fix: set sourcemap root to the workspace relative root_dir

examples/filegroup/BUILD.bazel

@@ -5,6 +5,7 @@ filegroup(
     srcs = [
         "a.ts",
         "b.ts",
+        "sub/c.ts",
     ],
 )
 

examples/filegroup/check_outputs.sh

@@ -5,10 +5,17 @@ cd "$TEST_SRCDIR/$TEST_WORKSPACE/$(dirname $TEST_TARGET)"
 
 grep "export var a" filegroup/a.js
 grep "sourceMappingURL=a.js.map" filegroup/a.js
-grep --fixed-strings '"sourceRoot":"examples/filegroup"' filegroup/a.js.map
+grep --fixed-strings '"sourceRoot":"examples/filegroup/"' filegroup/a.js.map
 grep --fixed-strings '"sources":["a.ts"]' filegroup/a.js.map
 
 grep "export var b" filegroup/b.js
 grep "sourceMappingURL=b.js.map" filegroup/b.js
-grep --fixed-strings '"sourceRoot":"examples/filegroup"' filegroup/b.js.map
-grep --fixed-strings '"sources":["b.ts"]' filegroup/b.js.map
+grep --fixed-strings '"sourceRoot":"examples/filegroup/"' filegroup/b.js.map
+grep --fixed-strings '"sources":["b.ts"]' filegroup/b.js.map
+
+grep "export var c" filegroup/sub/c.js
+grep "sourceMappingURL=c.js.map" filegroup/sub/c.js
+grep --fixed-strings '"sourceRoot":"examples/filegroup/"' filegroup/sub/c.js.map
+grep --fixed-strings '"sources":["sub/c.ts"]' filegroup/sub/c.js.map
+
+echo "SUCCESS"

examples/filegroup/sub/c.ts

@@ -0,0 +1 @@
+export const c: string = "c";

examples/root_dir/BUILD.bazel

@@ -1,28 +1,26 @@
+load("@aspect_bazel_lib//lib:write_source_files.bzl", "write_source_files")
 load("@aspect_rules_swc//swc:defs.bzl", "swc")
-load("@aspect_bazel_lib//lib:testing.bzl", "assert_outputs")
 
 swc(
     name = "compile",
     srcs = [
         "src/a.ts",
         "src/b.ts",
+        "src/sub/c.ts",
     ],
     out_dir = "out",
     root_dir = "src",
     source_maps = True,
 )
 
-# Since the srcs were in a filegroup, the swc macro cannot pre-declare the outputs.
-# So there is no label ":a.js" that we can reference from the build file.
-# However, a.js is still produced as one of the default outputs of the compile rule.
-# We can verify this in an action that depends on the ":compile" rule and reads the files.
-assert_outputs(
-    name = "check_outputs",
-    actual = "compile",
-    expected = [
-        "examples/root_dir/out/a.js",
-        "examples/root_dir/out/a.js.map",
-        "examples/root_dir/out/b.js",
-        "examples/root_dir/out/b.js.map",
-    ],
-)
+write_source_files(
+    name = "test",
+    files = {
+        "expected/a.js": ":out/a.js",
+        "expected/a.js.map": ":out/a.js.map",
+        "expected/b.js": ":out/b.js",
+        "expected/b.js.map": ":out/b.js.map",
+        "expected/sub/c.js": ":out/sub/c.js",
+        "expected/sub/c.js.map": ":out/sub/c.js.map",
+    },
+)

examples/root_dir/src/sub/c.ts

@@ -0,0 +1 @@
+export const c: string = "c";

swc/private/swc.bzl

@@ -83,6 +83,9 @@ def _relative_to_package(path, ctx):
     return path
 
 def _strip_root_dir(path, root_dir):
+    if not root_dir:
+        return path
+
     replace_pattern = root_dir + "/"
     if path.startswith("./"):
         path = path[len("./"):]
@@ -244,13 +247,15 @@ def _impl(ctx):
     else:
         output_sources = []
 
-        for src in ctx.files.srcs:
-            src_args = ctx.actions.args()
-            src_args.add("--source-file-name", src.basename)
-            src_args.add("--source-root", src.dirname)
+        root_dir = paths.join(ctx.label.package, ctx.attr.root_dir)
 
+        for src in ctx.files.srcs:
             src_path = _relative_to_package(src.path, ctx)
 
+            src_args = ctx.actions.args()
+            src_args.add("--source-file-name", _strip_root_dir(src_path, ctx.attr.root_dir))
+            src_args.add("--source-root", root_dir)
+
             js_out_path = _calculate_js_out(src_path, ctx.attr.out_dir, ctx.attr.root_dir, [_relative_to_package(f.path, ctx) for f in ctx.outputs.js_outs])
             if not js_out_path:
                 # This source file is not a supported src