E0379: Suggest impl const in diagnostic

Author: fmease

compiler/rustc_ast_passes/messages.ftl

@@ -243,6 +243,7 @@ ast_passes_trait_fn_const =
     } cannot be 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
 
 ast_passes_trait_object_single_bound = only a single explicit lifetime bound is permitted
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -54,7 +54,9 @@ struct AstValidator<'a> {
     extern_mod: Option<&'a Item>,
 
     /// Are we inside a trait impl?
-    in_trait_impl: bool,
+    ///
+    /// If so, this is the trait ref together with its polarity.
+    in_trait_impl: Option<(ImplPolarity, &'a TraitRef)>,
 
     /// Are we inside a const trait defn or impl?
     ///
@@ -80,15 +82,17 @@ struct AstValidator<'a> {
 impl<'a> AstValidator<'a> {
     fn with_in_trait_impl(
         &mut self,
-        is_in: bool,
-        constness: Option<Const>,
+        trait_ref: Option<(Const, ImplPolarity, &'a TraitRef)>,
         f: impl FnOnce(&mut Self),
     ) {
-        let old = mem::replace(&mut self.in_trait_impl, is_in);
+        let old = mem::replace(
+            &mut self.in_trait_impl,
+            trait_ref.map(|(_, polarity, trait_ref)| (polarity, trait_ref)),
+        );
         let old_const = mem::replace(
             &mut self.in_const_trait_or_impl,
-            match constness {
-                Some(Const::Yes(span)) => Some(span),
+            match trait_ref {
+                Some((Const::Yes(span), ..)) => Some(span),
                 _ => None,
             },
         );
@@ -301,11 +305,20 @@ impl<'a> AstValidator<'a> {
             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.in_const_trait_or_impl.is_none()
+            && let Some((ImplPolarity::Positive, trait_ref)) = self.in_trait_impl
+            && self.features.const_trait_impl
+        {
+            Some(trait_ref.path.span.shrink_to_lo())
+        } else {
+            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.
         self.dcx().emit_err(errors::TraitFnConst {
             span,
-            in_impl: self.in_trait_impl,
+            in_impl: self.in_trait_impl.is_some(),
             const_context_label: self.in_const_trait_or_impl,
             remove_const_sugg: (
                 self.session.source_map().span_extend_while(span, |c| c == ' ').unwrap_or(span),
@@ -314,6 +327,7 @@ impl<'a> AstValidator<'a> {
                     None => rustc_errors::Applicability::MaybeIncorrect,
                 },
             ),
+            make_impl_const_sugg,
         });
     }
 
@@ -837,7 +851,7 @@ 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,
@@ -1376,7 +1390,7 @@ 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 ctxt == AssocCtxt::Trait || self.in_trait_impl.is_none() {
             self.check_defaultness(item.span, item.kind.defaultness());
         }
 
@@ -1424,7 +1438,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             );
         }
 
-        if ctxt == AssocCtxt::Trait || self.in_trait_impl {
+        if ctxt == AssocCtxt::Trait || self.in_trait_impl.is_some() {
             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);
@@ -1453,8 +1467,7 @@ 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)),
         }
     }
 }
@@ -1570,7 +1583,7 @@ pub fn check_crate(
         session,
         features,
         extern_mod: None,
-        in_trait_impl: false,
+        in_trait_impl: None,
         in_const_trait_or_impl: None,
         has_proc_macro_decls: false,
         outer_impl_trait: None,

compiler/rustc_ast_passes/src/errors.rs

@@ -54,6 +54,12 @@ pub struct TraitFnConst {
     pub const_context_label: 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>,
 }
 
 #[derive(Diagnostic)]

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

@@ -10,4 +10,8 @@ impl const Trait for () {
     const fn fun() {} //~ ERROR functions in trait impls cannot be declared const
 }
 
+impl Trait for u32 {
+    const fn fun() {} //~ ERROR functions in trait impls cannot be declared const
+}
+
 fn main() {}

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

@@ -21,6 +21,22 @@ LL |     const fn fun() {}
    |     functions in trait impls cannot be const
    |     help: remove the `const`
 
-error: aborting due to 2 previous errors
+error[E0379]: functions in trait impls cannot be declared const
+  --> $DIR/trait-fn-const.rs:14:5
+   |
+LL |     const fn fun() {}
+   |     ^^^^^ functions in trait impls cannot be const
+   |
+help: remove the `const`
+   |
+LL -     const fn fun() {}
+LL +     fn fun() {}
+   |
+help: and declare the impl to be const instead
+   |
+LL | impl const Trait for u32 {
+   |      +++++
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0379`.