Fix ordering of --remap-path-prefix flags.

These flags are interpreted in reverse order (see
rust-lang/rust#82108), and if we have a set of
flags like:
```
  --remap-path-prefix=/a/b/c/d=.
  --remap-path-prefix=/a/b=.
```
then the first flag will never apply, because the path will be first
rewritten as ./c/d.

In this case, output_base is the outermost directory, so we need to make
sure it's remapped last.

Author: slackito

rust/private/rustc.bzl

@@ -1116,9 +1116,13 @@ def construct_arguments(
 
     # For determinism to help with build distribution and such
     if remap_path_prefix != None:
+        # `--remap-path-prefix` flags are applied in reverse order. We need to
+        # specify the outermost directory (output_base) first, so that it's
+        # remapped last. Otherwise we can end up with a partial rewrite where
+        # "/path/to/output_base/execroot" becomes "./execroot" rather than ".".
+        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=${{exec_root}}={}".format(remap_path_prefix))
-        rustc_flags.add("--remap-path-prefix=${{output_base}}={}".format(remap_path_prefix))
 
     emit_without_paths = []
     for kind in emit:

test/unit/remap_path_prefix/remap_path_prefix_test.bzl

@@ -17,9 +17,11 @@ def _remap_path_prefix_test_impl(ctx):
     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=${output_base}=.")
+    assert_list_contains_adjacent_elements(env, action.argv, [
+        "--remap-path-prefix=${output_base}=.",
+        "--remap-path-prefix=${pwd}=.",
+        "--remap-path-prefix=${exec_root}=.",
+    ])
 
     return analysistest.end(env)