Compute Swift toolchain root path dynamically for Bazel 9.

The canonical repo name for http_archive repos changed from
+_repo_rules2+ to +http_archive+ in Bazel 9. Instead of
hardcoding the path, use a repository rule to resolve it at
fetch time via label resolution.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sayrer

MODULE.bazel

@@ -199,7 +199,10 @@ exports_files(glob(["usr/**/*"]))
     urls = ["https://download.swift.org/swift-6.0.3-release/ubuntu2204-aarch64/swift-6.0.3-RELEASE/swift-6.0.3-RELEASE-ubuntu22.04-aarch64.tar.gz"],
 )
 
-register_toolchains("//tools/swift:hermetic_swift_linux_x86_64_toolchain")
+swift_toolchain_ext = use_extension("//tools/swift:extensions.bzl", "swift_toolchain_ext")
+use_repo(swift_toolchain_ext, "hermetic_swift_toolchain")
+
+register_toolchains("@hermetic_swift_toolchain//:hermetic_swift_linux_x86_64_toolchain")
 
 ###############################################################################
 # Hermetic LLVM Toolchain

tools/swift/BUILD.bazel

@@ -1,33 +1,8 @@
-"""Hermetic Swift toolchain configuration."""
+"""Hermetic Swift toolchain configuration.
 
-load("@rules_swift//swift/toolchains:swift_toolchain.bzl", "swift_toolchain")
+The actual swift_toolchain and toolchain rules are generated dynamically
+by the swift_toolchain_repo repository rule (swift_toolchain_wrapper.bzl)
+to avoid hardcoding the canonical repo path which changes between Bazel versions.
+"""
 
 package(default_visibility = ["//visibility:public"])
-
-# Hermetic Swift 6.0.3 toolchain for Linux x86_64
-swift_toolchain(
-    name = "hermetic_swift_linux_x86_64",
-    arch = "x86_64",
-    # Enable module wrapping (swift-modulewrap) instead of -add_ast_path which is Darwin-only
-    features = [
-        "swift.use_module_wrap",
-    ],
-    os = "linux",
-    root = "external/+_repo_rules2+swift_linux_x86_64/usr",
-    toolchain_files = "@swift_linux_x86_64//:usr",
-    version_file = "@swift_linux_x86_64//:version_file",
-)
-
-toolchain(
-    name = "hermetic_swift_linux_x86_64_toolchain",
-    exec_compatible_with = [
-        "@platforms//os:linux",
-        "@platforms//cpu:x86_64",
-    ],
-    target_compatible_with = [
-        "@platforms//os:linux",
-        "@platforms//cpu:x86_64",
-    ],
-    toolchain = ":hermetic_swift_linux_x86_64",
-    toolchain_type = "@rules_swift//toolchains:toolchain_type",
-)

tools/swift/extensions.bzl

@@ -0,0 +1,10 @@
+"""Module extension for hermetic Swift toolchain."""
+
+load("//tools/swift:swift_toolchain_wrapper.bzl", "swift_toolchain_repo")
+
+def _swift_toolchain_impl(ctx):
+    swift_toolchain_repo(name = "hermetic_swift_toolchain")
+
+swift_toolchain_ext = module_extension(
+    implementation = _swift_toolchain_impl,
+)

tools/swift/swift_toolchain_wrapper.bzl

@@ -1,18 +1,49 @@
-"""Repository rule to create a Swift toolchain with absolute paths."""
+"""Repository rule to create a Swift toolchain with the correct repo path."""
 
-def _swift_toolchain_path_impl(repository_ctx):
-    """Generate the absolute path to the Swift toolchain."""
-    # Get the path to the Swift toolchain
-    swift_path = str(repository_ctx.path(Label("@swift_linux_x86_64//:usr")).dirname.dirname)
+def _swift_toolchain_repo_impl(repository_ctx):
+    """Generate a swift_toolchain BUILD file with the correct root path."""
+    # Resolve the canonical path to the Swift toolchain repo
+    swift_repo_path = str(repository_ctx.path(Label("@swift_linux_x86_64//:BUILD.bazel")).dirname)
+
+    # Extract the canonical repo directory name (e.g., "+http_archive+swift_linux_x86_64")
+    # by finding the "external/" segment in the absolute path
+    parts = swift_repo_path.split("/external/")
+    repo_dir = parts[-1]
+
+    root = "external/{}/usr".format(repo_dir)
 
     repository_ctx.file("BUILD.bazel", """
+load("@rules_swift//swift/toolchains:swift_toolchain.bzl", "swift_toolchain")
+
 package(default_visibility = ["//visibility:public"])
 
-exports_files(["swift_root.txt"])
-""")
+swift_toolchain(
+    name = "hermetic_swift_linux_x86_64",
+    arch = "x86_64",
+    features = [
+        "swift.use_module_wrap",
+    ],
+    os = "linux",
+    root = "{root}",
+    toolchain_files = "@swift_linux_x86_64//:usr",
+    version_file = "@swift_linux_x86_64//:version_file",
+)
 
-    repository_ctx.file("swift_root.txt", swift_path + "/usr")
+toolchain(
+    name = "hermetic_swift_linux_x86_64_toolchain",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    toolchain = ":hermetic_swift_linux_x86_64",
+    toolchain_type = "@rules_swift//toolchains:toolchain_type",
+)
+""".format(root = root))
 
-swift_toolchain_path = repository_rule(
-    implementation = _swift_toolchain_path_impl,
+swift_toolchain_repo = repository_rule(
+    implementation = _swift_toolchain_repo_impl,
 )