Refactor to improve `spec format` speed (#43712)

When looking at where we spend our time in solver setup, I noticed a fair bit of time is spent
in `Spec.format()`, and `Spec.format()` is a pretty old, slow, convoluted method.

This PR does a number of things:
- [x] Consolidate most of what was being done manually with a character loop and several
      regexes into a single regex.
- [x] Precompile regexes where we keep them 
- [x] Remove the `transform=` argument to `Spec.format()` which was only used in one 
      place in the code (modules) to uppercase env var names, but added a lot of complexity
- [x] Avoid escaping and colorizing specs unless necessary
- [x] Refactor a lot of the colorization logic to avoid unnecessary object construction
- [x] Add type hints and remove some spots in the code where we were using nonexistent
      arguments to `format()`.
- [x] Add trivial cases to `__str__` in `VariantMap` and `VersionList` to avoid sorting
- [x] Avoid calling `isinstance()` in the main loop of `Spec.format()`
- [x] Don't bother constructing a `string` representation for the result of `_prev_version`
      as it is only used for comparisons.

In my timings (on all the specs formatted in a solve of `hdf5`), this is over 2.67x faster than the 
original `format()`, and it seems to reduce setup time by around a second (for `hdf5`).

12 files changed, 230 insertions, 274 deletions

diff --git a/lib/spack/llnl/util/tty/__init__.py b/lib/spack/llnl/util/tty/__init__.py
index 3f3ba1d462..807e50a159 100644
--- a/lib/spack/llnl/util/tty/__init__.py
+++ b/lib/spack/llnl/util/tty/__init__.py
@@ -12,7 +12,7 @@ import textwrap
 import traceback
 from datetime import datetime
 from sys import platform as _platform
-from typing import NoReturn
+from typing import Any, NoReturn
 
 if _platform != "win32":
     import fcntl
@@ -158,21 +158,22 @@ def get_timestamp(force=False):
         return ""
 
 
-def msg(message, *args, **kwargs):
+def msg(message: Any, *args: Any, newline: bool = True) -> None:
     if not msg_enabled():
         return
 
     if isinstance(message, Exception):
-        message = "%s: %s" % (message.__class__.__name__, str(message))
+        message = f"{message.__class__.__name__}: {message}"
+    else:
+        message = str(message)
 
-    newline = kwargs.get("newline", True)
     st_text = ""
     if _stacktrace:
         st_text = process_stacktrace(2)
-    if newline:
-        cprint("@*b{%s==>} %s%s" % (st_text, get_timestamp(), cescape(_output_filter(message))))
-    else:
-        cwrite("@*b{%s==>} %s%s" % (st_text, get_timestamp(), cescape(_output_filter(message))))
+
+    nl = "\n" if newline else ""
+    cwrite(f"@*b{{{st_text}==>}} {get_timestamp()}{cescape(_output_filter(message))}{nl}")
+
     for arg in args:
         print(indent + _output_filter(str(arg)))
 
diff --git a/lib/spack/llnl/util/tty/color.py b/lib/spack/llnl/util/tty/color.py
index bef7af8e58..a471505d34 100644
--- a/lib/spack/llnl/util/tty/color.py
+++ b/lib/spack/llnl/util/tty/color.py
@@ -62,6 +62,7 @@ To output an @, use '@@'.  To output a } inside braces, use '}}'.
 import re
 import sys
 from contextlib import contextmanager
+from typing import Optional
 
 
 class ColorParseError(Exception):
@@ -95,7 +96,7 @@ colors = {
 }  # white
 
 # Regex to be used for color formatting
-color_re = r"@(?:@|\.|([*_])?([a-zA-Z])?(?:{((?:[^}]|}})*)})?)"
+COLOR_RE = re.compile(r"@(?:(@)|(\.)|([*_])?([a-zA-Z])?(?:{((?:[^}]|}})*)})?)")
 
 # Mapping from color arguments to values for tty.set_color
 color_when_values = {"always": True, "auto": None, "never": False}
@@ -203,77 +204,64 @@ def color_when(value):
     set_color_when(old_value)
 
 
-class match_to_ansi:
-    def __init__(self, color=True, enclose=False, zsh=False):
-        self.color = _color_when_value(color)
-        self.enclose = enclose
-        self.zsh = zsh
-
-    def escape(self, s):
-        """Returns a TTY escape sequence for a color"""
-        if self.color:
-            if self.zsh:
-                result = rf"\e[0;{s}m"
-            else:
-                result = f"\033[{s}m"
-
-            if self.enclose:
-                result = rf"\[{result}\]"
-
-            return result
+def _escape(s: str, color: bool, enclose: bool, zsh: bool) -> str:
+    """Returns a TTY escape sequence for a color"""
+    if color:
+        if zsh:
+            result = rf"\e[0;{s}m"
         else:
-            return ""
+            result = f"\033[{s}m"
 
-    def __call__(self, match):
-        """Convert a match object generated by ``color_re`` into an ansi
-        color code. This can be used as a handler in ``re.sub``.
-        """
-        style, color, text = match.groups()
-        m = match.group(0)
-
-        if m == "@@":
-            return "@"
-        elif m == "@.":
-            return self.escape(0)
-        elif m == "@":
-            raise ColorParseError("Incomplete color format: '%s' in %s" % (m, match.string))
-
-        string = styles[style]
-        if color:
-            if color not in colors:
-                raise ColorParseError(
-                    "Invalid color specifier: '%s' in '%s'" % (color, match.string)
-                )
-            string += ";" + str(colors[color])
-
-        colored_text = ""
-        if text:
-            colored_text = text + self.escape(0)
+        if enclose:
+            result = rf"\[{result}\]"
 
-        return self.escape(string) + colored_text
+        return result
+    else:
+        return ""
 
 
-def colorize(string, **kwargs):
+def colorize(
+    string: str, color: Optional[bool] = None, enclose: bool = False, zsh: bool = False
+) -> str:
     """Replace all color expressions in a string with ANSI control codes.
 
     Args:
-        string (str): The string to replace
+        string: The string to replace
 
     Returns:
-        str: The filtered string
+        The filtered string
 
     Keyword Arguments:
-        color (bool): If False, output will be plain text without control
-            codes, for output to non-console devices.
-        enclose (bool): If True, enclose ansi color sequences with
+        color: If False, output will be plain text without control codes, for output to
+            non-console devices (default: automatically choose color or not)
+        enclose: If True, enclose ansi color sequences with
             square brackets to prevent misestimation of terminal width.
-        zsh (bool): If True, use zsh ansi codes instead of bash ones (for variables like PS1)
+        zsh: If True, use zsh ansi codes instead of bash ones (for variables like PS1)
     """
-    color = _color_when_value(kwargs.get("color", get_color_when()))
-    zsh = kwargs.get("zsh", False)
-    string = re.sub(color_re, match_to_ansi(color, kwargs.get("enclose")), string, zsh)
-    string = string.replace("}}", "}")
-    return string
+    color = color if color is not None else get_color_when()
+
+    def match_to_ansi(match):
+        """Convert a match object generated by ``COLOR_RE`` into an ansi
+        color code. This can be used as a handler in ``re.sub``.
+        """
+        escaped_at, dot, style, color_code, text = match.groups()
+
+        if escaped_at:
+            return "@"
+        elif dot:
+            return _escape(0, color, enclose, zsh)
+        elif not (style or color_code):
+            raise ColorParseError(
+                f"Incomplete color format: '{match.group(0)}' in '{match.string}'"
+            )
+
+        ansi_code = _escape(f"{styles[style]};{colors.get(color_code, '')}", color, enclose, zsh)
+        if text:
+            return f"{ansi_code}{text}{_escape(0, color, enclose, zsh)}"
+        else:
+            return ansi_code
+
+    return COLOR_RE.sub(match_to_ansi, string).replace("}}", "}")
 
 
 def clen(string):
@@ -305,7 +293,7 @@ def cprint(string, stream=None, color=None):
     cwrite(string + "\n", stream, color)
 
 
-def cescape(string):
+def cescape(string: str) -> str:
     """Escapes special characters needed for color codes.
 
     Replaces the following symbols with their equivalent literal forms:
@@ -321,10 +309,7 @@ def cescape(string):
     Returns:
         (str): the string with color codes escaped
     """
-    string = str(string)
-    string = string.replace("@", "@@")
-    string = string.replace("}", "}}")
-    return string
+    return string.replace("@", "@@").replace("}", "}}")
 
 
 class ColorStream:
diff --git a/lib/spack/spack/cmd/info.py b/lib/spack/spack/cmd/info.py
index b007c60516..3075107d07 100644
--- a/lib/spack/spack/cmd/info.py
+++ b/lib/spack/spack/cmd/info.py
@@ -263,8 +263,8 @@ def _fmt_name_and_default(variant):
     return color.colorize(f"@c{{{variant.name}}} @C{{[{_fmt_value(variant.default)}]}}")
 
 
-def _fmt_when(when, indent):
-    return color.colorize(f"{indent * ' '}@B{{when}} {color.cescape(when)}")
+def _fmt_when(when: "spack.spec.Spec", indent: int):
+    return color.colorize(f"{indent * ' '}@B{{when}} {color.cescape(str(when))}")
 
 
 def _fmt_variant_description(variant, width, indent):
@@ -441,7 +441,7 @@ def print_versions(pkg, args):
                 return "No URL"
 
         url = get_url(preferred) if pkg.has_code else ""
-        line = version("    {0}".format(pad(preferred))) + color.cescape(url)
+        line = version("    {0}".format(pad(preferred))) + color.cescape(str(url))
         color.cwrite(line)
 
         print()
@@ -464,7 +464,7 @@ def print_versions(pkg, args):
                 continue
 
             for v, url in vers:
-                line = version("    {0}".format(pad(v))) + color.cescape(url)
+                line = version("    {0}".format(pad(v))) + color.cescape(str(url))
                 color.cprint(line)
 
 
@@ -475,10 +475,7 @@ def print_virtuals(pkg, args):
     color.cprint(section_title("Virtual Packages: "))
     if pkg.provided:
         for when, specs in reversed(sorted(pkg.provided.items())):
-            line = "    %s provides %s" % (
-                when.colorized(),
-                ", ".join(s.colorized() for s in specs),
-            )
+            line = "    %s provides %s" % (when.cformat(), ", ".join(s.cformat() for s in specs))
             print(line)
 
     else:
@@ -497,7 +494,9 @@ def print_licenses(pkg, args):
         pad = padder(pkg.licenses, 4)
         for when_spec in pkg.licenses:
             license_identifier = pkg.licenses[when_spec]
-            line = license("    {0}".format(pad(license_identifier))) + color.cescape(when_spec)
+            line = license("    {0}".format(pad(license_identifier))) + color.cescape(
+                str(when_spec)
+            )
             color.cprint(line)
 
 
diff --git a/lib/spack/spack/modules/common.py b/lib/spack/spack/modules/common.py
index 20d75f61c1..5e46ca4c6c 100644
--- a/lib/spack/spack/modules/common.py
+++ b/lib/spack/spack/modules/common.py
@@ -83,6 +83,17 @@ _valid_tokens = (
 )
 
 
+_FORMAT_STRING_RE = re.compile(r"({[^}]*})")
+
+
+def _format_env_var_name(spec, var_name_fmt):
+    """Format the variable name, but uppercase any formatted fields."""
+    fmt_parts = _FORMAT_STRING_RE.split(var_name_fmt)
+    return "".join(
+        spec.format(part).upper() if _FORMAT_STRING_RE.match(part) else part for part in fmt_parts
+    )
+
+
 def _check_tokens_are_valid(format_string, message):
     """Checks that the tokens used in the format string are valid in
     the context of module file and environment variable naming.
@@ -737,20 +748,12 @@ class BaseContext(tengine.Context):
         exclude = self.conf.exclude_env_vars
 
         # We may have tokens to substitute in environment commands
-
-        # Prepare a suitable transformation dictionary for the names
-        # of the environment variables. This means turn the valid
-        # tokens uppercase.
-        transform = {}
-        for token in _valid_tokens:
-            transform[token] = lambda s, string: str.upper(string)
-
         for x in env:
             # Ensure all the tokens are valid in this context
             msg = "some tokens cannot be expanded in an environment variable name"
+
             _check_tokens_are_valid(x.name, message=msg)
-            # Transform them
-            x.name = self.spec.format(x.name, transform=transform)
+            x.name = _format_env_var_name(self.spec, x.name)
             if self.modification_needs_formatting(x):
                 try:
                     # Not every command has a value
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 301510336d..7d8266400e 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -51,7 +51,6 @@ line is a spec for a particular installation of the mpileaks package.
 import collections
 import collections.abc
 import enum
-import io
 import itertools
 import os
 import pathlib
@@ -59,7 +58,7 @@ import platform
 import re
 import socket
 import warnings
-from typing import Any, Callable, Dict, List, Optional, Set, Tuple, Union
+from typing import Any, Callable, Dict, List, Match, Optional, Set, Tuple, Union
 
 import llnl.path
 import llnl.string
@@ -121,36 +120,44 @@ __all__ = [
     "SpecDeprecatedError",
 ]
 
+
+SPEC_FORMAT_RE = re.compile(
+    r"(?:"  # this is one big or, with matches ordered by priority
+    # OPTION 1: escaped character (needs to be first to catch opening \{)
+    # Note that an unterminated \ at the end of a string is left untouched
+    r"(?:\\(.))"
+    r"|"  # or
+    # OPTION 2: an actual format string
+    r"{"  # non-escaped open brace {
+    r"([%@/]|arch=)?"  # optional sigil (to print sigil in color)
+    r"(?:\^([^}\.]+)\.)?"  # optional ^depname. (to get attr from dependency)
+    # after the sigil or depname, we can have a hash expression or another attribute
+    r"(?:"  # one of
+    r"(hash\b)(?:\:(\d+))?"  # hash followed by :<optional length>
+    r"|"  # or
+    r"([^}]*)"  # another attribute to format
+    r")"  # end one of
+    r"(})?"  # finish format string with non-escaped close brace }, or missing if not present
+    r"|"
+    # OPTION 3: mismatched close brace (option 2 would consume a matched open brace)
+    r"(})"  # brace
+    r")",
+    re.IGNORECASE,
+)
+
 #: Valid pattern for an identifier in Spack
 
 IDENTIFIER_RE = r"\w[\w-]*"
 
+# Coloring of specs when using color output. Fields are printed with
+# different colors to enhance readability.
+# See llnl.util.tty.color for descriptions of the color codes.
 COMPILER_COLOR = "@g"  #: color for highlighting compilers
 VERSION_COLOR = "@c"  #: color for highlighting versions
 ARCHITECTURE_COLOR = "@m"  #: color for highlighting architectures
-ENABLED_VARIANT_COLOR = "@B"  #: color for highlighting enabled variants
-DISABLED_VARIANT_COLOR = "r"  #: color for highlighting disabled varaints
-DEPENDENCY_COLOR = "@."  #: color for highlighting dependencies
+VARIANT_COLOR = "@B"  #: color for highlighting variants
 HASH_COLOR = "@K"  #: color for highlighting package hashes
 
-#: This map determines the coloring of specs when using color output.
-#: We make the fields different colors to enhance readability.
-#: See llnl.util.tty.color for descriptions of the color codes.
-COLOR_FORMATS = {
-    "%": COMPILER_COLOR,
-    "@": VERSION_COLOR,
-    "=": ARCHITECTURE_COLOR,
-    "+": ENABLED_VARIANT_COLOR,
-    "~": DISABLED_VARIANT_COLOR,
-    "^": DEPENDENCY_COLOR,
-    "#": HASH_COLOR,
-}
-
-#: Regex used for splitting by spec field separators.
-#: These need to be escaped to avoid metacharacters in
-#: ``COLOR_FORMATS.keys()``.
-_SEPARATORS = "[\\%s]" % "\\".join(COLOR_FORMATS.keys())
-
 #: Default format for Spec.format(). This format can be round-tripped, so that:
 #:     Spec(Spec("string").format()) == Spec("string)"
 DEFAULT_FORMAT = (
@@ -193,26 +200,7 @@ class InstallStatus(enum.Enum):
     missing = "@r{[-]}  "
 
 
-def colorize_spec(spec):
-    """Returns a spec colorized according to the colors specified in
-    COLOR_FORMATS."""
-
-    class insert_color:
-        def __init__(self):
-            self.last = None
-
-        def __call__(self, match):
-            # ignore compiler versions (color same as compiler)
-            sep = match.group(0)
-            if self.last == "%" and sep == "@":
-                return clr.cescape(sep)
-            self.last = sep
-
-            return "%s%s" % (COLOR_FORMATS[sep], clr.cescape(sep))
-
-    return clr.colorize(re.sub(_SEPARATORS, insert_color(), str(spec)) + "@.")
-
-
+# regexes used in spec formatting
 OLD_STYLE_FMT_RE = re.compile(r"\${[A-Z]+}")
 
 
@@ -4295,10 +4283,7 @@ class Spec:
 
         yield deps
 
-    def colorized(self):
-        return colorize_spec(self)
-
-    def format(self, format_string=DEFAULT_FORMAT, **kwargs):
+    def format(self, format_string: str = DEFAULT_FORMAT, color: Optional[bool] = False) -> str:
         r"""Prints out particular pieces of a spec, depending on what is
         in the format string.
 
@@ -4361,79 +4346,65 @@ class Spec:
         literal ``\`` character.
 
         Args:
-            format_string (str): string containing the format to be expanded
-
-        Keyword Args:
-            color (bool): True if returned string is colored
-            transform (dict): maps full-string formats to a callable \
-                that accepts a string and returns another one
-
+            format_string: string containing the format to be expanded
+            color: True for colorized result; False for no color; None for auto color.
         """
         ensure_modern_format_string(format_string)
-        color = kwargs.get("color", False)
-        transform = kwargs.get("transform", {})
-
-        out = io.StringIO()
-
-        def write(s, c=None):
-            f = clr.cescape(s)
-            if c is not None:
-                f = COLOR_FORMATS[c] + f + "@."
-            clr.cwrite(f, stream=out, color=color)
-
-        def write_attribute(spec, attribute, color):
-            attribute = attribute.lower()
-
-            sig = ""
-            if attribute.startswith(("@", "%", "/")):
-                # color sigils that are inside braces
-                sig = attribute[0]
-                attribute = attribute[1:]
-            elif attribute.startswith("arch="):
-                sig = " arch="  # include space as separator
-                attribute = attribute[5:]
 
-            current = spec
-            if attribute.startswith("^"):
-                attribute = attribute[1:]
-                dep, attribute = attribute.split(".", 1)
-                current = self[dep]
+        def safe_color(sigil: str, string: str, color_fmt: Optional[str]) -> str:
+            # avoid colorizing if there is no color or the string is empty
+            if (color is False) or not color_fmt or not string:
+                return sigil + string
+            # escape and add the sigil here to avoid multiple concatenations
+            if sigil == "@":
+                sigil = "@@"
+            return clr.colorize(f"{color_fmt}{sigil}{clr.cescape(string)}@.", color=color)
+
+        def format_attribute(match_object: Match) -> str:
+            (esc, sig, dep, hash, hash_len, attribute, close_brace, unmatched_close_brace) = (
+                match_object.groups()
+            )
+            if esc:
+                return esc
+            elif unmatched_close_brace:
+                raise SpecFormatStringError(f"Unmatched close brace: '{format_string}'")
+            elif not close_brace:
+                raise SpecFormatStringError(f"Missing close brace: '{format_string}'")
+
+            current = self if dep is None else self[dep]
+
+            # Hash attributes can return early.
+            # NOTE: we currently treat abstract_hash like an attribute and ignore
+            # any length associated with it. We may want to change that.
+            if hash:
+                if sig and sig != "/":
+                    raise SpecFormatSigilError(sig, "DAG hashes", hash)
+                try:
+                    length = int(hash_len) if hash_len else None
+                except ValueError:
+                    raise SpecFormatStringError(f"Invalid hash length: '{hash_len}'")
+                return safe_color(sig or "", current.dag_hash(length), HASH_COLOR)
 
             if attribute == "":
                 raise SpecFormatStringError("Format string attributes must be non-empty")
 
+            attribute = attribute.lower()
             parts = attribute.split(".")
             assert parts
 
             # check that the sigil is valid for the attribute.
-            if sig == "@" and parts[-1] not in ("versions", "version"):
+            if not sig:
+                sig = ""
+            elif sig == "@" and parts[-1] not in ("versions", "version"):
                 raise SpecFormatSigilError(sig, "versions", attribute)
             elif sig == "%" and attribute not in ("compiler", "compiler.name"):
                 raise SpecFormatSigilError(sig, "compilers", attribute)
-            elif sig == "/" and not re.match(r"(abstract_)?hash(:\d+)?$", attribute):
+            elif sig == "/" and attribute != "abstract_hash":
                 raise SpecFormatSigilError(sig, "DAG hashes", attribute)
-            elif sig == " arch=" and attribute not in ("architecture", "arch"):
-                raise SpecFormatSigilError(sig, "the architecture", attribute)
-
-            # find the morph function for our attribute
-            morph = transform.get(attribute, lambda s, x: x)
-
-            # Special cases for non-spec attributes and hashes.
-            # These must be the only non-dep component of the format attribute
-            if attribute == "spack_root":
-                write(morph(spec, spack.paths.spack_root))
-                return
-            elif attribute == "spack_install":
-                write(morph(spec, spack.store.STORE.layout.root))
-                return
-            elif re.match(r"hash(:\d)?", attribute):
-                col = "#"
-                if ":" in attribute:
-                    _, length = attribute.split(":")
-                    write(sig + morph(spec, current.dag_hash(int(length))), col)
-                else:
-                    write(sig + morph(spec, current.dag_hash()), col)
-                return
+            elif sig == "arch=":
+                if attribute not in ("architecture", "arch"):
+                    raise SpecFormatSigilError(sig, "the architecture", attribute)
+                sig = " arch="  # include space as separator
 
             # Iterate over components using getattr to get next element
             for idx, part in enumerate(parts):
@@ -4442,7 +4413,7 @@ class Spec:
                 if part.startswith("_"):
                     raise SpecFormatStringError("Attempted to format private attribute")
                 else:
-                    if isinstance(current, vt.VariantMap):
+                    if part == "variants" and isinstance(current, vt.VariantMap):
                         # subscript instead of getattr for variant names
                         current = current[part]
                     else:
@@ -4466,62 +4437,31 @@ class Spec:
                             raise SpecFormatStringError(m)
                         if isinstance(current, vn.VersionList):
                             if current == vn.any_version:
-                                # We don't print empty version lists
-                                return
+                                # don't print empty version lists
+                                return ""
 
                     if callable(current):
                         raise SpecFormatStringError("Attempted to format callable object")
+
                     if current is None:
-                        # We're not printing anything
-                        return
+                        # not printing anything
+                        return ""
 
             # Set color codes for various attributes
-            col = None
+            color = None
             if "variants" in parts:
-                col = "+"
+                color = VARIANT_COLOR
             elif "architecture" in parts:
-                col = "="
+                color = ARCHITECTURE_COLOR
             elif "compiler" in parts or "compiler_flags" in parts:
-                col = "%"
+                color = COMPILER_COLOR
             elif "version" in parts or "versions" in parts:
-                col = "@"
-
-            # Finally, write the output
-            write(sig + morph(spec, str(current)), col)
-
-        attribute = ""
-        in_attribute = False
-        escape = False
-
-        for c in format_string:
-            if escape:
-                out.write(c)
-                escape = False
-            elif c == "\\":
-                escape = True
-            elif in_attribute:
-                if c == "}":
-                    write_attribute(self, attribute, color)
-                    attribute = ""
-                    in_attribute = False
-                else:
-                    attribute += c
-            else:
-                if c == "}":
-                    raise SpecFormatStringError(
-                        "Encountered closing } before opening { in %s" % format_string
-                    )
-                elif c == "{":
-                    in_attribute = True
-                else:
-                    out.write(c)
-        if in_attribute:
-            raise SpecFormatStringError(
-                "Format string terminated while reading attribute." "Missing terminating }."
-            )
+                color = VERSION_COLOR
 
-        formatted_spec = out.getvalue()
-        return formatted_spec.strip()
+            # return colored output
+            return safe_color(sig, str(current), color)
+
+        return SPEC_FORMAT_RE.sub(format_attribute, format_string).strip()
 
     def cformat(self, *args, **kwargs):
         """Same as format, but color defaults to auto instead of False."""
@@ -4529,6 +4469,16 @@ class Spec:
         kwargs.setdefault("color", None)
         return self.format(*args, **kwargs)
 
+    @property
+    def spack_root(self):
+        """Special field for using ``{spack_root}`` in Spec.format()."""
+        return spack.paths.spack_root
+
+    @property
+    def spack_install(self):
+        """Special field for using ``{spack_install}`` in Spec.format()."""
+        return spack.store.STORE.layout.root
+
     def format_path(
         # self, format_string: str, _path_ctor: Optional[pathlib.PurePath] = None
         self,
@@ -4554,14 +4504,21 @@ class Spec:
 
         path_ctor = _path_ctor or pathlib.PurePath
         format_string_as_path = path_ctor(format_string)
-        if format_string_as_path.is_absolute():
+        if format_string_as_path.is_absolute() or (
+            # Paths that begin with a single "\" on windows are relative, but we still
+            # want to preserve the initial "\\" to be consistent with PureWindowsPath.
+            # Ensure that this '\' is not passed to polite_filename() so it's not converted to '_'
+            (os.name == "nt" or path_ctor == pathlib.PureWindowsPath)
+            and format_string_as_path.parts[0] == "\\"
+        ):
             output_path_components = [format_string_as_path.parts[0]]
             input_path_components = list(format_string_as_path.parts[1:])
         else:
             output_path_components = []
             input_path_components = list(format_string_as_path.parts)
+
         output_path_components += [
-            fs.polite_filename(self.format(x)) for x in input_path_components
+            fs.polite_filename(self.format(part)) for part in input_path_components
         ]
         return str(path_ctor(*output_path_components))
 
diff --git a/lib/spack/spack/test/bindist.py b/lib/spack/spack/test/bindist.py
index 8afd69e3c5..b30955641f 100644
--- a/lib/spack/spack/test/bindist.py
+++ b/lib/spack/spack/test/bindist.py
@@ -390,11 +390,11 @@ def test_built_spec_cache(mirror_dir):
         assert any([r["spec"] == s for r in results])
 
 
-def fake_dag_hash(spec):
+def fake_dag_hash(spec, length=None):
     # Generate an arbitrary hash that is intended to be different than
     # whatever a Spec reported before (to test actions that trigger when
     # the hash changes)
-    return "tal4c7h4z0gqmixb1eqa92mjoybxn5l6"
+    return "tal4c7h4z0gqmixb1eqa92mjoybxn5l6"[:length]
 
 
 @pytest.mark.usefixtures(
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index 76052eeb38..64fe403daa 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -1276,7 +1276,7 @@ def test_user_config_path_is_default_when_env_var_is_empty(working_env):
 
 def test_default_install_tree(monkeypatch, default_config):
     s = spack.spec.Spec("nonexistent@x.y.z %none@a.b.c arch=foo-bar-baz")
-    monkeypatch.setattr(s, "dag_hash", lambda: "abc123")
+    monkeypatch.setattr(s, "dag_hash", lambda length: "abc123")
     _, _, projections = spack.store.parse_install_tree(spack.config.get("config"))
     assert s.format(projections["all"]) == "foo-bar-baz/none-a.b.c/nonexistent-x.y.z-abc123"
 
diff --git a/lib/spack/spack/test/modules/lmod.py b/lib/spack/spack/test/modules/lmod.py
index 9038d8c9f6..5128518caa 100644
--- a/lib/spack/spack/test/modules/lmod.py
+++ b/lib/spack/spack/test/modules/lmod.py
@@ -146,9 +146,6 @@ class TestLmod:
 
         assert len([x for x in content if "depends_on(" in x]) == 5
 
-    @pytest.mark.skipif(
-        str(archspec.cpu.host().family) != "x86_64", reason="test data is specific for x86_64"
-    )
     def test_alter_environment(self, modulefile_content, module_configuration):
         """Tests modifications to run-time environment."""
 
diff --git a/lib/spack/spack/test/modules/tcl.py b/lib/spack/spack/test/modules/tcl.py
index 6630065373..76e854ced9 100644
--- a/lib/spack/spack/test/modules/tcl.py
+++ b/lib/spack/spack/test/modules/tcl.py
@@ -114,9 +114,6 @@ class TestTcl:
 
         assert len([x for x in content if "prereq" in x]) == 5
 
-    @pytest.mark.skipif(
-        str(archspec.cpu.host().family) != "x86_64", reason="test data is specific for x86_64"
-    )
     def test_alter_environment(self, modulefile_content, module_configuration):
         """Tests modifications to run-time environment."""
 
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index 2f4e7f43a4..171c194dc0 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -703,22 +703,25 @@ class TestSpecSemantics:
             actual = spec.format(named_str)
             assert expected == actual
 
-    def test_spec_formatting_escapes(self, default_mock_concretization):
-        spec = default_mock_concretization("multivalue-variant cflags=-O2")
-
-        sigil_mismatches = [
+    @pytest.mark.parametrize(
+        "fmt_str",
+        [
             "{@name}",
             "{@version.concrete}",
             "{%compiler.version}",
             "{/hashd}",
             "{arch=architecture.os}",
-        ]
+        ],
+    )
+    def test_spec_formatting_sigil_mismatches(self, default_mock_concretization, fmt_str):
+        spec = default_mock_concretization("multivalue-variant cflags=-O2")
 
-        for fmt_str in sigil_mismatches:
-            with pytest.raises(SpecFormatSigilError):
-                spec.format(fmt_str)
+        with pytest.raises(SpecFormatSigilError):
+            spec.format(fmt_str)
 
-        bad_formats = [
+    @pytest.mark.parametrize(
+        "fmt_str",
+        [
             r"{}",
             r"name}",
             r"\{name}",
@@ -728,11 +731,12 @@ class TestSpecSemantics:
             r"{dag_hash}",
             r"{foo}",
             r"{+variants.debug}",
-        ]
-
-        for fmt_str in bad_formats:
-            with pytest.raises(SpecFormatStringError):
-                spec.format(fmt_str)
+        ],
+    )
+    def test_spec_formatting_bad_formats(self, default_mock_concretization, fmt_str):
+        spec = default_mock_concretization("multivalue-variant cflags=-O2")
+        with pytest.raises(SpecFormatStringError):
+            spec.format(fmt_str)
 
     def test_combination_of_wildcard_or_none(self):
         # Test that using 'none' and another value raises
@@ -1138,12 +1142,12 @@ def _check_spec_format_path(spec_str, format_str, expected, path_ctor=None):
             r"\\hostname\sharename\{name}\{version}",
             r"\\hostname\sharename\git-test\git.foo_bar",
         ),
-        # Windows doesn't attribute any significance to a leading
-        # "/" so it is discarded
+        # leading '/' is preserved on windows but converted to '\'
+        # note that it's still not "absolute" -- absolute windows paths start with a drive.
         (
             "git-test@git.foo/bar",
             r"/installroot/{name}/{version}",
-            r"installroot\git-test\git.foo_bar",
+            r"\installroot\git-test\git.foo_bar",
         ),
     ],
 )
diff --git a/lib/spack/spack/variant.py b/lib/spack/spack/variant.py
index 07aafe8945..bb4138116f 100644
--- a/lib/spack/spack/variant.py
+++ b/lib/spack/spack/variant.py
@@ -638,6 +638,9 @@ class VariantMap(lang.HashableMap):
         return clone
 
     def __str__(self):
+        if not self:
+            return ""
+
         # print keys in order
         sorted_keys = sorted(self.keys())
 
diff --git a/lib/spack/spack/version/version_types.py b/lib/spack/spack/version/version_types.py
index b772cd9d90..f35192192d 100644
--- a/lib/spack/spack/version/version_types.py
+++ b/lib/spack/spack/version/version_types.py
@@ -146,13 +146,11 @@ class StandardVersion(ConcreteVersion):
 
     @staticmethod
     def typemin():
-        return StandardVersion("", ((), (ALPHA,)), ("",))
+        return _STANDARD_VERSION_TYPEMIN
 
     @staticmethod
     def typemax():
-        return StandardVersion(
-            "infinity", ((VersionStrComponent(len(infinity_versions)),), (FINAL,)), ("",)
-        )
+        return _STANDARD_VERSION_TYPEMAX
 
     def __bool__(self):
         return True
@@ -390,6 +388,13 @@ class StandardVersion(ConcreteVersion):
         return self[:index]
 
 
+_STANDARD_VERSION_TYPEMIN = StandardVersion("", ((), (ALPHA,)), ("",))
+
+_STANDARD_VERSION_TYPEMAX = StandardVersion(
+    "infinity", ((VersionStrComponent(len(infinity_versions)),), (FINAL,)), ("",)
+)
+
+
 class GitVersion(ConcreteVersion):
     """Class to represent versions interpreted from git refs.
 
@@ -1019,6 +1024,9 @@ class VersionList:
         return hash(tuple(self.versions))
 
     def __str__(self):
+        if not self.versions:
+            return ""
+
         return ",".join(
             f"={v}" if isinstance(v, StandardVersion) else str(v) for v in self.versions
         )
@@ -1127,7 +1135,9 @@ def _prev_version(v: StandardVersion) -> StandardVersion:
     components[1::2] = separators[: len(release)]
     if prerelease_type != FINAL:
         components.extend((PRERELEASE_TO_STRING[prerelease_type], *prerelease[1:]))
-    return StandardVersion("".join(str(c) for c in components), (release, prerelease), separators)
+
+    # this is only used for comparison functions, so don't bother making a string
+    return StandardVersion(None, (release, prerelease), separators)
 
 
 def Version(string: Union[str, int]) -> Union[GitVersion, StandardVersion]: