E0379: Suggest #[const_trait] in diagnostic

compiler/rustc_ast_passes/messages.ftl

@@ -244,6 +244,7 @@ ast_passes_trait_fn_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

@@ -63,6 +63,11 @@ struct AstValidator<'a> {
     /// If so, this is the span of the constness.
     in_const_trait_or_impl: Option<Span>,
 
+    /// Are we inside a trait?
+    ///
+    /// If so, this is the span of the trait.
+    in_trait: Option<Span>,
+
     has_proc_macro_decls: bool,
 
     /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
@@ -101,10 +106,12 @@ impl<'a> AstValidator<'a> {
         self.in_const_trait_or_impl = old_const;
     }
 
-    fn with_in_trait(&mut self, is_const: Option<Span>, f: impl FnOnce(&mut Self)) {
+    fn with_in_trait(&mut self, span: Span, is_const: Option<Span>, f: impl FnOnce(&mut Self)) {
         let old = mem::replace(&mut self.in_const_trait_or_impl, is_const);
+        let old_trait = mem::replace(&mut self.in_trait, Some(span));
         f(self);
         self.in_const_trait_or_impl = old;
+        self.in_trait = old_trait;
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
@@ -314,8 +321,15 @@ impl<'a> AstValidator<'a> {
             None
         };
 
-        // FIXME(const_trait_impl): If the trait is not const and feature `const_trait_impl`
-        // is enabled, provide a structured suggestion to make the trait const.
+        let make_trait_const_sugg = if self.in_const_trait_or_impl.is_none()
+            && let Some(trait_) = self.in_trait
+            && self.features.const_trait_impl
+        {
+            Some(trait_.shrink_to_lo())
+        } else {
+            None
+        };
+
         self.dcx().emit_err(errors::TraitFnConst {
             span,
             in_impl: self.in_trait_impl.is_some(),
@@ -328,6 +342,7 @@ impl<'a> AstValidator<'a> {
                 },
             ),
             make_impl_const_sugg,
+            make_trait_const_sugg,
         });
     }
 
@@ -999,7 +1014,7 @@ 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);
@@ -1585,6 +1600,7 @@ pub fn check_crate(
         extern_mod: None,
         in_trait_impl: None,
         in_const_trait_or_impl: None,
+        in_trait: None,
         has_proc_macro_decls: false,
         outer_impl_trait: None,
         disallow_tilde_const: Some(DisallowTildeConstContext::Item),

compiler/rustc_ast_passes/src/errors.rs

@@ -60,6 +60,12 @@ pub struct TraitFnConst {
         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/rfcs/rfc-2632-const-trait-impl/effects/trait-fn-const.rs

@@ -14,4 +14,8 @@ impl Trait for u32 {
     const fn fun() {} //~ ERROR functions in trait impls cannot be declared const
 }
 
+trait NonConst {
+    const fn fun(); //~ ERROR functions in traits cannot be declared const
+}
+
 fn main() {}

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

@@ -37,6 +37,23 @@ help: and declare the impl to be const instead
 LL | impl const Trait for u32 {
    |      +++++
 
-error: aborting due to 3 previous errors
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/trait-fn-const.rs:18:5
+   |
+LL |     const fn fun();
+   |     ^^^^^ functions in traits cannot be const
+   |
+help: remove the `const`
+   |
+LL -     const fn fun();
+LL +     fn fun();
+   |
+help: and declare the trait to be a `#[const_trait]` instead
+   |
+LL + #[const_trait]
+LL | trait NonConst {
+   |
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0379`.