fix: Disallow #[global_allocator] with #[thread_local]

[zhi-ping]

Summary

Disallow simultaneous use of #[global_allocator] and #[thread_local] on the same static item. This fixes a logical incompatibility where the global allocator (a single program-wide instance) cannot coexist with thread-local storage.

Test Plan

• Added a test case in src/test/ui/allocator/global-allocator-thread-local-conflict.rs that verifies the error is emitted when both attributes are present.
• Tested that existing valid uses of #[global_allocator] (without #[thread_local]) still compile successfully.

Related Issues

Fixes #85517.

Rationale

The global allocator is designed to be a single instance shared across all threads. Using #[thread_local] would create a separate instance per thread, which contradicts the purpose of the global allocator and could lead to unexpected behavior. Adding this check prevents such invalid code from compiling.

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  • 96a8466

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 11.338
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_builtin_macros/src/global_allocator.rs:37:
         ecx.dcx().emit_err(errors::AllocMustStatics { span: item.span() });
         return vec![orig_item];
     };
-    
+
     // Check if the static item also has the `#[thread_local]` attribute, which is incompatible with `#[global_allocator]`.
     if item.attrs.iter().any(|a| a.path().is_ident(sym::thread_local)) {
-        ecx.dcx().emit_err(errors::GlobalAllocatorThreadLocalConflict {
-            span: item.span,
-        });
+        ecx.dcx().emit_err(errors::GlobalAllocatorThreadLocalConflict { span: item.span });
         return vec![orig_item];
-    }    
-    
+    }
+
     // Generate a bunch of new items using the AllocFnFactory
     let span = ecx.with_def_site_ctxt(item.span);
     let f = AllocFnFactory { span, ty_span, global: ident, cx: ecx };
Diff in /checkout/compiler/rustc_builtin_macros/src/errors.rs:152:
 
 /// The `#[global_allocator]` attribute cannot be used with `#[thread_local]`.
 #[derive(Diagnostic)]
-#[diag(builtin_macros_global_allocator_thread_local_conflict)] 
-pub(crate) struct GlobalAllocatorThreadLocalConflict { 
-    #[primary_span] 
+#[diag(builtin_macros_global_allocator_thread_local_conflict)]
+pub(crate) struct GlobalAllocatorThreadLocalConflict {
+    #[primary_span]
     pub(crate) span: Span,
 }
 
fmt: checked 6570 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:51

[chenyukang]

Per our policy at rust-lang/compiler-team#893, I'm closing this PR.