Improve diagnostic for global_allocator + thread_local

Author: Fayti1703

compiler/rustc_builtin_macros/src/errors.rs

@@ -163,6 +163,9 @@ pub(crate) struct AllocMustStatics {
 pub(crate) struct AllocCannotThreadLocal {
     #[primary_span]
     pub(crate) span: Span,
+    #[label("marked `#[thread_local]` here")]
+    #[suggestion("remove this attribute", code = "", applicability = "maybe-incorrect")]
+    pub(crate) attr: Span,
 }
 
 pub(crate) use autodiff::*;

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -39,8 +39,11 @@ pub(crate) fn expand(
     };
 
     // Forbid `#[thread_local]` attributes on the item
-    if let Some(_) = item.attrs.iter().find(|x| { x.has_name(sym::thread_local) }) {
-        ecx.dcx().emit_err(errors::AllocCannotThreadLocal { span: item.span_with_attributes() });
+    if let Some(attr) = item.attrs.iter().find(|x| { x.has_name(sym::thread_local) }) {
+        ecx.dcx().emit_err(errors::AllocCannotThreadLocal {
+            span: item.span,
+            attr: attr.span
+        });
         return vec![orig_item];
     }
 

tests/ui/allocator/no-thread-local.rs

@@ -4,7 +4,7 @@ use std::alloc::System;
 
 #[global_allocator]
 #[thread_local]
-//~^ ERROR: allocators cannot be `#[thread_local]`
 static A: System = System;
+//~^ ERROR: allocators cannot be `#[thread_local]`
 
 fn main() {}

tests/ui/allocator/no-thread-local.stderr

@@ -1,10 +1,13 @@
 error: allocators cannot be `#[thread_local]`
-  --> $DIR/no-thread-local.rs:6:1
+  --> $DIR/no-thread-local.rs:7:1
    |
-LL | / #[thread_local]
-LL | |
-LL | | static A: System = System;
-   | |__________________________^
+LL | #[thread_local]
+   | ---------------
+   | |
+   | marked `#[thread_local]` here
+   | help: remove this attribute
+LL | static A: System = System;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error