Set Rust allocator mode on cgo rlib (#49)

## Summary
- wrap the import_extractor rlib with `with_cfg`
- set rules_rust's mangled allocator-symbol mode inside the transition
- apply the same setting to the staticlib wrapper for consistency
- remove the redundant global `.bazelrc` allocator setting

## Why
Consumers and BCR do not necessarily read this repo's root `.bazelrc`.
The cgo-linked Rust rlib should carry the allocator mode itself so Rust
1.95 mangled allocator references link with the matching allocator shim.

## Verification
- `bazel test //crates/import_extractor:test
//crates/import_extractor:ffi_symbols_test //py:py_test`
- `bazel build @gazelle_py//py/...
--repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
--repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
--@llvm//config:experimental_stub_libgcc_s=True` from `bcr_test/`

Author: longlho

.bazelrc

@@ -9,7 +9,6 @@ build --registry=https://bcr.bazel.build
 common --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
 common --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
 common --@llvm//config:experimental_stub_libgcc_s=True
-common --@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols=true
 
 # Go 1.27 (Aug 2026) makes Go binaries PIE-compatible — drop this then.
 # Until then, rules_go's external link via clang+lld needs -no-pie because

crates/import_extractor/BUILD.bazel

@@ -2,7 +2,7 @@ load("@rules_go//go:def.bzl", "go_test")
 load("@rules_rs//rs:rust_library.bzl", "rust_library")
 load("@rules_rs//rs:rust_test.bzl", "rust_test")
 load("@rules_rust_prost//:defs.bzl", "rust_prost_library")
-load(":opt.bzl", "opt_rust_static_library")
+load(":opt.bzl", "cgo_rust_library", "opt_rust_static_library")
 
 # proto_library lives at //proto:import_extractor_proto so that //py/proto can
 # depend on it without loading any rules_rs symbols.
@@ -31,15 +31,23 @@ _LIB_DEPS = [
     "@gazelle_py_crates//:rayon",
 ]
 
-# rlib variant — used by the unit tests.
-rust_library(
+# rlib variant — linked through cgo by //py and used by the unit tests.
+cgo_rust_library(
     name = "import_extractor",
     srcs = _LIB_SRCS,
     edition = "2021",
     visibility = ["//visibility:public"],
     deps = _LIB_DEPS,
 )
 
+rust_library(
+    name = "import_extractor_test_lib",
+    testonly = True,
+    srcs = _LIB_SRCS,
+    edition = "2021",
+    deps = _LIB_DEPS,
+)
+
 # staticlib variant — linked into the gazelle plugin's go_library via cgo.
 # Provides CcInfo so it can be passed directly as `cdeps` on go_library.
 # opt_rust_static_library is a with_cfg wrapper that pins compilation_mode
@@ -62,7 +70,7 @@ genrule(
 
 rust_test(
     name = "test",
-    crate = ":import_extractor",
+    crate = ":import_extractor_test_lib",
 )
 
 go_test(

crates/import_extractor/opt.bzl

@@ -1,31 +1,39 @@
-"""rust_static_library wrapper that pins compilation_mode and rustc flags.
+"""Rust target wrappers for cgo-compatible import_extractor builds.
 
-The gazelle plugin's go_library links the import_extractor staticlib via
-cgo. Without a transition, the staticlib's build flags would inherit the
-consumer's compilation_mode, so a consumer running plain `bazel build`
-would get a fastbuild rust binary inside their gazelle binary.
+The gazelle plugin's go_library links the import_extractor rlib via cgo.
+Rust 1.95's allocator symbols are mangled, so the rlib must be compiled
+with rules_rust's matching allocator-library setting. Pinning that here
+keeps consumers and BCR from needing a top-level command-line flag.
 
-with_cfg forces compilation_mode=opt and the release rustc flags
-(panic=abort, codegen-units=1, thin LTO, strip) on the staticlib and its
-transitive deps. The transition propagates so @gazelle_py_crates//
-crates are optimized too.
+The staticlib wrapper also forces compilation_mode=opt and the release
+rustc flags (panic=abort, codegen-units=1, thin LTO, strip).
 """
 
+load("@rules_rs//rs:rust_library.bzl", "rust_library")
 load("@rules_rs//rs:rust_static_library.bzl", "rust_static_library")
 load("@with_cfg.bzl", "with_cfg")
 
+_ALLOCATOR_MANGLED_SYMBOLS = Label("@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols")
+
+cgo_rust_library, _cgo_rust_library_internal = (
+    with_cfg(rust_library)
+        .set(_ALLOCATOR_MANGLED_SYMBOLS, True)
+        .build()
+)
+
 opt_rust_static_library, _opt_rust_static_library_internal = (
     with_cfg(rust_static_library)
         .set("compilation_mode", "opt")
+        .set(_ALLOCATOR_MANGLED_SYMBOLS, True)
         .set(
-            Label("@rules_rust//:extra_rustc_flags"),
-            [
-                "-Ccodegen-units=1",
-                "-Cpanic=abort",
-                "-Cstrip=symbols",
-                "-Clto=thin",
-                "-Cembed-bitcode=yes",
-            ],
-        )
+        Label("@rules_rust//:extra_rustc_flags"),
+        [
+            "-Ccodegen-units=1",
+            "-Cpanic=abort",
+            "-Cstrip=symbols",
+            "-Clto=thin",
+            "-Cembed-bitcode=yes",
+        ],
+    )
         .build()
 )