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

compiler/rustc_passes/messages.ftl

@@ -542,6 +542,10 @@ passes_only_has_effect_on =
         *[unspecified] (unspecified--this is a compiler bug)
     }
 
+passes_optimize_not_fn_or_closure =
+    attribute should be applied to function or closure
+    .label = not a function or closure
+
 passes_outer_crate_level_attr =
     crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
 

compiler/rustc_passes/src/check_attr.rs

@@ -124,6 +124,7 @@ fn check_attributes(
                 }
                 [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 @@ fn check_coverage(&self, attr: &Attribute, span: Span, target: Target) {
         }
     }
 
+    /// 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 {

tests/ui/attributes/optimize.rs

@@ -0,0 +1,28 @@
+#![feature(optimize_attribute)]
+#![feature(stmt_expr_attributes)]
+#![deny(unused_attributes)]
+#![allow(dead_code)]
+
+#[optimize(speed)] //~ ERROR attribute should be applied to function or closure
+struct F;
+
+fn invalid() {
+    #[optimize(speed)] //~ ERROR attribute should be applied to function or closure
+    {
+        1
+    };
+}
+
+#[optimize(speed)]
+fn valid() {}
+
+#[optimize(speed)]
+mod valid_module {}
+
+#[optimize(speed)]
+impl F {}
+
+fn main() {
+    let _ = #[optimize(speed)]
+    (|| 1);
+}

tests/ui/attributes/optimize.stderr

@@ -0,0 +1,20 @@
+error: attribute should be applied to function or closure
+  --> $DIR/optimize.rs:6:1
+   |
+LL | #[optimize(speed)]
+   | ^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/optimize.rs:3:9
+   |
+LL | #![deny(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: attribute should be applied to function or closure
+  --> $DIR/optimize.rs:10:5
+   |
+LL |     #[optimize(speed)]
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+