Port #[must_use] to new attribute parsing infrastructure

JonathanBrouwer · JonathanBrouwer · commit f42da01bef04 · 2025-06-22T12:09:14.000+02:00
Signed-off-by: Jonathan Brouwer &lt;jonathantbrouwer@gmail.com&gt;
diff --git a/compiler/rustc_attr_data_structures/src/attributes.rs b/compiler/rustc_attr_data_structures/src/attributes.rs
@@ -237,6 +237,13 @@ pub enum AttributeKind {
     /// Represents [`#[may_dangle]`](https://std-dev-guide.rust-lang.org/tricky/may-dangle.html).
     MayDangle(Span),
 
+    /// Represents `#[must_use]`.
+    MustUse {
+        span: Span,
+        /// must_use can optionally have a reason: `#[must_use = "reason this must be used"]`
+        reason: Option<Symbol>,
+    },
+
     /// Represents `#[optimize(size|speed)]`
     Optimize(OptimizeAttr, Span),
 
diff --git a/compiler/rustc_attr_parsing/src/attributes/mod.rs b/compiler/rustc_attr_parsing/src/attributes/mod.rs
@@ -33,6 +33,7 @@ pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod inline;
 pub(crate) mod lint_helpers;
+pub(crate) mod must_use;
 pub(crate) mod repr;
 pub(crate) mod semantics;
 pub(crate) mod stability;
diff --git a/compiler/rustc_attr_parsing/src/attributes/must_use.rs b/compiler/rustc_attr_parsing/src/attributes/must_use.rs
@@ -0,0 +1,40 @@
+use rustc_attr_data_structures::AttributeKind;
+use rustc_errors::DiagArgValue;
+use rustc_feature::{AttributeTemplate, template};
+use rustc_span::{Symbol, sym};
+
+use crate::attributes::{AttributeOrder, OnDuplicate, SingleAttributeParser};
+use crate::context::{AcceptContext, Stage};
+use crate::parser::ArgParser;
+use crate::session_diagnostics;
+
+pub(crate) struct MustUseParser;
+
+impl<S: Stage> SingleAttributeParser<S> for MustUseParser {
+    const PATH: &[Symbol] = &[sym::must_use];
+    const ATTRIBUTE_ORDER: AttributeOrder = AttributeOrder::KeepLast;
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::WarnButFutureError;
+    const TEMPLATE: AttributeTemplate = template!(Word, NameValueStr: "reason");
+
+    fn convert(cx: &mut AcceptContext<'_, '_, S>, args: &ArgParser<'_>) -> Option<AttributeKind> {
+        Some(AttributeKind::MustUse {
+            span: cx.attr_span,
+            reason: match args {
+                ArgParser::NoArgs => None,
+                ArgParser::NameValue(name_value) => name_value.value_as_str(),
+                ArgParser::List(_) => {
+                    let suggestions =
+                        <Self as SingleAttributeParser<S>>::TEMPLATE.suggestions(false, "must_use");
+                    cx.emit_err(session_diagnostics::MustUseIllFormedAttributeInput {
+                        num_suggestions: suggestions.len(),
+                        suggestions: DiagArgValue::StrListSepByAnd(
+                            suggestions.into_iter().map(|s| format!("`{s}`").into()).collect(),
+                        ),
+                        span: cx.attr_span,
+                    });
+                    return None;
+                }
+            },
+        })
+    }
+}
diff --git a/compiler/rustc_attr_parsing/src/context.rs b/compiler/rustc_attr_parsing/src/context.rs
@@ -20,6 +20,7 @@ use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
 use crate::attributes::inline::{InlineParser, RustcForceInlineParser};
 use crate::attributes::lint_helpers::AsPtrParser;
+use crate::attributes::must_use::MustUseParser;
 use crate::attributes::repr::{AlignParser, ReprParser};
 use crate::attributes::semantics::MayDangleParser;
 use crate::attributes::stability::{
@@ -112,6 +113,7 @@ attribute_parsers!(
         Single<DeprecationParser>,
         Single<InlineParser>,
         Single<MayDangleParser>,
+        Single<MustUseParser>,
         Single<OptimizeParser>,
         Single<RustcForceInlineParser>,
         Single<TransparencyParser>,
diff --git a/compiler/rustc_attr_parsing/src/session_diagnostics.rs b/compiler/rustc_attr_parsing/src/session_diagnostics.rs
@@ -436,6 +436,15 @@ pub(crate) struct IllFormedAttributeInput {
     pub suggestions: DiagArgValue,
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_parsing_ill_formed_attribute_input)]
+pub(crate) struct MustUseIllFormedAttributeInput {
+    #[primary_span]
+    pub span: Span,
+    pub num_suggestions: usize,
+    pub suggestions: DiagArgValue,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_parsing_stability_outside_std, code = E0734)]
 pub(crate) struct StabilityOutsideStd {
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
@@ -2,6 +2,7 @@ use std::iter;
 
 use rustc_ast::util::{classify, parser};
 use rustc_ast::{self as ast, ExprKind, HasAttrs as _, StmtKind};
+use rustc_attr_data_structures::{AttributeKind, find_attr};
 use rustc_errors::{MultiSpan, pluralize};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -368,10 +369,12 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
         }
 
         fn is_def_must_use(cx: &LateContext<'_>, def_id: DefId, span: Span) -> Option<MustUsePath> {
-            if let Some(attr) = cx.tcx.get_attr(def_id, sym::must_use) {
+            if let Some(reason) = find_attr!(
+                cx.tcx.get_all_attrs(def_id),
+                AttributeKind::MustUse { reason, .. } => reason
+            ) {
                 // check for #[must_use = "..."]
-                let reason = attr.value_str();
-                Some(MustUsePath::Def(span, def_id, reason))
+                Some(MustUsePath::Def(span, def_id, *reason))
             } else {
                 None
             }
diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
@@ -293,6 +293,7 @@ fn emit_malformed_attribute(
             | sym::deprecated
             | sym::optimize
             | sym::cold
+            | sym::must_use
     ) {
         return;
     }
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
@@ -166,6 +166,9 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 &Attribute::Parsed(AttributeKind::MayDangle(attr_span)) => {
                     self.check_may_dangle(hir_id, attr_span)
                 }
+                Attribute::Parsed(AttributeKind::MustUse { span, .. }) => {
+                    self.check_must_use(hir_id, *span, target)
+                }
                 Attribute::Unparsed(_) => {
                     match attr.path().as_slice() {
                         [sym::diagnostic, sym::do_not_recommend, ..] => {
@@ -238,7 +241,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         | [sym::const_trait, ..] => self.check_must_be_applied_to_trait(attr, span, target),
                         [sym::collapse_debuginfo, ..] => self.check_collapse_debuginfo(attr, span, target),
                         [sym::must_not_suspend, ..] => self.check_must_not_suspend(attr, span, target),
-                        [sym::must_use, ..] => self.check_must_use(hir_id, attr, target),
                         [sym::rustc_pass_by_value, ..] => self.check_pass_by_value(attr, span, target),
                         [sym::rustc_allow_incoherent_impl, ..] => {
                             self.check_allow_incoherent_impl(attr, span, target)
@@ -690,7 +692,8 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                             AttributeKind::Deprecation { .. }
                             | AttributeKind::Repr { .. }
                             | AttributeKind::Align { .. }
-                            | AttributeKind::Cold(..),
+                            | AttributeKind::Cold(..)
+                            | AttributeKind::MustUse { .. },
                         ) => {
                             continue;
                         }
@@ -1565,7 +1568,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     }
 
     /// Warns against some misuses of `#[must_use]`
-    fn check_must_use(&self, hir_id: HirId, attr: &Attribute, target: Target) {
+    fn check_must_use(&self, hir_id: HirId, attr_span: Span, target: Target) {
         if matches!(
             target,
             Target::Fn
@@ -1605,7 +1608,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         self.tcx.emit_node_span_lint(
             UNUSED_ATTRIBUTES,
             hir_id,
-            attr.span(),
+            attr_span,
             errors::MustUseNoEffect { article, target },
         );
     }
diff --git a/src/rustdoc-json-types/lib.rs b/src/rustdoc-json-types/lib.rs
@@ -37,8 +37,8 @@ pub type FxHashMap<K, V> = HashMap<K, V>; // re-export for use in src/librustdoc
 // will instead cause conflicts. See #94591 for more. (This paragraph and the "Latest feature" line
 // are deliberately not in a doc comment, because they need not be in public docs.)
 //
-// Latest feature: improve handling of generic args
-pub const FORMAT_VERSION: u32 = 51;
+// Latest feature: Pretty printing of must_use attributes changed
+pub const FORMAT_VERSION: u32 = 52;
 
 /// The root of the emitted JSON blob.
 ///
diff --git a/src/tools/clippy/clippy_lints/src/functions/must_use.rs b/src/tools/clippy/clippy_lints/src/functions/must_use.rs
@@ -15,14 +15,17 @@ use clippy_utils::ty::is_must_use_ty;
 use clippy_utils::visitors::for_each_expr_without_closures;
 use clippy_utils::{return_ty, trait_ref_of_method};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
+use rustc_attr_data_structures::AttributeKind;
+use rustc_span::Symbol;
+use rustc_attr_data_structures::find_attr;
 
 use core::ops::ControlFlow;
 
 use super::{DOUBLE_MUST_USE, MUST_USE_CANDIDATE, MUST_USE_UNIT};
 
 pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
     let attrs = cx.tcx.hir_attrs(item.hir_id());
-    let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
+    let attr = find_attr!(cx.tcx.hir_attrs(item.hir_id()), AttributeKind::MustUse { span, reason } => (span, reason));
     if let hir::ItemKind::Fn {
         ref sig,
         body: ref body_id,
@@ -31,8 +34,8 @@ pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>
     {
         let is_public = cx.effective_visibilities.is_exported(item.owner_id.def_id);
         let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
-        if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, attrs, sig);
+        if let Some((attr_span, reason)) = attr {
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, *attr_span, *reason, attrs, sig);
         } else if is_public && !is_proc_macro(attrs) && !attrs.iter().any(|a| a.has_name(sym::no_mangle)) {
             check_must_use_candidate(
                 cx,
@@ -52,9 +55,9 @@ pub(super) fn check_impl_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Imp
         let is_public = cx.effective_visibilities.is_exported(item.owner_id.def_id);
         let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
         let attrs = cx.tcx.hir_attrs(item.hir_id());
-        let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
-        if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, attrs, sig);
+        let attr = find_attr!(cx.tcx.hir_attrs(item.hir_id()), AttributeKind::MustUse { span, reason } => (span, reason));
+        if let Some((attr_span, reason)) = attr {
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, *attr_span, *reason, attrs, sig);
         } else if is_public && !is_proc_macro(attrs) && trait_ref_of_method(cx, item.owner_id).is_none() {
             check_must_use_candidate(
                 cx,
@@ -75,9 +78,9 @@ pub(super) fn check_trait_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Tr
         let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
 
         let attrs = cx.tcx.hir_attrs(item.hir_id());
-        let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
-        if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, attrs, sig);
+        let attr = find_attr!(cx.tcx.hir_attrs(item.hir_id()), AttributeKind::MustUse { span, reason } => (span, reason));
+        if let Some((attr_span, reason)) = attr {
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, *attr_span, *reason, attrs, sig);
         } else if let hir::TraitFn::Provided(eid) = *eid {
             let body = cx.tcx.hir_body(eid);
             if attr.is_none() && is_public && !is_proc_macro(attrs) {
@@ -103,7 +106,8 @@ fn check_needless_must_use(
     item_id: hir::OwnerId,
     item_span: Span,
     fn_header_span: Span,
-    attr: &Attribute,
+    attr_span: Span,
+    reason: Option<Symbol>,
     attrs: &[Attribute],
     sig: &FnSig<'_>,
 ) {
@@ -119,7 +123,7 @@ fn check_needless_must_use(
                 "this unit-returning function has a `#[must_use]` attribute",
                 |diag| {
                     diag.span_suggestion(
-                        attr.span(),
+                        attr_span,
                         "remove the attribute",
                         "",
                         Applicability::MachineApplicable,
@@ -137,11 +141,11 @@ fn check_needless_must_use(
                 MUST_USE_UNIT,
                 fn_header_span,
                 "this unit-returning function has a `#[must_use]` attribute",
-                Some(attr.span()),
+                Some(attr_span),
                 "remove `must_use`",
             );
         }
-    } else if attr.value_str().is_none() && is_must_use_ty(cx, return_ty(cx, item_id)) {
+    } else if reason.is_none() && is_must_use_ty(cx, return_ty(cx, item_id)) {
         // Ignore async functions unless Future::Output type is a must_use type
         if sig.header.is_async() {
             let infcx = cx.tcx.infer_ctxt().build(cx.typing_mode());
diff --git a/src/tools/clippy/clippy_lints/src/return_self_not_must_use.rs b/src/tools/clippy/clippy_lints/src/return_self_not_must_use.rs
@@ -6,7 +6,9 @@ use rustc_hir::intravisit::FnKind;
 use rustc_hir::{Body, FnDecl, OwnerId, TraitItem, TraitItemKind};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::declare_lint_pass;
-use rustc_span::{Span, sym};
+use rustc_span::{Span};
+use rustc_attr_data_structures::AttributeKind;
+use rustc_attr_data_structures::find_attr;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -74,7 +76,10 @@ fn check_method(cx: &LateContext<'_>, decl: &FnDecl<'_>, fn_def: LocalDefId, spa
         // We only show this warning for public exported methods.
         && cx.effective_visibilities.is_exported(fn_def)
         // We don't want to emit this lint if the `#[must_use]` attribute is already there.
-        && !cx.tcx.hir_attrs(owner_id.into()).iter().any(|attr| attr.has_name(sym::must_use))
+        && !find_attr!(
+            cx.tcx.hir_attrs(owner_id.into()),
+            AttributeKind::MustUse { .. }
+        )
         && cx.tcx.visibility(fn_def.to_def_id()).is_public()
         && let ret_ty = return_ty(cx, owner_id)
         && let self_arg = nth_arg(cx, owner_id, 0)
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -1886,7 +1886,10 @@ pub fn is_must_use_func_call(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
         _ => None,
     };
 
-    did.is_some_and(|did| cx.tcx.has_attr(did, sym::must_use))
+    did.is_some_and(|did| find_attr!(
+            cx.tcx.get_all_attrs(did),
+            AttributeKind::MustUse { ..}
+        ))
 }
 
 /// Checks if a function's body represents the identity function. Looks for bodies of the form:
diff --git a/src/tools/clippy/clippy_utils/src/ty/mod.rs b/src/tools/clippy/clippy_utils/src/ty/mod.rs
@@ -31,6 +31,8 @@ use rustc_trait_selection::traits::{Obligation, ObligationCause};
 use std::assert_matches::debug_assert_matches;
 use std::collections::hash_map::Entry;
 use std::iter;
+use rustc_attr_data_structures::find_attr;
+use rustc_attr_data_structures::AttributeKind;
 
 use crate::path_res;
 use crate::paths::{PathNS, lookup_path_str};
@@ -326,8 +328,14 @@ pub fn has_drop<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
 // Returns whether the type has #[must_use] attribute
 pub fn is_must_use_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
     match ty.kind() {
-        ty::Adt(adt, _) => cx.tcx.has_attr(adt.did(), sym::must_use),
-        ty::Foreign(did) => cx.tcx.has_attr(*did, sym::must_use),
+        ty::Adt(adt, _) => find_attr!(
+            cx.tcx.get_all_attrs(adt.did()),
+            AttributeKind::MustUse { ..}
+        ),
+        ty::Foreign(did) => find_attr!(
+            cx.tcx.get_all_attrs(*did),
+            AttributeKind::MustUse { ..}
+        ),
         ty::Slice(ty) | ty::Array(ty, _) | ty::RawPtr(ty, _) | ty::Ref(_, ty, _) => {
             // for the Array case we don't need to care for the len == 0 case
             // because we don't want to lint functions returning empty arrays
@@ -337,7 +345,7 @@ pub fn is_must_use_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
         ty::Alias(ty::Opaque, AliasTy { def_id, .. }) => {
             for (predicate, _) in cx.tcx.explicit_item_self_bounds(def_id).skip_binder() {
                 if let ty::ClauseKind::Trait(trait_predicate) = predicate.kind().skip_binder()
-                    && cx.tcx.has_attr(trait_predicate.trait_ref.def_id, sym::must_use)
+                    && find_attr!(cx.tcx.get_all_attrs(trait_predicate.trait_ref.def_id), AttributeKind::MustUse { ..})
                 {
                     return true;
                 }
@@ -347,7 +355,7 @@ pub fn is_must_use_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
         ty::Dynamic(binder, _, _) => {
             for predicate in *binder {
                 if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder()
-                    && cx.tcx.has_attr(trait_ref.def_id, sym::must_use)
+                    && find_attr!(cx.tcx.get_all_attrs(trait_ref.def_id), AttributeKind::MustUse { ..})
                 {
                     return true;
                 }
diff --git a/tests/rustdoc-json/attrs/must_use.rs b/tests/rustdoc-json/attrs/must_use.rs
@@ -1,9 +1,9 @@
 #![no_std]
 
-//@ is "$.index[?(@.name=='example')].attrs" '["#[must_use]"]'
+//@ is "$.index[?(@.name=='example')].attrs" '["#[attr = MustUse]"]'
 #[must_use]
 pub fn example() -> impl Iterator<Item = i64> {}
 
-//@ is "$.index[?(@.name=='explicit_message')].attrs" '["#[must_use = \"does nothing if you do not use it\"]"]'
+//@ is "$.index[?(@.name=='explicit_message')].attrs" '["#[attr = MustUse {reason: \"does nothing if you do not use it\"}]"]'
 #[must_use = "does nothing if you do not use it"]
 pub fn explicit_message() -> impl Iterator<Item = i64> {}
diff --git a/tests/ui/attributes/malformed-must_use.rs b/tests/ui/attributes/malformed-must_use.rs
@@ -0,0 +1,4 @@
+#[must_use()] //~ ERROR valid forms for the attribute are `#[must_use = "reason"]` and `#[must_use]`
+struct Test;
+
+fn main() {}
diff --git a/tests/ui/attributes/malformed-must_use.stderr b/tests/ui/attributes/malformed-must_use.stderr
@@ -0,0 +1,8 @@
+error: valid forms for the attribute are `#[must_use = "reason"]` and `#[must_use]`
+  --> $DIR/malformed-must_use.rs:1:1
+   |
+LL | #[must_use()]
+   | ^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.stderr b/tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.stderr
diff --git a/tests/ui/lint/unused/unused-attr-duplicate.stderr b/tests/ui/lint/unused/unused-attr-duplicate.stderr
diff --git a/tests/ui/parser/bad-lit-suffixes.rs b/tests/ui/parser/bad-lit-suffixes.rs
diff --git a/tests/ui/parser/bad-lit-suffixes.stderr b/tests/ui/parser/bad-lit-suffixes.stderr

Original file line number	Diff line number	Diff line change
`@@ -293,6 +293,7 @@ fn emit_malformed_attribute(`
`293`	`293`	`\| sym::deprecated`
`294`	`294`	`\| sym::optimize`
`295`	`295`	`\| sym::cold`
	`296`	`+ \| sym::must_use`
`296`	`297`	`) {`
`297`	`298`	`return;`
`298`	`299`	`}`