fix: remap bootstrap source paths in rustc

rust/private/rustc.bzl

@@ -1385,13 +1385,18 @@ 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=${{exec_root}}={}".format(remap_path_prefix))
+        if toolchain._bootstrapping:
+            # rustc resolves cwd itself, so bootstrap response files need no
+            # placeholder substitution to remap source paths in crate metadata.
+            rustc_flags.add(remap_path_prefix, format = "-Zremap-cwd-prefix=%s")
+        else:
+            # `--remap-path-prefix` flags are applied in reverse order. We need to
+            # specify the outermost directory (output_base) first, so that it's
+            # remapped last. Otherwise we can end up with a partial rewrite where
+            # "/path/to/output_base/execroot" becomes "./execroot" rather than ".".
+            rustc_flags.add("--remap-path-prefix=${output_base}=%s" % remap_path_prefix)
+            rustc_flags.add("--remap-path-prefix=${pwd}=%s" % remap_path_prefix)
+            rustc_flags.add("--remap-path-prefix=${exec_root}=%s" % remap_path_prefix)
 
     emit_without_paths = []
     redirect_link_to_metadata = build_metadata and crate_info.metadata != None
@@ -1661,6 +1666,11 @@ def construct_arguments(
     if hasattr(ctx.attr, "_extra_exec_rustc_env") and is_exec_configuration(ctx):
         env.update(ctx.attr._extra_exec_rustc_env[ExtraExecRustcEnvInfo].extra_exec_rustc_env)
 
+    if toolchain._bootstrapping and remap_path_prefix != None:
+        # -Zremap-cwd-prefix is unstable. Respect explicit RUSTC_BOOTSTRAP values;
+        # otherwise limit the opt-in to this bootstrap crate.
+        env.setdefault("RUSTC_BOOTSTRAP", ctx.configuration.default_shell_env.get("RUSTC_BOOTSTRAP", crate_info.name))
+
     # Strip any `-Zallow-features=` entries out of the toolchain's extra
     # rustc flags into `all_allowed_unstable_features` and, when
     # `unstable_rust_features_config` is configured to a concrete list,
@@ -2163,6 +2173,8 @@ def rustc_compile(
         use_json_output = bool(build_metadata) or bool(rustc_output) or bool(rustc_rmeta_output),
         skip_expanding_rustc_env = skip_expanding_rustc_env,
         require_explicit_unstable_features = require_explicit_unstable_features,
+        # _spawnvp in bootstrap_process_wrapper does not preserve embedded
+        # quotes in direct Windows arguments. rustc reads them from a response file.
         always_use_param_file = rust_toolchain._bootstrapping,
         inject_allow_features_guardrail = inject_allow_features_guardrail,
         allowed_unstable_rust_features = allowed_unstable_rust_features,

test/process_wrapper_bootstrap/BUILD.bazel

@@ -1,4 +1,20 @@
 load("//rust:defs.bzl", "rust_binary", "rust_test")
+load(":bootstrap_tinyjson.bzl", "bootstrap_tinyjson")
+
+bootstrap_tinyjson(
+    name = "bootstrap_tinyjson",
+)
+
+rust_test(
+    name = "bootstrap_reproducibility_test",
+    srcs = ["bootstrap_reproducibility_test.rs"],
+    data = [":bootstrap_tinyjson"],
+    edition = "2021",
+    env = {
+        "BOOTSTRAP_TINYJSON_RLOCATIONPATH": "$(rlocationpath :bootstrap_tinyjson)",
+    },
+    deps = ["//rust/runfiles"],
+)
 
 rust_binary(
     name = "bootstrap_process_wrapper_probe",

test/process_wrapper_bootstrap/bootstrap_reproducibility_test.rs

@@ -0,0 +1,19 @@
+//! Check the metadata of tinyjson compiled with the bootstrap process wrapper.
+
+#[test]
+fn bootstrap_tinyjson_does_not_embed_execution_paths() {
+    let rfiles = runfiles::Runfiles::create().unwrap();
+    let rlocation = std::env::var("BOOTSTRAP_TINYJSON_RLOCATIONPATH").unwrap();
+    let artifact = runfiles::rlocation!(rfiles, rlocation.as_str()).unwrap();
+    let bytes = std::fs::read(&artifact).unwrap();
+
+    for prefix in ["/execroot/", "\\execroot\\", "/sandbox/", "\\sandbox\\"] {
+        assert!(
+            !bytes
+                .windows(prefix.len())
+                .any(|part| part == prefix.as_bytes()),
+            "{} contains an unremapped execution path ({prefix})",
+            artifact.display(),
+        );
+    }
+}

test/process_wrapper_bootstrap/bootstrap_tinyjson.bzl

@@ -0,0 +1,21 @@
+"""Expose tinyjson from the process wrapper's bootstrap configuration."""
+
+load("//rust:defs.bzl", "rust_common")
+
+def _bootstrap_tinyjson_impl(ctx):
+    crates = ctx.attr.process_wrapper[rust_common.dep_info].transitive_crates.to_list()
+    tinyjson = [crate.output for crate in crates if crate.name == "tinyjson"]
+    if len(tinyjson) != 1:
+        fail("Expected one bootstrap tinyjson output, got %s" % tinyjson)
+    return [DefaultInfo(files = depset(tinyjson))]
+
+bootstrap_tinyjson = rule(
+    implementation = _bootstrap_tinyjson_impl,
+    attrs = {
+        "process_wrapper": attr.label(
+            cfg = "exec",
+            default = "//util/process_wrapper:process_wrapper",
+            providers = [rust_common.dep_info],
+        ),
+    },
+)