Make E0379 diagnostic more helpful

Author: fmease

compiler/rustc_ast_passes/messages.ftl

@@ -241,6 +241,8 @@ ast_passes_trait_fn_const =
         [true] trait impls
         *[false] traits
     } cannot be const
+    .const_context_label = this declares all functions implicitly const
+    .remove_const_sugg = remove the `const`
 
 ast_passes_trait_object_single_bound = only a single explicit lifetime bound is permitted
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -57,7 +57,9 @@ struct AstValidator<'a> {
     in_trait_impl: bool,
 
     /// Are we inside a const trait defn or impl?
-    in_const_trait_or_impl: bool,
+    ///
+    /// If so, this is the span of the constness.
+    in_const_trait_or_impl: Option<Span>,
 
     has_proc_macro_decls: bool,
 
@@ -85,14 +87,17 @@ impl<'a> AstValidator<'a> {
         let old = mem::replace(&mut self.in_trait_impl, is_in);
         let old_const = mem::replace(
             &mut self.in_const_trait_or_impl,
-            matches!(constness, Some(Const::Yes(_))),
+            match constness {
+                Some(Const::Yes(span)) => Some(span),
+                _ => None,
+            },
         );
         f(self);
         self.in_trait_impl = old;
         self.in_const_trait_or_impl = old_const;
     }
 
-    fn with_in_trait(&mut self, is_const: bool, f: impl FnOnce(&mut Self)) {
+    fn with_in_trait(&mut self, is_const: Option<Span>, f: impl FnOnce(&mut Self)) {
         let old = mem::replace(&mut self.in_const_trait_or_impl, is_const);
         f(self);
         self.in_const_trait_or_impl = old;
@@ -292,9 +297,18 @@ impl<'a> AstValidator<'a> {
     }
 
     fn check_trait_fn_not_const(&self, constness: Const) {
-        if let Const::Yes(span) = constness {
-            self.dcx().emit_err(errors::TraitFnConst { span, in_impl: self.in_trait_impl });
-        }
+        let Const::Yes(span) = constness else {
+            return;
+        };
+
+        // FIXME(const_trait_impl): If the trait or impl is not const and feature `const_trait_impl`
+        // is enabled, provide a structured suggestion to make the trait (impl) const.
+        self.dcx().emit_err(errors::TraitFnConst {
+            span,
+            in_impl: self.in_trait_impl,
+            const_context_label: self.in_const_trait_or_impl,
+            remove_const_sugg: self.in_const_trait_or_impl.map(|_| span),
+        });
     }
 
     fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
@@ -963,7 +977,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
             }
             ItemKind::Trait(box Trait { is_auto, generics, bounds, items, .. }) => {
-                let is_const_trait = attr::contains_name(&item.attrs, sym::const_trait);
+                let is_const_trait =
+                    attr::find_by_name(&item.attrs, sym::const_trait).map(|attr| attr.span);
                 self.with_in_trait(is_const_trait, |this| {
                     if *is_auto == IsAuto::Yes {
                         // Auto traits cannot have generics, super traits nor contain items.
@@ -977,8 +992,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     // context for the supertraits.
                     this.visit_vis(&item.vis);
                     this.visit_ident(item.ident);
-                    let disallowed =
-                        (!is_const_trait).then(|| DisallowTildeConstContext::Trait(item.span));
+                    let disallowed = is_const_trait
+                        .is_none()
+                        .then(|| DisallowTildeConstContext::Trait(item.span));
                     this.with_tilde_const(disallowed, |this| {
                         this.visit_generics(generics);
                         walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
@@ -1340,9 +1356,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             });
         }
 
-        let tilde_const_allowed =
-            matches!(fk.header(), Some(FnHeader { constness: ast::Const::Yes(_), .. }))
-                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)) if self.in_const_trait_or_impl);
+        let tilde_const_allowed = matches!(
+            fk.header(),
+            Some(FnHeader { constness: ast::Const::Yes(_), .. })
+        ) || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)) if self.in_const_trait_or_impl.is_some());
 
         let disallowed = (!tilde_const_allowed).then(|| DisallowTildeConstContext::Fn(fk));
         self.with_tilde_const(disallowed, |this| visit::walk_fn(this, fk));
@@ -1414,7 +1431,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
         match &item.kind {
             AssocItemKind::Fn(box Fn { sig, generics, body, .. })
-                if self.in_const_trait_or_impl
+                if self.in_const_trait_or_impl.is_some()
                     || ctxt == AssocCtxt::Trait
                     || matches!(sig.header.constness, Const::Yes(_)) =>
             {
@@ -1548,7 +1565,7 @@ pub fn check_crate(
         features,
         extern_mod: None,
         in_trait_impl: false,
-        in_const_trait_or_impl: false,
+        in_const_trait_or_impl: None,
         has_proc_macro_decls: false,
         outer_impl_trait: None,
         disallow_tilde_const: Some(DisallowTildeConstContext::Item),

compiler/rustc_ast_passes/src/errors.rs

@@ -50,6 +50,10 @@ pub struct TraitFnConst {
     #[label]
     pub span: Span,
     pub in_impl: bool,
+    #[label(ast_passes_const_context_label)]
+    pub const_context_label: Option<Span>,
+    #[suggestion(ast_passes_remove_const_sugg, code = "", applicability = "machine-applicable")]
+    pub remove_const_sugg: Option<Span>,
 }
 
 #[derive(Diagnostic)]

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/trait-fn-const.stderr

@@ -1,14 +1,25 @@
 error[E0379]: functions in traits cannot be declared const
   --> $DIR/trait-fn-const.rs:8:5
    |
+LL | #[const_trait]
+   | -------------- this declares all functions implicitly const
+LL | trait Trait {
 LL |     const fn fun();
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
 
 error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/trait-fn-const.rs:12:5
    |
+LL | impl const Trait for () {
+   |      ----- this declares all functions implicitly const
 LL |     const fn fun() {}
-   |     ^^^^^ functions in trait impls cannot be const
+   |     ^^^^^
+   |     |
+   |     functions in trait impls cannot be const
+   |     help: remove the `const`
 
 error: aborting due to 2 previous errors