compiler/rustc_ast_passes/messages.ftl

@@ -241,8 +241,10 @@ ast_passes_trait_fn_const =
         [true] trait impls
         *[false] traits
     } cannot be const
-    .const_context_label = this declares all functions implicitly const
+    .const_context_label = this declares all associated functions implicitly const
     .remove_const_sugg = remove the `const`
+    .make_impl_const_sugg = and declare the impl to be const instead
+    .make_trait_const_sugg = and declare the trait to be a `#[const_trait]` instead
 
 ast_passes_trait_object_single_bound = only a single explicit lifetime bound is permitted
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -46,20 +46,33 @@ enum DisallowTildeConstContext<'a> {
     Item,
 }
 
+enum TraitOrImpl<'a> {
+    Trait { span: Span, constness: Option<Span> },
+    Impl { trait_: Option<(Const, ImplPolarity, &'a TraitRef)> },
+}
+
+impl<'a> TraitOrImpl<'a> {
+    fn constness(&self) -> Option<Span> {
+        match self {
+            Self::Trait { constness: Some(span), .. }
+            | Self::Impl { trait_: Some((Const::Yes(span), ..)) } => Some(*span),
+            _ => None,
+        }
+    }
+
+    fn is_trait_or_trait_impl(&self) -> bool {
+        matches!(self, Self::Trait { .. } | Self::Impl { trait_: Some(_) })
+    }
+}
+
 struct AstValidator<'a> {
     session: &'a Session,
     features: &'a Features,
 
     /// The span of the `extern` in an `extern { ... }` block, if any.
     extern_mod: Option<&'a Item>,
 
-    /// Are we inside a trait impl?
-    in_trait_impl: bool,
-
-    /// Are we inside a const trait defn or impl?
-    ///
-    /// If so, this is the span of the constness.
-    in_const_trait_or_impl: Option<Span>,
+    outer_trait_or_impl: Option<TraitOrImpl<'a>>,
 
     has_proc_macro_decls: bool,
 
@@ -80,27 +93,24 @@ struct AstValidator<'a> {
 impl<'a> AstValidator<'a> {
     fn with_in_trait_impl(
         &mut self,
-        is_in: bool,
-        constness: Option<Const>,
+        trait_: Option<(Const, ImplPolarity, &'a TraitRef)>,
         f: impl FnOnce(&mut Self),
     ) {
-        let old = mem::replace(&mut self.in_trait_impl, is_in);
-        let old_const = mem::replace(
-            &mut self.in_const_trait_or_impl,
-            match constness {
-                Some(Const::Yes(span)) => Some(span),
-                _ => None,
-            },
+        let old = mem::replace(
+            &mut self.outer_trait_or_impl,
+            trait_.map(|trait_| TraitOrImpl::Impl { trait_: Some(trait_) }),
         );
         f(self);
-        self.in_trait_impl = old;
-        self.in_const_trait_or_impl = old_const;
+        self.outer_trait_or_impl = old;
     }
 
-    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);
+    fn with_in_trait(&mut self, span: Span, constness: Option<Span>, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(
+            &mut self.outer_trait_or_impl,
+            Some(TraitOrImpl::Trait { span, constness }),
+        );
         f(self);
-        self.in_const_trait_or_impl = old;
+        self.outer_trait_or_impl = old;
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
@@ -296,18 +306,42 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_trait_fn_not_const(&self, constness: Const) {
+    fn check_trait_fn_not_const(&self, constness: Const, trait_or_impl: &TraitOrImpl<'a>) {
         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.
+        let make_impl_const_sugg = if self.features.const_trait_impl
+            && let TraitOrImpl::Impl { trait_: Some(trait_) } = trait_or_impl
+            && let (Const::No, ImplPolarity::Positive, trait_ref) = trait_
+        {
+            Some(trait_ref.path.span.shrink_to_lo())
+        } else {
+            None
+        };
+
+        let make_trait_const_sugg = if self.features.const_trait_impl
+            && let TraitOrImpl::Trait { span, constness: None } = trait_or_impl
+        {
+            Some(span.shrink_to_lo())
+        } else {
+            None
+        };
+
+        let parent_constness = trait_or_impl.constness();
         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),
+            in_impl: matches!(trait_or_impl, TraitOrImpl::Impl { .. }),
+            const_context_label: parent_constness,
+            remove_const_sugg: (
+                self.session.source_map().span_extend_while(span, |c| c == ' ').unwrap_or(span),
+                match parent_constness {
+                    Some(_) => rustc_errors::Applicability::MachineApplicable,
+                    None => rustc_errors::Applicability::MaybeIncorrect,
+                },
+            ),
+            make_impl_const_sugg,
+            make_trait_const_sugg,
         });
     }
 
@@ -831,7 +865,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self_ty,
                 items,
             }) => {
-                self.with_in_trait_impl(true, Some(*constness), |this| {
+                self.with_in_trait_impl(Some((*constness, *polarity, t)), |this| {
                     this.visibility_not_permitted(
                         &item.vis,
                         errors::VisibilityNotPermittedNote::TraitImpl,
@@ -979,7 +1013,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             ItemKind::Trait(box Trait { is_auto, generics, bounds, items, .. }) => {
                 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| {
+                self.with_in_trait(item.span, is_const_trait, |this| {
                     if *is_auto == IsAuto::Yes {
                         // Auto traits cannot have generics, super traits nor contain items.
                         this.deny_generic_params(generics, item.ident.span);
@@ -1356,10 +1390,10 @@ 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.is_some());
+        let tilde_const_allowed =
+            matches!(fk.header(), Some(FnHeader { constness: ast::Const::Yes(_), .. }))
+                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)))
+                    && self.outer_trait_or_impl.as_ref().and_then(TraitOrImpl::constness).is_some();
 
         let disallowed = (!tilde_const_allowed).then(|| DisallowTildeConstContext::Fn(fk));
         self.with_tilde_const(disallowed, |this| visit::walk_fn(this, fk));
@@ -1370,7 +1404,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             self.check_nomangle_item_asciionly(item.ident, item.span);
         }
 
-        if ctxt == AssocCtxt::Trait || !self.in_trait_impl {
+        if self.outer_trait_or_impl.as_ref().is_some_and(TraitOrImpl::is_trait_or_trait_impl) {
             self.check_defaultness(item.span, item.kind.defaultness());
         }
 
@@ -1418,10 +1452,12 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             );
         }
 
-        if ctxt == AssocCtxt::Trait || self.in_trait_impl {
+        if let Some(trait_or_impl) = &self.outer_trait_or_impl
+            && trait_or_impl.is_trait_or_trait_impl()
+        {
             self.visibility_not_permitted(&item.vis, errors::VisibilityNotPermittedNote::TraitImpl);
             if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
-                self.check_trait_fn_not_const(sig.header.constness);
+                self.check_trait_fn_not_const(sig.header.constness, trait_or_impl);
             }
         }
 
@@ -1431,7 +1467,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
         match &item.kind {
             AssocItemKind::Fn(box Fn { sig, generics, body, .. })
-                if self.in_const_trait_or_impl.is_some()
+                if self.outer_trait_or_impl.as_ref().and_then(TraitOrImpl::constness).is_some()
                     || ctxt == AssocCtxt::Trait
                     || matches!(sig.header.constness, Const::Yes(_)) =>
             {
@@ -1447,8 +1483,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 );
                 self.visit_fn(kind, item.span, item.id);
             }
-            _ => self
-                .with_in_trait_impl(false, None, |this| visit::walk_assoc_item(this, item, ctxt)),
+            _ => self.with_in_trait_impl(None, |this| visit::walk_assoc_item(this, item, ctxt)),
         }
     }
 }
@@ -1564,8 +1599,7 @@ pub fn check_crate(
         session,
         features,
         extern_mod: None,
-        in_trait_impl: false,
-        in_const_trait_or_impl: None,
+        outer_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

@@ -1,7 +1,7 @@
 //! Errors emitted by ast_passes.
 
 use rustc_ast::ParamKindOrd;
-use rustc_errors::AddToDiagnostic;
+use rustc_errors::{AddToDiagnostic, Applicability};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
 
@@ -52,8 +52,20 @@ pub struct TraitFnConst {
     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>,
+    #[suggestion(ast_passes_remove_const_sugg, code = "")]
+    pub remove_const_sugg: (Span, Applicability),
+    #[suggestion(
+        ast_passes_make_impl_const_sugg,
+        code = "const ",
+        applicability = "maybe-incorrect"
+    )]
+    pub make_impl_const_sugg: Option<Span>,
+    #[suggestion(
+        ast_passes_make_trait_const_sugg,
+        code = "#[const_trait]\n",
+        applicability = "maybe-incorrect"
+    )]
+    pub make_trait_const_sugg: Option<Span>,
 }
 
 #[derive(Diagnostic)]

tests/ui/consts/const-fn-mismatch.stderr

@@ -2,7 +2,10 @@ error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/const-fn-mismatch.rs:11:5
    |
 LL |     const fn f() -> u32 {
-   |     ^^^^^ functions in trait impls cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in trait impls cannot be const
+   |     help: remove the `const`
 
 error: aborting due to 1 previous error
 

tests/ui/consts/const-fn-not-in-trait.stderr

@@ -2,13 +2,19 @@ error[E0379]: functions in traits cannot be declared const
   --> $DIR/const-fn-not-in-trait.rs:5:5
    |
 LL |     const fn f() -> u32;
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
 
 error[E0379]: functions in traits cannot be declared const
   --> $DIR/const-fn-not-in-trait.rs:7:5
    |
 LL |     const fn g() -> u32 {
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
 
 error: aborting due to 2 previous errors
 

tests/ui/consts/issue-54954.stderr

@@ -2,7 +2,10 @@ error[E0379]: functions in traits cannot be declared const
   --> $DIR/issue-54954.rs:5:5
    |
 LL |     const fn const_val<T: Sized>() -> usize {
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
 
 error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
   --> $DIR/issue-54954.rs:1:24

tests/ui/feature-gates/feature-gate-min_const_fn.stderr

@@ -2,19 +2,28 @@ error[E0379]: functions in traits cannot be declared const
   --> $DIR/feature-gate-min_const_fn.rs:6:5
    |
 LL |     const fn foo() -> u32;
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
 
 error[E0379]: functions in traits cannot be declared const
   --> $DIR/feature-gate-min_const_fn.rs:7:5
    |
 LL |     const fn bar() -> u32 { 0 }
-   |     ^^^^^ 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/feature-gate-min_const_fn.rs:11:5
    |
 LL |     const fn foo() -> u32 { 0 }
-   |     ^^^^^ functions in trait impls cannot be const
+   |     ^^^^^-
+   |     |
+   |     functions in trait impls cannot be const
+   |     help: remove the `const`
 
 error: aborting due to 3 previous errors
 

tests/ui/mismatched_types/const-fn-in-trait.stderr

@@ -2,13 +2,19 @@ error[E0379]: functions in traits cannot be declared const
   --> $DIR/const-fn-in-trait.rs:3:5
    |
 LL |     const fn g();
-   |     ^^^^^ 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/const-fn-in-trait.rs:7:5
    |
 LL |     const fn f() -> u32 { 22 }
-   |     ^^^^^ 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