warn on unused linker_messages warning attributes

Author: jyn514

compiler/rustc_passes/messages.ftl

@@ -798,6 +798,9 @@ passes_unused_duplicate =
 passes_unused_empty_lints_note =
     attribute `{$name}` with an empty list has no effect
 
+passes_unused_linker_warnings_note =
+    the `linker_warnings` lint can only be controlled at the root of a crate with `--crate-type bin`
+
 passes_unused_multiple =
     multiple `{$name}` attributes
     .suggestion = remove this attribute

compiler/rustc_passes/src/check_attr.rs

@@ -2271,6 +2271,16 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             && item.path == sym::reason
         {
             errors::UnusedNote::NoLints { name: attr.name_or_empty() }
+        } else if matches!(
+            attr.name_or_empty(),
+            sym::allow | sym::warn | sym::deny | sym::forbid | sym::expect
+        ) && let Some(meta) = attr.meta_item_list()
+            && meta.iter().any(|meta| {
+                meta.meta_item().map_or(false, |item| item.path == sym::linker_messages)
+            })
+            && hir_id != CRATE_HIR_ID
+        {
+            errors::UnusedNote::LinkerWarningsBinaryCrateOnly
         } else if attr.name_or_empty() == sym::default_method_body_is_const {
             errors::UnusedNote::DefaultMethodBodyConst
         } else {

compiler/rustc_passes/src/errors.rs

@@ -762,6 +762,8 @@ pub(crate) enum UnusedNote {
     NoLints { name: Symbol },
     #[note(passes_unused_default_method_body_const_note)]
     DefaultMethodBodyConst,
+    #[note(passes_unused_linker_warnings_note)]
+    LinkerWarningsBinaryCrateOnly,
 }
 
 #[derive(LintDiagnostic)]

compiler/rustc_span/src/symbol.rs

@@ -1153,6 +1153,7 @@ symbols! {
         link_section,
         linkage,
         linker,
+        linker_messages,
         lint_reasons,
         literal,
         load,