Remap relative bin-dir prefix for file!() and log macros

When transform_sources() symlinks sources into bazel-out/<config>/bin/,
file!(), Location::caller(), and tracing/log macros embed the literal
relative path rustc received on the command line. The existing ${pwd}
remap only matches absolute paths in debug info.

Add a second --remap-path-prefix for the relative bazel-out/<config>/bin/
prefix (without ${pwd}) so these compile-time paths are also clean.

Add analysis tests verifying the flag is present for generated sources
and absent for plain sources, plus a runtime test confirming file!()
returns a clean path.

Author: Tamas Vajk

rust/private/rustc.bzl

@@ -1193,6 +1193,14 @@ def construct_arguments(
         rustc_flags.add("--remap-path-prefix=${{output_base}}={}".format(remap_path_prefix))
         rustc_flags.add("--remap-path-prefix=${{pwd}}={}".format(remap_path_prefix))
         rustc_flags.add("--remap-path-prefix=${{exec_root}}={}".format(remap_path_prefix))
+        if not crate_info.root.is_source:
+            # Also remap the relative bazel-out/<config>/bin/ prefix.
+            # file!(), Location::caller(), and tracing/log macros embed
+            # the literal relative path that rustc received on the command
+            # line.  The ${pwd} remap above only matches absolute paths in
+            # debug info, so a second remap without ${pwd} is needed to
+            # strip the bin-dir prefix from these compile-time paths.
+            rustc_flags.add("--remap-path-prefix={}/=".format(ctx.bin_dir.path))
 
     emit_without_paths = []
     for kind in emit:

test/unit/remap_path_prefix/BUILD.bazel

@@ -1,3 +1,4 @@
+load("@bazel_skylib//rules:write_file.bzl", "write_file")
 load("//rust:defs.bzl", "rust_binary", "rust_library", "rust_test")
 load(":debug_transition.bzl", "dbg_rust_binary")
 load(":remap_path_prefix_test.bzl", "remap_path_prefix_test_suite")
@@ -37,6 +38,32 @@ rust_test(
     deps = ["//rust/runfiles"],
 )
 
+# Library with generated source to trigger transform_sources().
+# Exposes file!() so a runtime test can verify the path is clean.
+write_file(
+    name = "remap_file_macro_lib_src",
+    out = "remap_file_macro_lib.rs",
+    content = [
+        "pub fn get_file_path() -> &'static str {",
+        "    file!()",
+        "}",
+        "",
+    ],
+)
+
+rust_library(
+    name = "remap_file_macro_lib",
+    srcs = [":remap_file_macro_lib.rs"],
+    edition = "2021",
+)
+
+rust_test(
+    name = "file_macro_test",
+    srcs = ["file_macro_test.rs"],
+    edition = "2021",
+    deps = [":remap_file_macro_lib"],
+)
+
 remap_path_prefix_test_suite(
     name = "remap_path_prefix_test_suite",
 )

test/unit/remap_path_prefix/file_macro_test.rs

@@ -0,0 +1,8 @@
+#[test]
+fn file_macro_has_no_bazel_out_prefix() {
+    let path = remap_file_macro_lib::get_file_path();
+    assert!(
+        !path.contains("bazel-out"),
+        "file!() should not contain 'bazel-out' prefix, got: {path}",
+    );
+}

test/unit/remap_path_prefix/remap_path_prefix_test.bzl

@@ -6,6 +6,9 @@ load("//rust:defs.bzl", "rust_binary", "rust_library")
 load(
     "//test/unit:common.bzl",
     "assert_action_mnemonic",
+    "assert_argv_contains",
+    "assert_argv_contains_prefix_not",
+    "assert_argv_contains_prefix_suffix",
     "assert_list_contains_adjacent_elements",
 )
 
@@ -26,6 +29,38 @@ def _remap_path_prefix_test_impl(ctx):
 
 _remap_path_prefix_test = analysistest.make(_remap_path_prefix_test_impl)
 
+def _remap_path_prefix_source_test_impl(ctx):
+    """Verify that source targets do NOT get the relative bin-dir remap."""
+    env = analysistest.begin(ctx)
+    target = analysistest.target_under_test(env)
+
+    action = target.actions[0]
+    assert_action_mnemonic(env, action, "Rustc")
+
+    assert_argv_contains(env, action, "--remap-path-prefix=${pwd}=.")
+    assert_argv_contains(env, action, "--remap-path-prefix=${exec_root}=.")
+    assert_argv_contains(env, action, "--remap-path-prefix=${output_base}=.")
+    assert_argv_contains_prefix_not(env, action, "--remap-path-prefix=bazel-out/")
+
+    return analysistest.end(env)
+
+_remap_path_prefix_source_test = analysistest.make(_remap_path_prefix_source_test_impl)
+
+def _remap_file_macro_generated_test_impl(ctx):
+    """Verify that generated-source targets get a relative bin-dir remap for file!()."""
+    env = analysistest.begin(ctx)
+    target = analysistest.target_under_test(env)
+
+    action = target.actions[0]
+    assert_action_mnemonic(env, action, "Rustc")
+
+    # Should have the relative bin-dir remap (e.g. --remap-path-prefix=bazel-out/k8-fastbuild/bin/=)
+    assert_argv_contains_prefix_suffix(env, action, "--remap-path-prefix=bazel-out/", "/bin/=")
+
+    return analysistest.end(env)
+
+_remap_file_macro_generated_test = analysistest.make(_remap_file_macro_generated_test_impl)
+
 def _subst_flags_test_impl(ctx):
     """Verify that process wrapper --subst flags are present."""
     env = analysistest.begin(ctx)
@@ -88,6 +123,23 @@ def remap_path_prefix_test_suite(name):
         target_under_test = ":remap_bin",
     )
 
+    # Verify source targets do NOT get the relative bin-dir remap.
+    _remap_path_prefix_source_test(
+        name = "remap_path_prefix_source_lib_test",
+        target_under_test = ":dep",
+    )
+
+    # Verify generated-source targets get the relative bin-dir remap for file!().
+    _remap_file_macro_generated_test(
+        name = "remap_file_macro_generated_lib_test",
+        target_under_test = ":remap_lib",
+    )
+
+    _remap_file_macro_generated_test(
+        name = "remap_file_macro_generated_bin_test",
+        target_under_test = ":remap_bin",
+    )
+
     _subst_flags_test(
         name = "subst_flags_lib_test",
         target_under_test = ":remap_lib",
@@ -101,6 +153,9 @@ def remap_path_prefix_test_suite(name):
     tests = [
         ":remap_path_prefix_lib_test",
         ":remap_path_prefix_bin_test",
+        ":remap_path_prefix_source_lib_test",
+        ":remap_file_macro_generated_lib_test",
+        ":remap_file_macro_generated_bin_test",
         ":subst_flags_lib_test",
         ":subst_flags_bin_test",
     ]