Emit an error if #[optimize] is applied to an incompatible item

compiler/rustc_passes/src/check_attr.rs

@@ -124,6 +124,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 }
                 [sym::inline] => self.check_inline(hir_id, attr, span, target),
                 [sym::coverage] => self.check_coverage(attr, span, target),
+                [sym::optimize] => self.check_optimize(hir_id, attr, target),
                 [sym::non_exhaustive] => self.check_non_exhaustive(hir_id, attr, span, target),
                 [sym::marker] => self.check_marker(hir_id, attr, span, target),
                 [sym::target_feature] => {
@@ -373,6 +374,27 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    /// Checks that `#[optimize(..)]` is applied to a function/closure/method,
+    /// or to an impl block or module.
+    fn check_optimize(&self, hir_id: HirId, attr: &Attribute, target: Target) {
+        match target {
+            Target::Fn
+            | Target::Closure
+            | Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent)
+            | Target::Impl
+            | Target::Mod => {}
+
+            _ => {
+                self.tcx.emit_node_span_lint(
+                    UNUSED_ATTRIBUTES,
+                    hir_id,
+                    attr.span,
+                    errors::OptimizeNotFnOrClosure,
+                );
+            }
+        }
+    }
+
     fn check_generic_attr(
         &self,
         hir_id: HirId,

compiler/rustc_passes/src/errors.rs

@@ -68,6 +68,10 @@ pub struct CoverageNotFnOrClosure {
     pub defn_span: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(passes_optimize_not_fn_or_closure)]
+pub struct OptimizeNotFnOrClosure;
+
 #[derive(Diagnostic)]
 #[diag(passes_should_be_applied_to_fn)]
 pub struct AttrShouldBeAppliedToFn {