Don't ICE on malformed rustc_args_required_const attribute

compiler/rustc_passes/src/check_attr.rs

@@ -545,9 +545,24 @@ impl CheckAttrVisitor<'tcx> {
         target: Target,
         item: Option<ItemLike<'_>>,
     ) -> bool {
-        if let Target::Fn | Target::Method(..) | Target::ForeignFn = target {
+        let is_function = matches!(target, Target::Fn | Target::Method(..) | Target::ForeignFn);
+        if !is_function {
+            self.tcx
+                .sess
+                .struct_span_err(attr.span, "attribute should be applied to a function")
+                .span_label(*span, "not a function")
+                .emit();
+            return false;
+        }
+
+        let list = match attr.meta_item_list() {
+            // The attribute form is validated on AST.
+            None => return false,
+            Some(it) => it,
+        };
+
         let mut invalid_args = vec![];
-            for meta in attr.meta_item_list().expect("no meta item list") {
+        for meta in list {
             if let Some(LitKind::Int(val, _)) = meta.literal().map(|lit| &lit.kind) {
                 if let Some(ItemLike::Item(Item {
                     kind: ItemKind::Fn(FnSig { decl, .. }, ..),
@@ -583,6 +598,7 @@ impl CheckAttrVisitor<'tcx> {
                 invalid_args.push(meta.span());
             }
         }
+
         if !invalid_args.is_empty() {
             self.tcx
                 .sess
@@ -592,14 +608,6 @@ impl CheckAttrVisitor<'tcx> {
         } else {
             true
         }
-        } else {
-            self.tcx
-                .sess
-                .struct_span_err(attr.span, "attribute should be applied to a function")
-                .span_label(*span, "not a function")
-                .emit();
-            false
-        }
     }
 
     /// Checks if `#[link_section]` is applied to a function or static.

src/test/ui/invalid-rustc_args_required_const-arguments.rs

@@ -23,4 +23,10 @@ extern {
     fn foo7(_: u8);
 }
 
+#[rustc_args_required_const] //~ ERROR malformed `rustc_args_required_const` attribute
+fn bar1() {}
+
+#[rustc_args_required_const = 1] //~ ERROR malformed `rustc_args_required_const` attribute
+fn bar2() {}
+
 fn main() {}

src/test/ui/invalid-rustc_args_required_const-arguments.stderr

@@ -6,6 +6,18 @@ LL | #[rustc_args_required_const(0usize)]
    |
    = help: instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), use an unsuffixed version (`1`, `1.0`, etc.)
 
+error: malformed `rustc_args_required_const` attribute input
+  --> $DIR/invalid-rustc_args_required_const-arguments.rs:26:1
+   |
+LL | #[rustc_args_required_const]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[rustc_args_required_const(N)]`
+
+error: malformed `rustc_args_required_const` attribute input
+  --> $DIR/invalid-rustc_args_required_const-arguments.rs:29:1
+   |
+LL | #[rustc_args_required_const = 1]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[rustc_args_required_const(N)]`
+
 error: index exceeds number of arguments
   --> $DIR/invalid-rustc_args_required_const-arguments.rs:3:29
    |
@@ -44,5 +56,5 @@ error: index exceeds number of arguments
 LL |     #[rustc_args_required_const(1)]
    |                                 ^ there is only 1 argument
 
-error: aborting due to 7 previous errors
+error: aborting due to 9 previous errors