`TOOLCHAINS=<swift snapshot bundle id>` emits unresolved Swift toolchain linker search path on macOS

[bc-lee]

Using a system-wide installed Swift snapshot on macOS can emit a nonexistent linker search path.

I installed this snapshot:

https://download.swift.org/development/xcode/swift-DEVELOPMENT-SNAPSHOT-2026-03-16-a/swift-DEVELOPMENT-SNAPSHOT-2026-03-16-a-osx.pkg

Then I built one of my Swift projects with:

bazel build ... --action_env=TOOLCHAINS=org.swift.64202603161a

The bundle identifier was extracted with:

plutil -extract CFBundleIdentifier raw -o - \
  /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2026-03-16-a.xctoolchain/Info.plist

Basic compilation works, but the link step emits this warning:

ld: warning: search path '__BAZEL_SWIFT_TOOLCHAIN_PATH__/usr/lib/swift/macosx' not found

Root cause

After digging into this, the issue appears to be a placeholder mismatch inside rules_swift.

rules_swift emits __BAZEL_SWIFT_TOOLCHAIN_PATH__ into linker flags for the custom toolchain path, but apple_support only substitutes __BAZEL_CUSTOM_XCODE_TOOLCHAIN_PATH__ during link argument processing.

That leaves this linker flag unresolved:

-L__BAZEL_SWIFT_TOOLCHAIN_PATH__/usr/lib/swift/macosx

Expected behavior

When TOOLCHAINS is used with an installed Swift snapshot toolchain, rules_swift should emit a linker-side placeholder that is actually resolved during link argument processing, so no nonexistent search path warning is produced.

Proposed fix

Use the Swift placeholder for Swift-side worker/tool behavior, but use the linker-compatible custom Xcode toolchain placeholder when constructing linker search paths for custom toolchains.

I have a small patch and regression test for this and can open a PR.