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

This change adds a check in the #[global_allocator] macro to prevent
simultaneous use with #[thread_local], which is logically incompatible.
The global allocator must be a single instance across all threads.

Fixes #85517

Author: zhi-ping

compiler/rustc_builtin_macros/messages.ftl

@@ -306,3 +306,9 @@ builtin_macros_unexpected_lit = expected path to a trait, found literal
     .other = for example, write `#[derive(Debug)]` for `Debug`
 
 builtin_macros_unnameable_test_items = cannot test inner items
+
+# New: Disallow simultaneous use of #[global_allocator] and #[thread_local]
+builtin_macros_global_allocator_thread_local_conflict =
+    `#[global_allocator]` cannot be used with `#[thread_local]` on the same static
+    .note = The global allocator must be a single, program-wide instance.
+    .help = Remove either the `#[global_allocator]` or `#[thread_local]` attribute.

compiler/rustc_builtin_macros/src/errors.rs

@@ -150,6 +150,14 @@ pub(crate) struct AllocMustStatics {
     pub(crate) span: Span,
 }
 
+/// 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] 
+    pub(crate) span: Span,
+}
+
 pub(crate) use autodiff::*;
 
 mod autodiff {

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -37,7 +37,15 @@ pub(crate) fn expand(
         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,
+        });
+        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 };