Account for associated types in Sized bound error

Author: estebank

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1935,8 +1935,58 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ObligationCauseCode::ImplicitSizedObligation(item_def_id, span) => {
                 let item_name = tcx.def_path_str(item_def_id);
                 let mut sp: MultiSpan = span.into();
-                sp.push_span_label(span, format!("required by this bound in `{}`", item_name));
-                err.span_note(sp, "type parameters have an implicit `Sized` obligation");
+                match self.tcx.hir().get_if_local(item_def_id) {
+                    Some(hir::Node::TraitItem(hir::TraitItem {
+                        kind: hir::TraitItemKind::Type(bounds, _),
+                        ident,
+                        ..
+                    })) => {
+                        sp.push_span_label(
+                            span,
+                            format!("required by associated type `{}`", item_name),
+                        );
+                        err.span_note(sp, "associated types have an implicit `Sized` obligation");
+
+                        let sized_trait = self.tcx.lang_items().sized_trait();
+                        if bounds.len() == 0 {
+                            err.span_suggestion_verbose(
+                                ident.span.shrink_to_hi(),
+                                "consider relaxing the `Sized` obligation",
+                                ": ?Sized".to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        } else if bounds.iter().all(|bound| {
+                            bound.trait_ref().and_then(|tr| tr.trait_def_id()) != sized_trait
+                        }) {
+                            err.span_suggestion_verbose(
+                                bounds.iter().last().unwrap().span().shrink_to_hi(),
+                                "consider relaxing the `Sized` obligation",
+                                " + ?Sized".to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                    Some(hir::Node::ImplItem(hir::ImplItem {
+                        kind: hir::ImplItemKind::TyAlias(_),
+                        ..
+                    })) => {
+                        let msg = "associated types on `impl` blocks for types, have an implicit \
+                            mandatory `Sized` obligation; associated types from `trait`s can be \
+                            relaxed to `?Sized`";
+                        sp.push_span_label(
+                            span,
+                            format!("required by associated type `{}`", item_name),
+                        );
+                        err.span_note(sp, msg);
+                    }
+                    _ => {
+                        sp.push_span_label(
+                            span,
+                            format!("required by this bound in `{}`", item_name),
+                        );
+                        err.span_note(sp, "type parameters have an implicit `Sized` obligation");
+                    }
+                }
             }
             ObligationCauseCode::BindingObligation(item_def_id, span) => {
                 let item_name = tcx.def_path_str(item_def_id);
@@ -1953,7 +2003,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     }
                 }
                 if span != DUMMY_SP {
-                        err.span_label(span, &msg);
+                    err.span_label(span, &msg);
                 } else {
                     err.note(&msg);
                 }

compiler/rustc_typeck/src/check/compare_method.rs

@@ -1239,11 +1239,16 @@ pub fn check_type_bounds<'tcx>(
 
         let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
         let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
-        let mk_cause = |span| {
+        let mk_cause = |bound, span| {
             ObligationCause::new(
                 impl_ty_span,
                 impl_ty_hir_id,
-                ObligationCauseCode::BindingObligation(trait_ty.def_id, span),
+                match bound {
+                    ty::PredicateKind::Trait(_, _, ty::ImplicitTraitPredicate::Yes) => {
+                        traits::ImplicitSizedObligation(trait_ty.def_id, span)
+                    }
+                    _ => ObligationCauseCode::BindingObligation(trait_ty.def_id, span),
+                },
             )
         };
 
@@ -1254,7 +1259,11 @@ pub fn check_type_bounds<'tcx>(
                 let concrete_ty_bound = bound.subst(tcx, rebased_substs);
                 debug!("check_type_bounds: concrete_ty_bound = {:?}", concrete_ty_bound);
 
-                traits::Obligation::new(mk_cause(span), param_env, concrete_ty_bound)
+                traits::Obligation::new(
+                    mk_cause(bound.kind().skip_binder(), span),
+                    param_env,
+                    concrete_ty_bound,
+                )
             })
             .collect();
         debug!("check_type_bounds: item_bounds={:?}", obligations);

src/test/ui/associated-types/issue-63593.stderr

@@ -2,15 +2,21 @@ error[E0277]: the size for values of type `Self` cannot be known at compilation
   --> $DIR/issue-63593.rs:9:17
    |
 LL |     type This = Self;
-   |     ------------^^^^-
-   |     |           |
-   |     |           doesn't have a size known at compile-time
-   |     required by this bound in `MyTrait::This`
+   |                 ^^^^ doesn't have a size known at compile-time
    |
+note: associated types have an implicit `Sized` obligation
+  --> $DIR/issue-63593.rs:9:5
+   |
+LL |     type This = Self;
+   |     ^^^^^^^^^^^^^^^^^ required by associated type `MyTrait::This`
 help: consider further restricting `Self`
    |
 LL | trait MyTrait: Sized {
    |              ^^^^^^^
+help: consider relaxing the `Sized` obligation
+   |
+LL |     type This: ?Sized = Self;
+   |              ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/generic-associated-types/issue-74816.stderr

@@ -15,15 +15,21 @@ error[E0277]: the size for values of type `Self` cannot be known at compilation
   --> $DIR/issue-74816.rs:10:31
    |
 LL |     type Associated: Trait1 = Self;
-   |     --------------------------^^^^-
-   |     |                         |
-   |     |                         doesn't have a size known at compile-time
-   |     required by this bound in `Trait2::Associated`
+   |                               ^^^^ doesn't have a size known at compile-time
    |
+note: associated types have an implicit `Sized` obligation
+  --> $DIR/issue-74816.rs:10:5
+   |
+LL |     type Associated: Trait1 = Self;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by associated type `Trait2::Associated`
 help: consider further restricting `Self`
    |
 LL | trait Trait2: Sized {
    |             ^^^^^^^
+help: consider relaxing the `Sized` obligation
+   |
+LL |     type Associated: Trait1 + ?Sized = Self;
+   |                             ^^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/traits/issue-65673.stderr

@@ -1,13 +1,19 @@
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
   --> $DIR/issue-65673.rs:9:16
    |
-LL |     type Ctx;
-   |     --------- required by this bound in `WithType::Ctx`
-...
 LL |     type Ctx = dyn Alias<T>;
    |                ^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Trait + 'static)`
+note: associated types have an implicit `Sized` obligation
+  --> $DIR/issue-65673.rs:4:5
+   |
+LL |     type Ctx;
+   |     ^^^^^^^^^ required by associated type `WithType::Ctx`
+help: consider relaxing the `Sized` obligation
+   |
+LL |     type Ctx: ?Sized;
+   |             ^^^^^^^^
 
 error: aborting due to previous error