configure: specify labels to sysroots in sysroot_label_map

This change has the same motivation and caveats as the `toolchain_roots`
change (previous commit).

For `sysroot` I considered just flipping the map since I think forcing a
`1:1` mapping would be more acceptable but:
  - this is a potentially more confusing breaking change
  - this would force us to drop support for absolute paths to a sysroot

tests/MODULE.bazel

@@ -158,7 +158,10 @@ llvm.toolchain(
     name = "llvm_toolchain_with_sysroot",
     llvm_versions = LLVM_VERSIONS,
     sysroot = {
-        "linux-x86_64": "@org_chromium_sysroot_linux_x64//:sysroot",
+        "linux-x86_64": "chromium_x64_sysroot",
+    },
+    sysroot_label_map = {
+        "@org_chromium_sysroot_linux_x64//:sysroot": "chromium_x64_sysroot",
     },
     # We can share the downloaded LLVM distribution with the first configuration.
     toolchain_roots = {

tests/WORKSPACE

@@ -120,7 +120,10 @@ llvm_toolchain(
     name = "llvm_toolchain_with_sysroot",
     llvm_versions = LLVM_VERSIONS,
     sysroot = {
-        "linux-x86_64": "@org_chromium_sysroot_linux_x64//:sysroot",
+        "linux-x86_64": "chromium_x64_sysroot",
+    },
+    sysroot_label_map = {
+        "@org_chromium_sysroot_linux_x64//:sysroot": "chromium_x64_sysroot",
     },
     # We can share the downloaded LLVM distribution with the first configuration.
     toolchain_roots = {

toolchain/internal/common.bzl

@@ -140,7 +140,7 @@ BZLMOD_ENABLED = "@@" in str(Label("//:unused"))
 
 def pkg_name_from_label(label):
     if label.workspace_name:
-        return "@" + label.workspace_name + "//" + label.package
+        return ("@@" if BZLMOD_ENABLED else "@") + label.workspace_name + "//" + label.package
     else:
         return label.package
 

toolchain/internal/configure.bzl

@@ -145,9 +145,12 @@ def llvm_register_toolchains():
         tools_path_prefix = llvm_dist_path_prefix + "bin/"
         symlinked_tools_str = ""
 
+    # Flip the sysroot label map (note: no error on duplicate "keys" for now)
+    _sysroot_label_map = { key: label for label, key in rctx.attr.sysroot_label_map.items() }
     sysroot_paths_dict, sysroot_labels_dict = _sysroot_paths_dict(
         rctx,
         rctx.attr.sysroot,
+        _sysroot_label_map,
         use_absolute_paths_sysroot,
     )
 

toolchain/internal/repo.bzl

@@ -111,9 +111,18 @@ _compiler_configuration_attrs = {
                "({}), ".format(_target_pairs) +
                "used to indicate the set of files that form the sysroot for the compiler. " +
                "If the value begins with exactly one forward slash '/', then the value is " +
-               "assumed to be a system path. Else, the value will be assumed to be a label " +
-               "containing the files and the sysroot path will be taken as the path to the " +
-               "package of this label."),
+               "assumed to be a system path. Else, the value will be assumed to be a key into " +
+               "`sysroot_label_map` that then points to a label containing the files. The " +
+               "sysroot path will be taken as the path to the package of this label."),
+    ),
+    # NOTE: see the comments on `toolchain_roots_label_map`; this exists for
+    # similar reasons.
+    "sysroot_label_map": attr.label_keyed_string_dict(
+        mandatory = False,
+        doc = ("Inverted map from sysroot Label to string identifier.\n" +
+               "This attribute is to be used with `sysroot` in order to specify sysroots " +
+               "that come from a Bazel package. Keys can be any identifier that do not start " +
+               "with a forward slash '/`.\n"),
     ),
     "cxx_builtin_include_directories": attr.string_list_dict(
         mandatory = False,

toolchain/internal/sysroot.bzl

@@ -39,7 +39,7 @@ def default_sysroot_path(rctx, os):
         return ""
 
 # Return the sysroot path and the label to the files, if sysroot is not a system path.
-def _sysroot_path(sysroot_dict, os, arch):
+def _sysroot_path(sysroot_dict, label_map, os, arch):
     sysroot = sysroot_dict.get(_os_arch_pair(os, arch))
     if not sysroot:
         return (None, None)
@@ -50,18 +50,26 @@ def _sysroot_path(sysroot_dict, os, arch):
     if sysroot[0] == "/" and (len(sysroot) == 1 or sysroot[1] != "/"):
         return (sysroot, None)
 
-    label = Label(sysroot)
+    # Otherwise, consult the label map:
+    if sysroot not in label_map:
+        fail(("Value in `sysroot` {key} was interpreted as a `sysroot_label_map` " +
+            "key but it is not present in the map: {map}").format(
+            key = sysroot, map = label_map,
+        ))
+    label = label_map[sysroot]
+
     sysroot_path = _pkg_path_from_label(label)
     return (sysroot_path, label)
 
 # Return dictionaries for paths (relative or absolute) and labels if the
 # sysroot needs to be included in the build sandbox.
-def sysroot_paths_dict(rctx, sysroot_dict, use_absolute_paths):
+def sysroot_paths_dict(rctx, sysroot_dict, sysroot_label_map, use_absolute_paths):
     paths_dict = dict()
     labels_dict = dict()
     for (target_os, target_arch) in _supported_targets:
         path, label = _sysroot_path(
             sysroot_dict,
+            sysroot_label_map,
             target_os,
             target_arch,
         )
@@ -71,6 +79,8 @@ def sysroot_paths_dict(rctx, sysroot_dict, use_absolute_paths):
         if label and use_absolute_paths:
             # Get a label for a regular file in the sysroot package.
             # Exact target does not matter.
+            #
+            # NOTE: this path has to exist
             label = Label(_pkg_name_from_label(label) + ":BUILD.bazel")
             path = _canonical_dir_path(str(rctx.path(label).dirname))
             label = None