Rollup merge of #142964 - GrigorenkoPV:attributes/argless, r=oli-obk

Attribute rework: a parser for single attributes without arguments

Part of rust-lang/rust#131229

r? `@jdonszelmann`

I think code (with comments) speaks for itself.

The only subtlety: now `#[cold]`, `#[no_mangle]`, & `#[track_caller]` do not get thrown away when malformed (i.e. have arguments). This doesn't matter too much (I think), because an error gets emitted either way, so the compilation will not finish.

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -17,7 +17,7 @@
 use std::marker::PhantomData;
 
 use rustc_attr_data_structures::AttributeKind;
-use rustc_feature::AttributeTemplate;
+use rustc_feature::{AttributeTemplate, template};
 use rustc_span::{Span, Symbol};
 use thin_vec::ThinVec;
 
@@ -229,6 +229,41 @@ pub(crate) enum AttributeOrder {
     KeepLast,
 }
 
+/// An even simpler version of [`SingleAttributeParser`]:
+/// now automatically check that there are no arguments provided to the attribute.
+///
+/// [`WithoutArgs<T> where T: NoArgsAttributeParser`](WithoutArgs) implements [`SingleAttributeParser`].
+//
+pub(crate) trait NoArgsAttributeParser<S: Stage>: 'static {
+    const PATH: &[Symbol];
+    const ON_DUPLICATE: OnDuplicate<S>;
+
+    /// Create the [`AttributeKind`] given attribute's [`Span`].
+    const CREATE: fn(Span) -> AttributeKind;
+}
+
+pub(crate) struct WithoutArgs<T: NoArgsAttributeParser<S>, S: Stage>(PhantomData<(S, T)>);
+
+impl<T: NoArgsAttributeParser<S>, S: Stage> Default for WithoutArgs<T, S> {
+    fn default() -> Self {
+        Self(Default::default())
+    }
+}
+
+impl<T: NoArgsAttributeParser<S>, S: Stage> SingleAttributeParser<S> for WithoutArgs<T, S> {
+    const PATH: &[Symbol] = T::PATH;
+    const ATTRIBUTE_ORDER: AttributeOrder = AttributeOrder::KeepLast;
+    const ON_DUPLICATE: OnDuplicate<S> = T::ON_DUPLICATE;
+    const TEMPLATE: AttributeTemplate = template!(Word);
+
+    fn convert(cx: &mut AcceptContext<'_, '_, S>, args: &ArgParser<'_>) -> Option<AttributeKind> {
+        if let Err(span) = args.no_args() {
+            cx.expected_no_args(span);
+        }
+        Some(T::CREATE(cx.attr_span))
+    }
+}
+
 type ConvertFn<E> = fn(ThinVec<E>) -> AttributeKind;
 
 /// Alternative to [`AttributeParser`] that automatically handles state management.