[DRAFT] try building the shims on stable

ffi/rs/allocator_library/BUILD.bazel

@@ -18,11 +18,18 @@ srcs = select({
     "//conditions:default": ["allocator_library.rs"],
 })
 
+# This is hacky, but lets us compile the shims even on stable.
+rustc_env = {"RUSTC_BOOTSTRAP": "1"}
+
+rustc_flags = []
+
 rust_library(
     name = "allocator_library",
     srcs = srcs,
     allocator_libraries = "@rules_rust//ffi/rs:empty_allocator_libraries",
     edition = "2024",
+    rustc_env = rustc_env,
+    rustc_flags = rustc_flags,
     tags = ["manual"],
 )
 
@@ -31,5 +38,7 @@ rust_library_without_process_wrapper(
     srcs = srcs,
     allocator_libraries = "@rules_rust//ffi/rs:empty_allocator_libraries",
     edition = "2024",
+    rustc_env = rustc_env,
+    rustc_flags = rustc_flags,
     tags = ["manual"],
 )

ffi/rs/global_allocator_library/BUILD.bazel

@@ -17,5 +17,10 @@ rust_library(
     srcs = srcs,
     allocator_libraries = "@rules_rust//ffi/rs:empty_allocator_libraries",
     edition = "2024",
+    rustc_env = {"RUSTC_BOOTSTRAP": "1"},
+    rustc_flags = select({
+        "//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols.legacy.rg_oom": ["--cfg=rules_rust_allocator_legacy_rg_oom"],
+        "//conditions:default": [],
+    }),
     tags = ["manual"],
 )

ffi/rs/global_allocator_library/global_allocator_library.rs

@@ -23,11 +23,13 @@
 #![feature(linkage)]
 
 unsafe extern "C" {
+    #[cfg(rules_rust_allocator_legacy_rg_oom)]
     #[rustc_std_internal_symbol]
     fn __rg_oom(size: usize, align: usize) -> *mut u8;
 }
 
 #[linkage = "weak"]
+#[cfg(rules_rust_allocator_legacy_rg_oom)]
 #[rustc_std_internal_symbol]
 fn __rust_alloc_error_handler(size: usize, align: usize) {
     unsafe {
@@ -41,7 +43,19 @@ fn __rust_alloc_error_handler(size: usize, align: usize) {
 #[rustc_std_internal_symbol]
 static mut __rust_alloc_error_handler_should_panic: u8 = 1;
 
+// See  https://github.com/rust-lang/rust/pull/143387.
+#[linkage = "weak"]
+#[rustc_std_internal_symbol]
+fn __rust_alloc_error_handler_should_panic_v2() -> u8 {
+    return 1;
+}
+
 // See https://github.com/rust-lang/rust/issues/73632#issuecomment-1563462239
 #[linkage = "weak"]
 #[rustc_std_internal_symbol]
 static mut __rust_no_alloc_shim_is_unstable: u8 = 0;
+
+// See https://github.com/rust-lang/rust/pull/141061.
+#[linkage = "weak"]
+#[rustc_std_internal_symbol]
+fn __rust_no_alloc_shim_is_unstable_v2() {}

rust/settings/settings.bzl

@@ -3,6 +3,7 @@
 Definitions for all `@rules_rust//rust` settings
 """
 
+load("@bazel_skylib//lib:selects.bzl", "selects")
 load(
     "@bazel_skylib//rules:common_settings.bzl",
     "bool_flag",
@@ -221,8 +222,6 @@ def experimental_use_allocator_libraries_with_mangled_symbols(name):
     the result of building the rust allocator libraries via a provider, which
     can be consumed by the rust build actions. We attach an instance of this
     as a common attribute to the rust rule set.
-
-    TODO: how this interacts with stdlibs
     """
     bool_flag(
         name = name,
@@ -243,6 +242,42 @@ def experimental_use_allocator_libraries_with_mangled_symbols(name):
         },
     )
 
+    legacy_configs = [{
+        # For rust toolchains prior to this rust commit:
+        #   Support #[alloc_error_handler] without the allocator shim
+        #   https://github.com/rust-lang/rust/commit/116f4ae171e292423304ee6842a11c48a4fff5ba
+        "name": "rg_oom",
+        "latest_versions": ["1.91.1", "nightly/2025-10-15", "beta/2025-10-26"],
+    }]
+
+    legacy_names = [config["name"] for config in legacy_configs]
+
+    legacy_flag_name = "%s.legacy" % name
+
+    string_flag(
+        name = legacy_flag_name,
+        values = legacy_names + ["latest"],
+        build_setting_default = "latest",
+    )
+
+    for name in legacy_names:
+        native.config_setting(
+            name = "%s.%s_only" % (legacy_flag_name, name),
+            flag_values = {
+                legacy_flag_name: name,
+            },
+        )
+
+    for index in range(len(legacy_names)):
+        match_any = [
+            "%s.%s_only" % (legacy_flag_name, later)
+            for later in legacy_names[index:]
+        ]
+        selects.config_setting_group(
+            name = "%s.%s" % (legacy_flag_name, legacy_names[index]),
+            match_any = match_any,
+        )
+
 def experimental_use_coverage_metadata_files():
     """A flag to have coverage tooling added as `coverage_common.instrumented_files_info.metadata_files` instead of \
     reporting tools like `llvm-cov` and `llvm-profdata` as runfiles to each test.

test/integration/cc_common_link/MODULE.bazel

@@ -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")
 

test/integration/cc_common_link_with_global_alloc/MODULE.bazel

@@ -22,7 +22,7 @@ rust.toolchain(
 # 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/2025-04-08"
+VERSION = "nightly/2026-01-25"
 
 rust.repository_set(
     name = "rust_with_alloc_mangling_linux_x86_64",