Point out if a local trait has no implementations

Author: compiler-errors

compiler/rustc_middle/src/ty/trait_def.rs

@@ -90,6 +90,10 @@ pub struct TraitImpls {
 }
 
 impl TraitImpls {
+    pub fn is_empty(&self) -> bool {
+        self.blanket_impls.is_empty() && self.non_blanket_impls.is_empty()
+    }
+
     pub fn blanket_impls(&self) -> &[DefId] {
         self.blanket_impls.as_slice()
     }

compiler/rustc_trait_selection/messages.ftl

@@ -40,5 +40,7 @@ trait_selection_no_value_in_rustc_on_unimplemented = this attribute must have a
     .label = expected value here
     .note = eg `#[rustc_on_unimplemented(message="foo")]`
 
+trait_selection_trait_has_no_impls = this trait has no implementations, consider adding one
+
 trait_selection_ty_alias_overflow = in case this is a recursive type alias, consider using a struct, enum, or union instead
 trait_selection_unable_to_construct_constant_value = unable to construct a constant value for the unevaluated constant {$unevaluated}

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

@@ -3009,10 +3009,10 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         // Try to report a help message
         if is_fn_trait
             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
-            obligation.param_env,
-            trait_ref.self_ty(),
-            trait_predicate.skip_binder().polarity,
-        )
+                obligation.param_env,
+                trait_ref.self_ty(),
+                trait_predicate.skip_binder().polarity,
+            )
         {
             self.add_help_message_for_fn_trait(trait_ref, err, implemented_kind, params);
         } else if !trait_ref.has_non_region_infer()
@@ -3031,6 +3031,15 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 None,
                 obligation.cause.body_id,
             );
+        } else if trait_ref.def_id().is_local()
+            && self.tcx.trait_impls_of(trait_ref.def_id()).is_empty()
+            && !self.tcx.trait_is_auto(trait_ref.def_id())
+            && !self.tcx.trait_is_alias(trait_ref.def_id())
+        {
+            err.span_help(
+                self.tcx.def_span(trait_ref.def_id()),
+                crate::fluent_generated::trait_selection_trait_has_no_impls,
+            );
         } else if !suggested && !unsatisfied_const {
             // Can't show anything else useful, try to find similar impls.
             let impl_candidates = self.find_similar_impl_candidates(*trait_predicate);
@@ -3041,7 +3050,12 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 err,
                 true,
             ) {
-                self.report_similar_impl_candidates_for_root_obligation(&obligation, *trait_predicate, body_def_id, err);
+                self.report_similar_impl_candidates_for_root_obligation(
+                    &obligation,
+                    *trait_predicate,
+                    body_def_id,
+                    err,
+                );
             }
 
             self.suggest_convert_to_slice(

tests/ui/associated-consts/associated-const-array-len.stderr

@@ -3,6 +3,12 @@ error[E0277]: the trait bound `i32: Foo` is not satisfied
    |
 LL | const X: [i32; <i32 as Foo>::ID] = [0, 1, 2];
    |                 ^^^ the trait `Foo` is not implemented for `i32`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/associated-const-array-len.rs:1:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^
 
 error: aborting due to previous error
 

tests/ui/associated-consts/issue-105330.stderr

@@ -51,6 +51,11 @@ error[E0277]: the trait bound `Demo: TraitWAssocConst` is not satisfied
 LL |     foo::<Demo>()();
    |           ^^^^ the trait `TraitWAssocConst` is not implemented for `Demo`
    |
+help: this trait has no implementations, consider adding one
+  --> $DIR/issue-105330.rs:1:1
+   |
+LL | pub trait TraitWAssocConst {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `foo`
   --> $DIR/issue-105330.rs:11:11
    |
@@ -87,6 +92,11 @@ error[E0277]: the trait bound `Demo: TraitWAssocConst` is not satisfied
 LL |     foo::<Demo>();
    |           ^^^^ the trait `TraitWAssocConst` is not implemented for `Demo`
    |
+help: this trait has no implementations, consider adding one
+  --> $DIR/issue-105330.rs:1:1
+   |
+LL | pub trait TraitWAssocConst {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `foo`
   --> $DIR/issue-105330.rs:11:11
    |

tests/ui/associated-types/associated-types-ICE-when-projecting-out-of-err.stderr

@@ -3,6 +3,12 @@ error[E0277]: the trait bound `(): Add<A>` is not satisfied
    |
 LL |     r = r + a;
    |           ^ the trait `Add<A>` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/associated-types-ICE-when-projecting-out-of-err.rs:15:1
+   |
+LL | trait Add<RHS=Self> {
+   | ^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

tests/ui/associated-types/associated-types-no-suitable-supertrait.stderr

@@ -3,6 +3,12 @@ error[E0277]: the trait bound `(T, U): Get` is not satisfied
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <(T, U) as Get>::Value) {}
    |                                        ^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `(T, U)`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/associated-types-no-suitable-supertrait.rs:12:1
+   |
+LL | trait Get {
+   | ^^^^^^^^^
 
 error[E0277]: the trait bound `Self: Get` is not satisfied
   --> $DIR/associated-types-no-suitable-supertrait.rs:17:40

tests/ui/associated-types/defaults-suitability.stderr

@@ -58,6 +58,11 @@ error[E0277]: the trait bound `(): Foo<Self>` is not satisfied
 LL |     type Assoc: Foo<Self> = ();
    |                             ^^ the trait `Foo<Self>` is not implemented for `()`
    |
+help: this trait has no implementations, consider adding one
+  --> $DIR/defaults-suitability.rs:27:1
+   |
+LL | trait Foo<T> {
+   | ^^^^^^^^^^^^
 note: required by a bound in `Bar::Assoc`
   --> $DIR/defaults-suitability.rs:34:17
    |

tests/ui/associated-types/issue-59324.stderr

@@ -48,6 +48,12 @@ error[E0277]: the trait bound `(): Foo` is not satisfied
    |
 LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
    |                             ^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/issue-59324.rs:3:1
+   |
+LL | pub trait Foo: NotFoo {
+   | ^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `Bug: Foo` is not satisfied
   --> $DIR/issue-59324.rs:19:10

tests/ui/associated-types/issue-64855.stderr

@@ -3,6 +3,12 @@ error[E0277]: the trait bound `Bar<T>: Foo` is not satisfied
    |
 LL | pub struct Bar<T>(<Self as Foo>::Type) where Self: ;
    |                   ^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bar<T>`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/issue-64855.rs:1:1
+   |
+LL | pub trait Foo {
+   | ^^^^^^^^^^^^^
 
 error: aborting due to previous error