mockery,
+                                  mock_fetch, mutable_mock_env_path):
+    """
+    Test non-root specs in an environment are properly marked for dev builds.
+    """
+    # setup dev-build-test-install package for dev build
+    build_dir = tmpdir.mkdir('build')
+    spec = spack.spec.Spec('dependent-of-dev-build@0.0.0')
+    dep_spec = spack.spec.Spec('dev-build-test-install')
+
+    with build_dir.as_cwd():
+        with open(dep_spec.package.filename, 'w') as f:
+            f.write(dep_spec.package.original_string)
+
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - dependent-of-dev-build@0.0.0
+
+  develop:
+    dev-build-test-install:
+      spec: dev-build-test-install@0.0.0
+      path: %s
+""" % build_dir)
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            # concretize in the environment to get the dev build info
+            # equivalent to setting dev_build and dev_path variants
+            # on all specs above
+            spec.concretize()
+            dep_spec.concretize()
+            install()
+
+    # Ensure that both specs installed properly
+    assert dep_spec.package.filename in os.listdir(dep_spec.prefix)
+    assert os.path.exists(spec.prefix)
+
+    # Ensure variants set properly
+    for dep in (dep_spec, spec['dev-build-test-install']):
+        assert dep.satisfies('dev_path=%s' % build_dir)
+    assert spec.satisfies('^dev_path=any')
+
+
+@pytest.mark.parametrize('test_spec', ['dev-build-test-install',
+                                       'dependent-of-dev-build'])
+def test_dev_build_rebuild_on_source_changes(
+        test_spec, tmpdir, mock_packages, install_mockery,
+        mutable_mock_env_path, mock_fetch):
+    """Test dev builds rebuild on changes to source code.
+
+    ``test_spec = dev-build-test-install`` tests rebuild for changes to package
+    ``test_spec = dependent-of-dev-build`` tests rebuild for changes to dep
+    """
+    # setup dev-build-test-install package for dev build
+    build_dir = tmpdir.mkdir('build')
+    spec = spack.spec.Spec('dev-build-test-install@0.0.0 dev_path=%s' %
+                           build_dir)
+    spec.concretize()
+
+    def reset_string():
+        with build_dir.as_cwd():
+            with open(spec.package.filename, 'w') as f:
+                f.write(spec.package.original_string)
+
+    reset_string()
+
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - %s@0.0.0
+
+  develop:
+    dev-build-test-install:
+      spec: dev-build-test-install@0.0.0
+      path: %s
+""" % (test_spec, build_dir))
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            install()
+
+            reset_string()  # so the package will accept rebuilds
+
+            fs.touch(os.path.join(str(build_dir), 'test'))
+            output = install()
+
+    assert 'Installing %s' % test_spec in output
diff --git a/lib/spack/spack/test/cmd/develop.py b/lib/spack/spack/test/cmd/develop.py
new file mode 100644
index 0000000000..c539cd91c0
--- /dev/null
+++ b/lib/spack/spack/test/cmd/develop.py
@@ -0,0 +1,100 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import pytest
+import os
+import shutil
+import llnl.util.filesystem as fs
+
+import spack.spec
+import spack.environment as ev
+from spack.main import SpackCommand
+
+develop = SpackCommand('develop')
+env = SpackCommand('env')
+
+
+@pytest.mark.usefixtures(
+    'mutable_mock_env_path', 'mock_packages', 'mock_fetch')
+class TestDevelop(object):
+    def check_develop(self, env, spec, path=None):
+        path = path or spec.name
+
+        # check in memory representation
+        assert spec.name in env.dev_specs
+        dev_specs_entry = env.dev_specs[spec.name]
+        assert dev_specs_entry['path'] == path
+        assert dev_specs_entry['spec'] == str(spec)
+
+        # check yaml representation
+        yaml = ev.config_dict(env.yaml)
+        assert spec.name in yaml['develop']
+        yaml_entry = yaml['develop'][spec.name]
+        assert yaml_entry['spec'] == str(spec)
+        if path == spec.name:
+            # default paths aren't written out
+            assert 'path' not in yaml_entry
+        else:
+            assert yaml_entry['path'] == path
+
+    def test_develop_no_path_no_clone(self):
+        env('create', 'test')
+        with ev.read('test') as e:
+            # develop checks that the path exists
+            fs.mkdirp(os.path.join(e.path, 'mpich'))
+            develop('--no-clone', 'mpich@1.0')
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'))
+
+    def test_develop_no_clone(self, tmpdir):
+        env('create', 'test')
+        with ev.read('test') as e:
+            develop('--no-clone', '-p', str(tmpdir), 'mpich@1.0')
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'), str(tmpdir))
+
+    def test_develop(self):
+        env('create', 'test')
+        with ev.read('test') as e:
+            develop('mpich@1.0')
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'))
+
+    def test_develop_no_args(self):
+        env('create', 'test')
+        with ev.read('test') as e:
+            # develop and remove it
+            develop('mpich@1.0')
+            shutil.rmtree(os.path.join(e.path, 'mpich'))
+
+            # test develop with no args
+            develop()
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'))
+
+    def test_develop_twice(self):
+        env('create', 'test')
+        with ev.read('test') as e:
+            develop('mpich@1.0')
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'))
+
+            develop('mpich@1.0')
+            # disk representation isn't updated unless we write
+            # second develop command doesn't change it, so we don't write
+            # but we check disk representation
+            e.write()
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'))
+            assert len(e.dev_specs) == 1
+
+    def test_develop_update_path(self, tmpdir):
+        env('create', 'test')
+        with ev.read('test') as e:
+            develop('mpich@1.0')
+            develop('-p', str(tmpdir), 'mpich@1.0')
+            self.check_develop(e, spack.spec.Spec('mpich@1.0'), str(tmpdir))
+            assert len(e.dev_specs) == 1
+
+    def test_develop_update_spec(self):
+        env('create', 'test')
+        with ev.read('test') as e:
+            develop('mpich@1.0')
+            develop('mpich@2.0')
+            self.check_develop(e, spack.spec.Spec('mpich@2.0'))
+            assert len(e.dev_specs) == 1
diff --git a/lib/spack/spack/test/cmd/undevelop.py b/lib/spack/spack/test/cmd/undevelop.py
new file mode 100644
index 0000000000..2ebc554f29
--- /dev/null
+++ b/lib/spack/spack/test/cmd/undevelop.py
@@ -0,0 +1,67 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import spack.spec
+import spack.environment as ev
+from spack.main import SpackCommand
+
+undevelop = SpackCommand('undevelop')
+env = SpackCommand('env')
+concretize = SpackCommand('concretize')
+
+
+def test_undevelop(tmpdir, mock_packages, mutable_mock_env_path):
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - mpich
+
+  develop:
+    mpich:
+      spec: mpich@1.0
+      path: /fake/path
+""")
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test'):
+            before = spack.spec.Spec('mpich').concretized()
+            undevelop('mpich')
+            after = spack.spec.Spec('mpich').concretized()
+
+    # Removing dev spec from environment changes concretization
+    assert before.satisfies('dev_path=any')
+    assert not after.satisfies('dev_path=any')
+
+
+def test_undevelop_nonexistent(tmpdir, mock_packages, mutable_mock_env_path):
+    # setup environment
+    envdir = tmpdir.mkdir('env')
+    with envdir.as_cwd():
+        with open('spack.yaml', 'w') as f:
+            f.write("""\
+env:
+  specs:
+  - mpich
+
+  develop:
+    mpich:
+      spec: mpich@1.0
+      path: /fake/path
+""")
+
+        env('create', 'test', './spack.yaml')
+        with ev.read('test') as e:
+            concretize()
+            before = e.specs_by_hash
+            undevelop('package-not-in-develop')  # does nothing
+            concretize('-f')
+            after = e.specs_by_hash
+
+    # nothing should have changed
+    assert before == after
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index 6e86d7c914..65ac93105a 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -195,7 +195,7 @@ class TestConcretize(object):
             s.satisfies('mpich2') for s in repo.providers_for('mpi@3')
         )
 
-    def test_provides_handles_multiple_providers_of_same_vesrion(self):
+    def test_provides_handles_multiple_providers_of_same_version(self):
         """
         """
         providers = spack.repo.path.providers_for('mpi@3.0')
@@ -644,3 +644,12 @@ class TestConcretize(object):
         with pytest.raises(spack.error.SpecError):
             s = Spec('+variant')
             s.concretize()
+
+    def test_concretize_anonymous_dep(self):
+        with pytest.raises(spack.error.SpecError):
+            s = Spec('mpileaks ^%gcc')
+            s.concretize()
+
+        with pytest.raises(spack.error.SpecError):
+            s = Spec('mpileaks ^cflags=-g')
+            s.concretize()
diff --git a/lib/spack/spack/test/concretize_preferences.py b/lib/spack/spack/test/concretize_preferences.py
index c941ccad94..3910d528e9 100644
--- a/lib/spack/spack/test/concretize_preferences.py
+++ b/lib/spack/spack/test/concretize_preferences.py
@@ -9,6 +9,7 @@ import stat
 import spack.package_prefs
 import spack.repo
 import spack.util.spack_yaml as syaml
+from spack.concretize import NonDeterministicVariantError
 from spack.config import ConfigScope, ConfigError
 from spack.spec import Spec
 from spack.version import Version
@@ -84,6 +85,23 @@ class TestConcretizePreferences(object):
             'mpileaks', debug=True, opt=True, shared=False, static=False
         )
 
+    def test_preferred_variants_from_any(self):
+        """
+        Test that 'foo=any' concretizes to any non-none value
+
+        Test that concretization of variants raises an error attempting
+        non-deterministic concretization from 'any' when preferred value is
+        'none'.
+        """
+        update_packages('multivalue-variant', 'variants', 'foo=bar')
+        assert_variant_values(
+            'multivalue-variant foo=any', foo=('bar',)
+        )
+
+        update_packages('multivalue-variant', 'variants', 'foo=none')
+        with pytest.raises(NonDeterministicVariantError):
+            concretize('multivalue-variant foo=any')
+
     def test_preferred_compilers(self):
         """Test preferred compilers are applied correctly
         """
diff --git a/lib/spack/spack/test/install.py b/lib/spack/spack/test/install.py
index 6d15dff7f6..47ec06bbc4 100644
--- a/lib/spack/spack/test/install.py
+++ b/lib/spack/spack/test/install.py
@@ -512,12 +512,6 @@ def test_unconcretized_install(install_mockery, mock_fetch, mock_packages):
     with pytest.raises(ValueError, match="only install concrete packages"):
         spec.package.do_install()
 
-    with pytest.raises(ValueError, match="only fetch concrete packages"):
-        spec.package.do_fetch()
-
-    with pytest.raises(ValueError, match="only stage concrete packages"):
-        spec.package.do_stage()
-
     with pytest.raises(ValueError, match="only patch concrete packages"):
         spec.package.do_patch()
 
diff --git a/lib/spack/spack/test/modules/conftest.py b/lib/spack/spack/test/modules/conftest.py
index 4ba25bd073..365a5a7092 100644
--- a/lib/spack/spack/test/modules/conftest.py
+++ b/lib/spack/spack/test/modules/conftest.py
@@ -34,7 +34,6 @@ def filename_dict(file_registry, monkeypatch):
         if not mode == 'w':
             raise RuntimeError('opening mode must be "w" [stringio_open]')
 
-        file_registry[filename] = StringIO()
         try:
             yield file_registry[filename]
         finally:
@@ -63,6 +62,7 @@ def modulefile_content(filename_dict, request):
 
         # Get its filename
         filename = generator.layout.filename
+
         # Retrieve the content
         content = filename_dict[filename].split('\n')
         generator.remove()
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index e4d111a64f..a258e45826 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -7,6 +7,7 @@ import sys
 import pytest
 
 from spack.error import SpecError, UnsatisfiableSpecError
+from spack.spec import UnconstrainableDependencySpecError
 from spack.spec import Spec, SpecFormatSigilError, SpecFormatStringError
 from spack.variant import InvalidVariantValueError, UnknownVariantError
 from spack.variant import MultipleValuesInExclusiveVariantError
@@ -80,7 +81,8 @@ def check_constrain_not_changed(spec, constraint):
 def check_invalid_constraint(spec, constraint):
     spec = Spec(spec)
     constraint = Spec(constraint)
-    with pytest.raises(UnsatisfiableSpecError):
+    with pytest.raises((UnsatisfiableSpecError,
+                        UnconstrainableDependencySpecError)):
         spec.constrain(constraint)
 
 
@@ -272,6 +274,8 @@ class TestSpecSematics(object):
         check_satisfies('mpich foo=true', 'mpich+foo')
         check_satisfies('mpich~foo', 'mpich foo=FALSE')
         check_satisfies('mpich foo=False', 'mpich~foo')
+        check_satisfies('mpich foo=any', 'mpich~foo')
+        check_satisfies('mpich +foo', 'mpich foo=any')
 
     def test_satisfies_multi_value_variant(self):
         # Check quoting
@@ -284,6 +288,12 @@ class TestSpecSematics(object):
 
         # A more constrained spec satisfies a less constrained one
         check_satisfies('multivalue-variant foo="bar,baz"',
+                        'multivalue-variant foo=any')
+
+        check_satisfies('multivalue-variant foo=any',
+                        'multivalue-variant foo="bar,baz"')
+
+        check_satisfies('multivalue-variant foo="bar,baz"',
                         'multivalue-variant foo="bar"')
 
         check_satisfies('multivalue-variant foo="bar,baz"',
@@ -307,6 +317,7 @@ class TestSpecSematics(object):
         a.concretize()
 
         assert a.satisfies('foobar=bar')
+        assert a.satisfies('foobar=any')
 
         # Assert that an autospec generated from a literal
         # gives the right result for a single valued variant
@@ -441,6 +452,10 @@ class TestSpecSematics(object):
         check_unsatisfiable('mpich', 'mpich~foo', True)
         check_unsatisfiable('mpich', 'mpich foo=1', True)
 
+        # None and any do not satisfy each other
+        check_unsatisfiable('foo=none', 'foo=any')
+        check_unsatisfiable('foo=any', 'foo=none')
+
     def test_unsatisfiable_variant_mismatch(self):
         # No matchi in specs
         check_unsatisfiable('mpich~foo', 'mpich+foo')
@@ -608,6 +623,11 @@ class TestSpecSematics(object):
             'multivalue-variant foo="baz"'
         )
 
+        check_constrain(
+            'libelf foo=bar,baz', 'libelf foo=bar,baz', 'libelf foo=any')
+        check_constrain(
+            'libelf foo=bar,baz', 'libelf foo=any', 'libelf foo=bar,baz')
+
     def test_constrain_compiler_flags(self):
         check_constrain(
             'libelf cflags="-O3" cppflags="-Wall"',
@@ -648,12 +668,15 @@ class TestSpecSematics(object):
         check_invalid_constraint('libelf+debug', 'libelf~debug')
         check_invalid_constraint('libelf+debug~foo', 'libelf+debug+foo')
         check_invalid_constraint('libelf debug=True', 'libelf debug=False')
+        check_invalid_constraint('libelf foo=none', 'libelf foo=any')
+        check_invalid_constraint('libelf foo=any', 'libelf foo=none')
 
         check_invalid_constraint(
             'libelf cppflags="-O3"', 'libelf cppflags="-O2"')
         check_invalid_constraint(
             'libelf platform=test target=be os=be', 'libelf target=fe os=fe'
         )
+        check_invalid_constraint('libdwarf', '^%gcc')
 
     def test_constrain_changed(self):
         check_constrain_changed('libelf', '@1.0')
@@ -661,6 +684,7 @@ class TestSpecSematics(object):
         check_constrain_changed('libelf', '%gcc')
         check_constrain_changed('libelf%gcc', '%gcc@4.5')
         check_constrain_changed('libelf', '+debug')
+        check_constrain_changed('libelf', 'debug=any')
         check_constrain_changed('libelf', '~debug')
         check_constrain_changed('libelf', 'debug=2')
         check_constrain_changed('libelf', 'cppflags="-O3"')
@@ -680,6 +704,7 @@ class TestSpecSematics(object):
         check_constrain_not_changed('libelf+debug', '+debug')
         check_constrain_not_changed('libelf~debug', '~debug')
         check_constrain_not_changed('libelf debug=2', 'debug=2')
+        check_constrain_not_changed('libelf debug=2', 'debug=any')
         check_constrain_not_changed(
             'libelf cppflags="-O3"', 'cppflags="-O3"')
 
@@ -893,13 +918,14 @@ class TestSpecSematics(object):
                 for x in ('cflags', 'cxxflags', 'fflags')
             )
 
-    def test_any_combination_of(self):
-        # Test that using 'none' and another value raise during concretization
-        spec = Spec('multivalue-variant foo=none,bar')
-        with pytest.raises(spack.error.SpecError) as exc_info:
-            spec.concretize()
+    def test_combination_of_any_or_none(self):
+        # Test that using 'none' and another value raises
+        with pytest.raises(spack.variant.InvalidVariantValueCombinationError):
+            Spec('multivalue-variant foo=none,bar')
 
-        assert "is mutually exclusive with any of the" in str(exc_info.value)
+        # Test that using 'any' and another value raises
+        with pytest.raises(spack.variant.InvalidVariantValueCombinationError):
+            Spec('multivalue-variant foo=any,bar')
 
     @pytest.mark.skipif(
         sys.version_info[0] == 2, reason='__wrapped__ requires python 3'
diff --git a/lib/spack/spack/variant.py b/lib/spack/spack/variant.py
index e43a002182..d5d6acf6ec 100644
--- a/lib/spack/spack/variant.py
+++ b/lib/spack/spack/variant.py
@@ -25,6 +25,8 @@ try:
 except ImportError:
     from collections import Sequence
 
+special_variant_values = [None, 'none', 'any']
+
 
 class Variant(object):
     """Represents a variant in a package, as declared in the
@@ -119,13 +121,14 @@ class Variant(object):
 
         # Check and record the values that are not allowed
         not_allowed_values = [
-            x for x in value if self.single_value_validator(x) is False
+            x for x in value
+            if x != 'any' and self.single_value_validator(x) is False
         ]
         if not_allowed_values:
             raise InvalidVariantValueError(self, not_allowed_values, pkg)
 
         # Validate the group of values if needed
-        if self.group_validator is not None:
+        if self.group_validator is not None and value != ('any',):
             self.group_validator(pkg.name, self.name, value)
 
     @property
@@ -267,6 +270,14 @@ class AbstractVariant(object):
             # Tuple is necessary here instead of list because the
             # values need to be hashed
             value = re.split(r'\s*,\s*', str(value))
+            value = list(map(lambda x: 'any' if str(x).upper() == 'ANY' else x,
+                             value))
+
+        for val in special_variant_values:
+            if val in value and len(value) > 1:
+                msg = "'%s' cannot be combined with other variant" % val
+                msg += " values."
+                raise InvalidVariantValueCombinationError(msg)
 
         # With multi-value variants it is necessary
         # to remove duplicates and give an order
@@ -302,7 +313,15 @@ class AbstractVariant(object):
         """
         # If names are different then `self` does not satisfy `other`
         # (`foo=bar` will never satisfy `baz=bar`)
-        return other.name == self.name
+        if other.name != self.name:
+            return False
+        # If the variant is already set to none, it can't satisfy any
+        if ('none' in self or None in self) and 'any' in other:
+            return False
+        # If the variant is set to any, it can't be constrained by none
+        if 'any' in self and ('none' in other or None in other):
+            return False
+        return True
 
     @implicit_variant_conversion
     def compatible(self, other):
@@ -317,8 +336,17 @@ class AbstractVariant(object):
         Returns:
             bool: True or False
         """
-        # If names are different then they are not compatible
-        return other.name == self.name
+        # If names are different then `self` is not compatible with `other`
+        # (`foo=bar` is incompatible with `baz=bar`)
+        if other.name != self.name:
+            return False
+        # If the variant is already set to none, incompatible with any
+        if ('none' in self or None in self) and 'any' in other:
+            return False
+        # If the variant is set to any, it can't be compatible with none
+        if 'any' in self and ('none' in other or None in other):
+            return False
+        return True
 
     @implicit_variant_conversion
     def constrain(self, other):
@@ -336,7 +364,13 @@ class AbstractVariant(object):
             raise ValueError('variants must have the same name')
 
         old_value = self.value
-        self.value = ','.join(sorted(set(self.value + other.value)))
+
+        values = list(sorted(set(self.value + other.value)))
+        # If we constraint any by another value, just take value
+        if 'any' in values and len(values) > 1:
+            values.remove('any')
+
+        self.value = ','.join(values)
         return old_value != self.value
 
     def __contains__(self, item):
@@ -367,16 +401,16 @@ class MultiValuedVariant(AbstractVariant):
         Returns:
             bool: True or False
         """
-        # If names are different then `self` does not satisfy `other`
-        # (`foo=bar` does not satisfy `baz=bar`)
-        if other.name != self.name:
-            return False
+        # If it doesn't satisfy as an AbstractVariant, it doesn't satisfy as a
+        # MultiValuedVariant this handles conflicts between none and any
+        super_sat = super(MultiValuedVariant, self).satisfies(other)
 
         # Otherwise we want all the values in `other` to be also in `self`
-        return all(v in self.value for v in other.value)
+        return super_sat and (all(v in self.value for v in other.value) or
+                              'any' in other or 'any' in self)
 
 
-class SingleValuedVariant(MultiValuedVariant):
+class SingleValuedVariant(AbstractVariant):
     """A variant that can hold multiple values, but one at a time."""
 
     def _value_setter(self, value):
@@ -393,12 +427,12 @@ class SingleValuedVariant(MultiValuedVariant):
 
     @implicit_variant_conversion
     def satisfies(self, other):
-        # If names are different then `self` does not satisfy `other`
-        # (`foo=bar` does not satisfy `baz=bar`)
-        if other.name != self.name:
-            return False
+        # If it doesn't satisfy as an AbstractVariant, it doesn't satisfy as a
+        # SingleValuedVariant this handles conflicts between none and any
+        abstract_sat = super(SingleValuedVariant, self).satisfies(other)
 
-        return self.value == other.value
+        return abstract_sat and (self.value == other.value or
+                                 other.value == 'any' or self.value == 'any')
 
     def compatible(self, other):
         return self.satisfies(other)
@@ -408,6 +442,13 @@ class SingleValuedVariant(MultiValuedVariant):
         if self.name != other.name:
             raise ValueError('variants must have the same name')
 
+        if self.value == 'any':
+            self.value = other.value
+            return self.value != other.value
+
+        if other.value == 'any' and self.value not in ('none', None):
+            return False
+
         if self.value != other.value:
             raise UnsatisfiableVariantSpecError(other.value, self.value)
         return False
@@ -420,7 +461,10 @@ class SingleValuedVariant(MultiValuedVariant):
 
 
 class BoolValuedVariant(SingleValuedVariant):
-    """A variant that can hold either True or False."""
+    """A variant that can hold either True or False.
+
+    BoolValuedVariant can also hold the value 'any', for coerced
+    comparisons between ``foo=any`` and ``+foo`` or ``~foo``."""
 
     def _value_setter(self, value):
         # Check the string representation of the value and turn
@@ -431,6 +475,9 @@ class BoolValuedVariant(SingleValuedVariant):
         elif str(value).upper() == 'FALSE':
             self._original_value = value
             self._value = False
+        elif str(value).upper() == 'ANY':
+            self._original_value = value
+            self._value = 'any'
         else:
             msg = 'cannot construct a BoolValuedVariant for "{0}" from '
             msg += 'a value that does not represent a bool'
@@ -604,6 +651,9 @@ def substitute_abstract_variants(spec):
     failed = []
     for name, v in spec.variants.items():
         if name in spack.directives.reserved_names:
+            if name == 'dev_path':
+                new_variant = SingleValuedVariant(name, v._original_value)
+                spec.variants.substitute(new_variant)
             continue
         pkg_variant = spec.package_class.variants.get(name, None)
         if not pkg_variant:
@@ -823,6 +873,10 @@ class MultipleValuesInExclusiveVariantError(error.SpecError, ValueError):
         )
 
 
+class InvalidVariantValueCombinationError(error.SpecError):
+    """Raised when a variant has values 'any' or 'none' with other values."""
+
+
 class InvalidVariantValueError(error.SpecError):
     """Raised when a valid variant has at least an invalid value."""
 
diff --git a/share/spack/spack-completion.bash b/share/spack/spack-completion.bash
index 6b4b83b489..17bac4034f 100755
--- a/share/spack/spack-completion.bash
+++ b/share/spack/spack-completion.bash
@@ -320,7 +320,7 @@ _spack() {
     then
         SPACK_COMPREPLY="-h --help -H --all-help --color -C --config-scope -d --debug --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
     else
-        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup spec stage test uninstall unload url verify versions view"
+        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup spec stage test undevelop uninstall unload url verify versions view"
     fi
 }
 
@@ -725,6 +725,15 @@ _spack_dev_build() {
     fi
 }
 
+_spack_develop() {
+    if $list_options
+    then
+        SPACK_COMPREPLY="-h --help -p --path --no-clone --clone -f --force"
+    else
+        _all_packages
+    fi
+}
+
 _spack_docs() {
     SPACK_COMPREPLY="-h --help"
 }
@@ -1482,6 +1491,15 @@ _spack_test() {
     fi
 }
 
+_spack_undevelop() {
+    if $list_options
+    then
+        SPACK_COMPREPLY="-h --help -a --all"
+    else
+        _all_packages
+    fi
+}
+
 _spack_uninstall() {
     if $list_options
     then
diff --git a/var/spack/repos/builtin.mock/packages/dependent-of-dev-build/package.py b/var/spack/repos/builtin.mock/packages/dependent-of-dev-build/package.py
new file mode 100644
index 0000000000..9079fa766a
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/dependent-of-dev-build/package.py
@@ -0,0 +1,17 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+class DependentOfDevBuild(Package):
+    homepage = "example.com"
+    url = "fake.com"
+
+    version('0.0.0', sha256='0123456789abcdefgh')
+
+    depends_on('dev-build-test-install')
+
+    def install(self, spec, prefix):
+        with open(prefix.filename, 'w') as f:
+            f.write("This file is installed")
diff --git a/var/spack/repos/builtin.mock/packages/dev-build-test-dependent/package.py b/var/spack/repos/builtin.mock/packages/dev-build-test-dependent/package.py
new file mode 100644
index 0000000000..e6a5b8351f
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/dev-build-test-dependent/package.py
@@ -0,0 +1,29 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+class DevBuildTestDependent(Package):
+    homepage = "example.com"
+    url = "fake.com"
+
+    version('0.0.0', sha256='0123456789abcdefgh')
+
+    phases = ['edit', 'install']
+
+    filename = 'dev-build-test-file.txt'
+    original_string = "This file should be edited"
+    replacement_string = "This file has been edited"
+
+    depends_on('dev-build-test-install')
+
+    def edit(self, spec, prefix):
+        with open(self.filename, 'r+') as f:
+            assert f.read() == self.original_string
+            f.seek(0)
+            f.truncate()
+            f.write(self.replacement_string)
+
+    def install(self, spec, prefix):
+        install(self.filename, prefix)