use an empty staticlib as an allocator library for cc_common.link (#3864)

No functional changes intended.

As suggested on multiple channels, using an empty staticlib is a good
effective way to ensure we get the rust allocator shims generated
consistently. This is simpler than what we have and also looks like it
uniformly supports stable and nightly across versions.

closes #3459

Author: krasimirgg

ffi/rs/allocator_library/BUILD.bazel

@@ -1,34 +1,30 @@
-load("@rules_rust//rust:defs.bzl", "rust_library")
+load("@rules_rust//rust:defs.bzl", "rust_static_library")
 
 # buildifier: disable=bzl-visibility
 load(
     "@rules_rust//rust/private:rust.bzl",
-    "rust_library_without_process_wrapper",
+    "rust_static_library_without_process_wrapper",
 )
 
 package(
     default_visibility = ["@rules_rust//ffi/rs:__subpackages__"],
 )
 
-srcs = select({
-    # Windows doesn't support weak symbol linkage.
-    # If someone can make this work on Windows, please do!
-    # For now we will silently not supply any symbols, because it would be very messy to conditionally define the default allocator library on toolchains depending on the platform.
-    "@platforms//os:windows": ["empty.rs"],
-    "//conditions:default": ["allocator_library.rs"],
-})
+# When building a staticlib crate, rustc injects the default allocator
+# redirections (e.g., __rust_alloc -> __rdl_alloc).
+# So we use an empty static library to provide these.
 
-rust_library(
+rust_static_library(
     name = "allocator_library",
-    srcs = srcs,
+    srcs = ["empty.rs"],
     allocator_libraries = "@rules_rust//ffi/rs:empty_allocator_libraries",
     edition = "2024",
     tags = ["manual"],
 )
 
-rust_library_without_process_wrapper(
+rust_static_library_without_process_wrapper(
     name = "allocator_library_without_process_wrapper",
-    srcs = srcs,
+    srcs = ["empty.rs"],
     allocator_libraries = "@rules_rust//ffi/rs:empty_allocator_libraries",
     edition = "2024",
     tags = ["manual"],

ffi/rs/allocator_library/allocator_library.rs

@@ -1382,6 +1382,26 @@ rust_library_without_process_wrapper = rule(
     ],
 )
 
+def _rust_static_library_without_process_wrapper_impl(ctx):
+    providers = _rust_static_library_impl(ctx)
+    return providers + [_RustBuiltWithoutProcessWrapperInfo()]
+
+rust_static_library_without_process_wrapper = rule(
+    implementation = _rust_static_library_without_process_wrapper_impl,
+    doc = "A variant of `rust_static_library` that uses a minimal process wrapper for `Rustc` actions.",
+    attrs = dict(_common_attrs_for_binary_without_process_wrapper(_common_attrs).items()),
+    fragments = ["cpp"],
+    toolchains = [
+        str(Label("//rust:toolchain_type")),
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
+    ],
+    provides = [
+        CcInfo,
+        rust_common.test_crate_info,
+        _RustBuiltWithoutProcessWrapperInfo,
+    ],
+)
+
 def _test_attrs_for_binary_without_process_wrapper(attrs):
     new_attrs = {}
     new_attrs.update(attrs)

rust/private/rust.bzl

@@ -1980,6 +1980,9 @@ def establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_co
             cc_infos.append(libstd_and_allocator_cc_info)
 
     providers = [cc_common.merge_cc_infos(cc_infos = cc_infos)]
+    if crate_info.type == "staticlib":
+        # The static archive is the output.
+        dot_a = crate_info.output
     if dot_a:
         providers.append(AllocatorLibrariesImplInfo(static_archive = dot_a))
     return providers

rust/private/rustc.bzl

@@ -16,30 +16,6 @@ bazel_dep(name = "platforms", version = "1.0.0")
 rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
 rust.toolchain(
     edition = "2018",
-    target_settings = [
-        "@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols_off",
-    ],
-)
-
-# Generate a toolchain to be used for rust-based allocator symbols.
-
-# A recent enough version of rustc that mangles the internal allocator symbols.
-VERSION = "nightly/2025-07-08"
-
-rust.repository_set(
-    name = "rust_with_alloc_mangling_linux_x86_64",
-    allocator_library = "@rules_rust//ffi/rs:empty",
-    edition = "2021",
-    exec_triple = "x86_64-unknown-linux-gnu",
-    target_compatible_with = [
-        "@platforms//cpu:x86_64",
-        "@platforms//os:linux",
-    ],
-    target_settings = [
-        "@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols_on",
-    ],
-    target_triple = "x86_64-unknown-linux-gnu",
-    versions = [VERSION],
 )
 use_repo(rust, "rust_toolchains")