Add macros support for Linux (#1108)

This uses Swift's builtin feature detection to check what flags are
available. This probably means previous toolchains where macros weren't
complete would return true for this, but that's probably fine.

This also requires absolutizing TMPDIR since that is validated in a case
on Linux and errors otherwise. Hoping this doesn't cause issues.

swift/internal/swift_autoconfiguration.bzl

@@ -40,6 +40,7 @@ load(
     "SWIFT_FEATURE_USE_MODULE_WRAP",
     "SWIFT_FEATURE_USE_OLD_DRIVER",
     "SWIFT_FEATURE_USE_RESPONSE_FILES",
+    "SWIFT_FEATURE__SUPPORTS_MACROS",
 )
 
 def _scratch_file(repository_ctx, temp_dir, name, content = ""):
@@ -170,6 +171,30 @@ def _write_swift_version(repository_ctx, swiftc_path):
     repository_ctx.file(filename, contents, executable = False)
     return filename
 
+def _fetch_supported_features(repository_ctx, swiftc_path):
+    """Fetch the json config of supported features from Swift
+
+    This can be used to flip rules specific features
+
+    Args:
+        repository_ctx: The repository context.
+        swiftc_path: The `path` to the `swiftc` executable.
+
+    Returns:
+        The list of supported features, or an empty array if it fails
+    """
+    repository_ctx.file("empty.swift")
+    result = repository_ctx.execute([
+        swiftc_path,
+        "-frontend",
+        "-emit-supported-features",
+        "empty.swift",
+    ])
+    if result.return_code == 0:
+        return json.decode(result.stdout.strip()).get("SupportedArguments", [])
+
+    return []
+
 def _compute_feature_values(repository_ctx, swiftc_path):
     """Computes a list of supported/unsupported features by running checks.
 
@@ -252,6 +277,10 @@ def _create_linux_toolchain(repository_ctx):
     feature_values.append(SWIFT_FEATURE_USE_AUTOLINK_EXTRACT)
     feature_values.append(SWIFT_FEATURE_USE_MODULE_WRAP)
 
+    swift_features_config = _fetch_supported_features(repository_ctx, path_to_swiftc)
+    if "load-plugin-executable" in swift_features_config:
+        feature_values.append(SWIFT_FEATURE__SUPPORTS_MACROS)
+
     repository_ctx.file(
         "BUILD",
         """

tools/worker/swift_runner.cc

@@ -301,7 +301,18 @@ bool SwiftRunner::ProcessArgument(
       } else if (StripPrefix("-macro-expansion-dir=", new_arg)) {
         changed = true;
         std::filesystem::create_directories(new_arg);
+#if __APPLE__
         job_env_["TMPDIR"] = new_arg;
+#else
+        // TEMPDIR is read by C++ but not Swift. Swift requires the temprorary
+        // directory to be an absolute path and otherwise fails (or ignores it
+        // silently on macOS) so we need to set one that Swift does not read.
+        // C++ prioritizes TMPDIR over TEMPDIR so we need to wipe out the other
+        // one. The downside is that anything else reading TMPDIR will not use
+        // the one potentially set by the user.
+        job_env_["TEMPDIR"] = new_arg;
+        job_env_.erase("TMPDIR");
+#endif
       } else if (new_arg == "-ephemeral-module-cache") {
         // Create a temporary directory to hold the module cache, which will be
         // deleted after compilation is finished.