Add a "requires" directive, extend functionality of package requirements (#36286)

Add a "require" directive to packages, which functions exactly like
requirements specified in packages.yaml (uses the same fact-generation
logic); update both to allow making the requirement conditional.

* Packages may now use "require" to add constraints. This can be useful
  for something like "require(%gcc)" (where before we had to add a
  conflict for every compiler except gcc).
* Requirements (in packages.yaml or in a "require" directive) can be
  conditional on a spec, e.g. "require(%gcc, when=@1.0.0)" (version
  1.0.0 can only build with gcc).
* Requirements may include a message which clarifies why they are needed.
  The concretizer assigns a high priority to errors which generate these
  messages (in particular over errors for unsatisfied requirements that
  do not produce messages, but also over a number of more-generic
  errors).

2 files changed, 36 insertions, 0 deletions

diff --git a/var/spack/repos/builtin.mock/packages/requires_clang/package.py b/var/spack/repos/builtin.mock/packages/requires_clang/package.py
new file mode 100644
index 0000000000..9f1c2d0ba4
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/requires_clang/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2023 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)
+
+from spack.package import *
+
+
+class RequiresClang(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    version("0.9", md5="abcd456789abcdef0123456789abcdef")
+
+    requires("%clang", msg="can only be compiled with Clang")
diff --git a/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py b/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py
new file mode 100644
index 0000000000..18f924e92f
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/requires_clang_or_gcc/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2023 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)
+
+from spack.package import *
+
+
+class RequiresClangOrGcc(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/b-1.0.tar.gz"
+
+    version("1.0", md5="0123456789abcdef0123456789abcdef")
+    version("0.9", md5="abcd456789abcdef0123456789abcdef")
+
+    requires("%gcc", "%clang", policy="one_of")