Fix mojo_library with generated sources with path stripping enabled

.github/workflows/ci.yaml

@@ -35,4 +35,7 @@ jobs:
       - name: CPU Info
         run: |
           lscpu || true
-      - run: bazel test //...
+      # Test twice, once with path stripping + path mapping, and once without
+      - run: |
+          bazel test //...
+          bazel test --experimental_output_paths=strip //...

mojo/mojo_library.bzl

@@ -8,6 +8,9 @@ load("//mojo/private:utils.bzl", "MOJO_EXTENSIONS", "collect_mojoinfo", "format_
 def _format_include(arg):
     return ["-I", arg.dirname]
 
+def _format_root(arg):
+    return arg.dirname
+
 def _mojo_library_implementation(ctx):
     mojo_toolchain = ctx.toolchains["//:toolchain_type"].mojo_toolchain_info
     build_env = getattr(ctx.toolchains["//:toolchain_type"], "build_env", {})
@@ -43,7 +46,7 @@ def _mojo_library_implementation(ctx):
         args.add("--experimental-export-fixit", fixits_file)
 
     file_args.add_all(import_paths, map_each = format_import)
-    file_args.add(root_directory)
+    file_args.add_all([ctx.files.srcs[0]], map_each = _format_root)
     ctx.actions.run(
         executable = mojo_toolchain.mojo,
         inputs = depset(ctx.files.srcs + ctx.files.additional_compiler_inputs, transitive = [transitive_mojodeps]),

tests/generated/BUILD.bazel

@@ -0,0 +1,37 @@
+# Regression test to ensure mojo_library and mojo_binary targets work with generated sources
+
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+load("//mojo:mojo_binary.bzl", "mojo_binary")
+load("//mojo:mojo_library.bzl", "mojo_library")
+
+genrule(
+    name = "srcs",
+    outs = [
+        "__init__.mojo",
+        "main.mojo",
+    ],
+    cmd = """cat << 'EOF' > $(location main.mojo)
+def main():
+    pass
+EOF
+touch $(location __init__.mojo)
+""",
+)
+
+mojo_library(
+    name = "lib",
+    srcs = [":__init__.mojo"],
+)
+
+mojo_binary(
+    name = "bin",
+    srcs = [":main.mojo"],
+)
+
+build_test(
+    name = "build_targets",
+    targets = [
+        ":lib",
+        ":bin",
+    ],
+)