Rollup merge of #143252 - JonathanBrouwer:rewrite_empty_attribute, r=jdonszelmann

Rewrite empty attribute lint for new attribute parser

cc `@jdonszelmann`

Author: matthiaskrgr

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -67,8 +67,6 @@ pub enum ReprAttr {
     ReprSimd,
     ReprTransparent,
     ReprAlign(Align),
-    // this one is just so we can emit a lint for it
-    ReprEmpty,
 }
 pub use ReprAttr::*;
 
@@ -304,7 +302,7 @@ pub enum AttributeKind {
     PubTransparent(Span),
 
     /// Represents [`#[repr]`](https://doc.rust-lang.org/stable/reference/type-layout.html#representations).
-    Repr(ThinVec<(ReprAttr, Span)>),
+    Repr { reprs: ThinVec<(ReprAttr, Span)>, first_span: Span },
 
     /// Represents `#[rustc_layout_scalar_valid_range_end]`.
     RustcLayoutScalarValidRangeEnd(Box<u128>, Span),

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -41,7 +41,7 @@ impl AttributeKind {
             Optimize(..) => No,
             PassByValue(..) => Yes,
             PubTransparent(..) => Yes,
-            Repr(..) => No,
+            Repr { .. } => No,
             RustcLayoutScalarValidRangeEnd(..) => Yes,
             RustcLayoutScalarValidRangeStart(..) => Yes,
             RustcObjectLifetimeDefault => No,

compiler/rustc_attr_data_structures/src/lints.rs

@@ -12,4 +12,5 @@ pub struct AttributeLint<Id> {
 pub enum AttributeLintKind {
     UnusedDuplicate { this: Span, other: Span, warning: bool },
     IllFormedAttributeInput { suggestions: Vec<String> },
+    EmptyAttribute { first_span: Span },
 }

compiler/rustc_attr_parsing/messages.ftl

@@ -6,6 +6,10 @@ attr_parsing_deprecated_item_suggestion =
     .help = add `#![feature(deprecated_suggestion)]` to the crate root
     .note = see #94785 for more details
 
+attr_parsing_empty_attribute =
+    unused attribute
+    .suggestion = remove this attribute
+
 attr_parsing_empty_confusables =
     expected at least one confusable name
 attr_parsing_expected_one_cfg_pattern =

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -298,6 +298,10 @@ impl<S: Stage> CombineAttributeParser<S> for TargetFeatureParser {
             cx.expected_list(cx.attr_span);
             return features;
         };
+        if list.is_empty() {
+            cx.warn_empty_attribute(cx.attr_span);
+            return features;
+        }
         for item in list.mixed() {
             let Some(name_value) = item.meta_item() else {
                 cx.expected_name_value(item.span(), Some(sym::enable));

compiler/rustc_attr_parsing/src/attributes/repr.rs

@@ -23,7 +23,8 @@ pub(crate) struct ReprParser;
 impl<S: Stage> CombineAttributeParser<S> for ReprParser {
     type Item = (ReprAttr, Span);
     const PATH: &[Symbol] = &[sym::repr];
-    const CONVERT: ConvertFn<Self::Item> = |items, _| AttributeKind::Repr(items);
+    const CONVERT: ConvertFn<Self::Item> =
+        |items, first_span| AttributeKind::Repr { reprs: items, first_span };
     // FIXME(jdonszelmann): never used
     const TEMPLATE: AttributeTemplate =
         template!(List: "C | Rust | align(...) | packed(...) | <integer type> | transparent");
@@ -40,8 +41,8 @@ impl<S: Stage> CombineAttributeParser<S> for ReprParser {
         };
 
         if list.is_empty() {
-            // this is so validation can emit a lint
-            reprs.push((ReprAttr::ReprEmpty, cx.attr_span));
+            cx.warn_empty_attribute(cx.attr_span);
+            return reprs;
         }
 
         for param in list.mixed() {

compiler/rustc_attr_parsing/src/context.rs

@@ -165,6 +165,7 @@ mod private {
 #[allow(private_interfaces)]
 pub trait Stage: Sized + 'static + Sealed {
     type Id: Copy;
+    const SHOULD_EMIT_LINTS: bool;
 
     fn parsers() -> &'static group_type!(Self);
 
@@ -175,6 +176,7 @@ pub trait Stage: Sized + 'static + Sealed {
 #[allow(private_interfaces)]
 impl Stage for Early {
     type Id = NodeId;
+    const SHOULD_EMIT_LINTS: bool = false;
 
     fn parsers() -> &'static group_type!(Self) {
         &early::ATTRIBUTE_PARSERS
@@ -188,6 +190,7 @@ impl Stage for Early {
 #[allow(private_interfaces)]
 impl Stage for Late {
     type Id = HirId;
+    const SHOULD_EMIT_LINTS: bool = true;
 
     fn parsers() -> &'static group_type!(Self) {
         &late::ATTRIBUTE_PARSERS
@@ -228,6 +231,9 @@ impl<'f, 'sess: 'f, S: Stage> SharedContext<'f, 'sess, S> {
     /// must be delayed until after HIR is built. This method will take care of the details of
     /// that.
     pub(crate) fn emit_lint(&mut self, lint: AttributeLintKind, span: Span) {
+        if !S::SHOULD_EMIT_LINTS {
+            return;
+        }
         let id = self.target_id;
         (self.emit_lint)(AttributeLint { id, span, kind: lint });
     }
@@ -409,6 +415,10 @@ impl<'f, 'sess: 'f, S: Stage> AcceptContext<'f, 'sess, S> {
             },
         })
     }
+
+    pub(crate) fn warn_empty_attribute(&mut self, span: Span) {
+        self.emit_lint(AttributeLintKind::EmptyAttribute { first_span: span }, span);
+    }
 }
 
 impl<'f, 'sess, S: Stage> Deref for AcceptContext<'f, 'sess, S> {

compiler/rustc_attr_parsing/src/lints.rs

@@ -28,5 +28,11 @@ pub fn emit_attribute_lint<L: LintEmitter>(lint: &AttributeLint<HirId>, lint_emi
                 },
             );
         }
+        AttributeLintKind::EmptyAttribute { first_span } => lint_emitter.emit_node_span_lint(
+            rustc_session::lint::builtin::UNUSED_ATTRIBUTES,
+            *id,
+            *first_span,
+            session_diagnostics::EmptyAttributeList { attr_span: *first_span },
+        ),
     }
 }

compiler/rustc_attr_parsing/src/session_diagnostics.rs

@@ -473,6 +473,13 @@ pub(crate) struct EmptyConfusables {
     pub span: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(attr_parsing_empty_attribute)]
+pub(crate) struct EmptyAttributeList {
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub attr_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_parsing_invalid_alignment_value, code = E0589)]
 pub(crate) struct InvalidAlignmentValue {

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -485,7 +485,7 @@ impl<'a> TraitDef<'a> {
             Annotatable::Item(item) => {
                 let is_packed = matches!(
                     AttributeParser::parse_limited(cx.sess, &item.attrs, sym::repr, item.span, item.id),
-                    Some(Attribute::Parsed(AttributeKind::Repr(r))) if r.iter().any(|(x, _)| matches!(x, ReprPacked(..)))
+                    Some(Attribute::Parsed(AttributeKind::Repr { reprs, .. })) if reprs.iter().any(|(x, _)| matches!(x, ReprPacked(..)))
                 );
 
                 let newitem = match &item.kind {