On type mismatch involving associated type, suggest constraint

src/librustc_infer/infer/error_reporting/mod.rs

@@ -1388,6 +1388,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         terr: &TypeError<'tcx>,
     ) {
         let span = cause.span(self.tcx);
+        debug!("note_type_err cause={:?} values={:?}, terr={:?}", cause, values, terr);
 
         // For some types of errors, expected-found does not make
         // sense, so just ignore the values we were given.
@@ -1599,11 +1600,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 self.tcx.hir().body_owner_def_id(hir::BodyId { hir_id: cause.body_id })
             });
         self.check_and_note_conflicting_crates(diag, terr);
-        self.tcx.note_and_explain_type_err(diag, terr, span, body_owner_def_id.to_def_id());
+        self.tcx.note_and_explain_type_err(diag, terr, cause, span, body_owner_def_id.to_def_id());
 
         // It reads better to have the error origin as the final
         // thing.
-        self.note_error_origin(diag, &cause, exp_found);
+        self.note_error_origin(diag, cause, exp_found);
     }
 
     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,

src/librustc_middle/traits/mod.rs

@@ -193,6 +193,9 @@ pub enum ObligationCauseCode<'tcx> {
 
     DerivedObligation(DerivedObligationCause<'tcx>),
 
+    /// Error derived when matching traits/impls; see ObligationCause for more details
+    CompareImplConstObligation,
+
     /// Error derived when matching traits/impls; see ObligationCause for more details
     CompareImplMethodObligation {
         item_name: ast::Name,

src/librustc_middle/traits/structural_impls.rs

@@ -164,6 +164,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 tcx.lift(cause).map(super::ImplDerivedObligation)
             }
             super::DerivedObligation(ref cause) => tcx.lift(cause).map(super::DerivedObligation),
+            super::CompareImplConstObligation => Some(super::CompareImplConstObligation),
             super::CompareImplMethodObligation {
                 item_name,
                 impl_item_def_id,

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -1738,6 +1738,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     predicate
                 ));
             }
+            ObligationCauseCode::CompareImplConstObligation => {
+                err.note(&format!(
+                    "the requirement `{}` appears on the associated impl constant \
+                     but not on the corresponding associated trait constant",
+                    predicate
+                ));
+            }
             ObligationCauseCode::ReturnType
             | ObligationCauseCode::ReturnValue(_)
             | ObligationCauseCode::BlockTailExpression(_) => (),

src/librustc_typeck/check/compare_method.rs

@@ -966,6 +966,7 @@ crate fn compare_const_impl<'tcx>(
         let impl_ty = tcx.type_of(impl_c.def_id);
         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
         let mut cause = ObligationCause::misc(impl_c_span, impl_c_hir_id);
+        cause.code = ObligationCauseCode::CompareImplConstObligation;
 
         // There is no "body" here, so just pass dummy id.
         let impl_ty =

src/test/ui/associated-const/associated-const-generic-obligations.stderr

@@ -9,8 +9,6 @@ LL |     const FROM: &'static str = "foo";
    |
    = note: expected associated type `<T as Foo>::Out`
                     found reference `&'static str`
-   = note: consider constraining the associated type `<T as Foo>::Out` to `&'static str` or calling a method that returns `<T as Foo>::Out`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-eq-3.stderr

@@ -8,8 +8,10 @@ LL |     let _: Bar = x.boo();
    |
    = note:       expected struct `Bar`
            found associated type `<I as Foo>::A`
-   = note: consider constraining the associated type `<I as Foo>::A` to `Bar`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<I as Foo>::A` to `Bar`
+   |
+LL | fn foo2<I: Foo<A = Bar>>(x: I) {
+   |               ^^^^^^^^^
 
 error[E0271]: type mismatch resolving `<isize as Foo>::A == Bar`
   --> $DIR/associated-types-eq-3.rs:38:5

src/test/ui/associated-types/associated-types-issue-20346.stderr

@@ -12,8 +12,6 @@ LL |     is_iterator_of::<Option<T>, _>(&adapter);
    |
    = note: expected enum `std::option::Option<T>`
               found type `T`
-   = help: type parameters must be constrained to match other types
-   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr

@@ -9,8 +9,10 @@ LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
    |
    = note:         expected type `i32`
            found associated type `<T as Foo>::Y`
-   = note: consider constraining the associated type `<T as Foo>::Y` to `i32`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<T as Foo>::Y` to `i32`
+   |
+LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T)
+   |                             ^^^^^^^^^
 
 error[E0271]: type mismatch resolving `<T as Foo>::X == u32`
   --> $DIR/associated-types-multiple-types-one-trait.rs:18:5
@@ -23,8 +25,10 @@ LL | fn want_x<T:Foo<X=u32>>(t: &T) { }
    |
    = note:         expected type `u32`
            found associated type `<T as Foo>::X`
-   = note: consider constraining the associated type `<T as Foo>::X` to `u32`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<T as Foo>::X` to `u32`
+   |
+LL | fn have_y_want_x<T:Foo<Y=i32, X = u32>>(t: &T)
+   |                             ^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/associated-types/defaults-in-other-trait-items.rs

@@ -3,15 +3,13 @@
 // Associated type defaults may not be assumed inside the trait defining them.
 // ie. they only resolve to `<Self as Tr>::A`, not the actual type `()`
 trait Tr {
-    type A = ();
+    type A = (); //~ NOTE associated type defaults can't be assumed inside the trait defining them
 
     fn f(p: Self::A) {
         let () = p;
         //~^ ERROR mismatched types
         //~| NOTE expected associated type, found `()`
         //~| NOTE expected associated type `<Self as Tr>::A`
-        //~| NOTE consider constraining the associated type
-        //~| NOTE for more information, visit
     }
 }
 
@@ -31,15 +29,13 @@ impl Tr for u8 {
 }
 
 trait AssocConst {
-    type Ty = u8;
+    type Ty = u8; //~ NOTE associated type defaults can't be assumed inside the trait defining them
 
     // Assoc. consts also cannot assume that default types hold
     const C: Self::Ty = 0u8;
     //~^ ERROR mismatched types
     //~| NOTE expected associated type, found `u8`
     //~| NOTE expected associated type `<Self as AssocConst>::Ty`
-    //~| NOTE consider constraining the associated type
-    //~| NOTE for more information, visit
 }
 
 // An impl can, however

src/test/ui/associated-types/defaults-in-other-trait-items.stderr

@@ -1,24 +1,26 @@
 error[E0308]: mismatched types
   --> $DIR/defaults-in-other-trait-items.rs:9:13
    |
+LL |     type A = ();
+   |     ------------ associated type defaults can't be assumed inside the trait defining them
+...
 LL |         let () = p;
    |             ^^ expected associated type, found `()`
    |
    = note: expected associated type `<Self as Tr>::A`
                     found unit type `()`
-   = note: consider constraining the associated type `<Self as Tr>::A` to `()` or calling a method that returns `<Self as Tr>::A`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
-  --> $DIR/defaults-in-other-trait-items.rs:37:25
+  --> $DIR/defaults-in-other-trait-items.rs:35:25
    |
+LL |     type Ty = u8;
+   |     ------------- associated type defaults can't be assumed inside the trait defining them
+...
 LL |     const C: Self::Ty = 0u8;
    |                         ^^^ expected associated type, found `u8`
    |
    = note: expected associated type `<Self as AssocConst>::Ty`
                          found type `u8`
-   = note: consider constraining the associated type `<Self as AssocConst>::Ty` to `u8` or calling a method that returns `<Self as AssocConst>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to 2 previous errors
 

src/test/ui/associated-types/defaults-specialization.stderr

@@ -9,35 +9,35 @@ LL |     fn make() -> u8 { 0 }
    |
    = note: expected fn pointer `fn() -> <A<T> as Tr>::Ty`
               found fn pointer `fn() -> u8`
-   = note: consider constraining the associated type `<A<T> as Tr>::Ty` to `u8` or calling a method that returns `<A<T> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0053]: method `make` has an incompatible type for trait
   --> $DIR/defaults-specialization.rs:34:18
    |
 LL |     fn make() -> Self::Ty {
    |                  -------- type in trait
 ...
+LL |     default type Ty = bool;
+   |     ----------------------- expected this associated type
+LL | 
 LL |     fn make() -> bool { true }
    |                  ^^^^ expected associated type, found `bool`
    |
    = note: expected fn pointer `fn() -> <B<T> as Tr>::Ty`
               found fn pointer `fn() -> bool`
-   = note: consider constraining the associated type `<B<T> as Tr>::Ty` to `bool` or calling a method that returns `<B<T> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:9:9
    |
+LL |     type Ty = u8;
+   |     ------------- associated type defaults can't be assumed inside the trait defining them
+LL | 
 LL |     fn make() -> Self::Ty {
    |                  -------- expected `<Self as Tr>::Ty` because of return type
 LL |         0u8
    |         ^^^ expected associated type, found `u8`
    |
    = note: expected associated type `<Self as Tr>::Ty`
                          found type `u8`
-   = note: consider constraining the associated type `<Self as Tr>::Ty` to `u8` or calling a method that returns `<Self as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:25:29
@@ -49,21 +49,22 @@ LL |     fn make() -> Self::Ty { 0u8 }
    |
    = note: expected associated type `<A2<T> as Tr>::Ty`
                          found type `u8`
-   = note: consider constraining the associated type `<A2<T> as Tr>::Ty` to `u8` or calling a method that returns `<A2<T> as Tr>::Ty`
+   = help: consider constraining the associated type `<A2<T> as Tr>::Ty` to `u8` or calling a method that returns `<A2<T> as Tr>::Ty`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:43:29
    |
+LL |     default type Ty = bool;
+   |     ----------------------- expected this associated type
+LL | 
 LL |     fn make() -> Self::Ty { true }
    |                  --------   ^^^^ expected associated type, found `bool`
    |                  |
    |                  expected `<B2<T> as Tr>::Ty` because of return type
    |
    = note: expected associated type `<B2<T> as Tr>::Ty`
                          found type `bool`
-   = note: consider constraining the associated type `<B2<T> as Tr>::Ty` to `bool` or calling a method that returns `<B2<T> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:86:32
@@ -75,8 +76,11 @@ LL |     let _: <B<()> as Tr>::Ty = 0u8;
    |
    = note: expected associated type `<B<()> as Tr>::Ty`
                          found type `u8`
-   = note: consider constraining the associated type `<B<()> as Tr>::Ty` to `u8` or calling a method that returns `<B<()> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: a method is available that returns `<B<()> as Tr>::Ty`
+  --> $DIR/defaults-specialization.rs:8:5
+   |
+LL |     fn make() -> Self::Ty {
+   |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:87:32
@@ -88,8 +92,11 @@ LL |     let _: <B<()> as Tr>::Ty = true;
    |
    = note: expected associated type `<B<()> as Tr>::Ty`
                          found type `bool`
-   = note: consider constraining the associated type `<B<()> as Tr>::Ty` to `bool` or calling a method that returns `<B<()> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: a method is available that returns `<B<()> as Tr>::Ty`
+  --> $DIR/defaults-specialization.rs:8:5
+   |
+LL |     fn make() -> Self::Ty {
+   |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:88:33
@@ -101,8 +108,11 @@ LL |     let _: <B2<()> as Tr>::Ty = 0u8;
    |
    = note: expected associated type `<B2<()> as Tr>::Ty`
                          found type `u8`
-   = note: consider constraining the associated type `<B2<()> as Tr>::Ty` to `u8` or calling a method that returns `<B2<()> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: a method is available that returns `<B2<()> as Tr>::Ty`
+  --> $DIR/defaults-specialization.rs:8:5
+   |
+LL |     fn make() -> Self::Ty {
+   |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error[E0308]: mismatched types
   --> $DIR/defaults-specialization.rs:89:33
@@ -114,8 +124,11 @@ LL |     let _: <B2<()> as Tr>::Ty = true;
    |
    = note: expected associated type `<B2<()> as Tr>::Ty`
                          found type `bool`
-   = note: consider constraining the associated type `<B2<()> as Tr>::Ty` to `bool` or calling a method that returns `<B2<()> as Tr>::Ty`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: a method is available that returns `<B2<()> as Tr>::Ty`
+  --> $DIR/defaults-specialization.rs:8:5
+   |
+LL |     fn make() -> Self::Ty {
+   |     ^^^^^^^^^^^^^^^^^^^^^ consider calling `Tr::make`
 
 error: aborting due to 9 previous errors
 

src/test/ui/associated-types/impl-trait-return-missing-constraint.rs

@@ -0,0 +1,32 @@
+trait Foo {
+    type Item;
+}
+
+trait Bar: Foo {}
+
+struct S;
+
+impl Foo for S {
+    type Item = i32;
+}
+impl Bar for S {}
+
+struct T;
+
+impl Foo for T {
+    type Item = u32;
+}
+impl Bar for T {}
+
+fn bar() -> impl Bar {
+    T
+}
+
+fn baz() -> impl Bar<Item = i32> {
+//~^ ERROR type mismatch resolving `<impl Bar as Foo>::Item == i32`
+    bar()
+}
+
+fn main() {
+    let _ = baz();
+}

src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr

@@ -0,0 +1,20 @@
+error[E0271]: type mismatch resolving `<impl Bar as Foo>::Item == i32`
+  --> $DIR/impl-trait-return-missing-constraint.rs:25:13
+   |
+LL | fn bar() -> impl Bar {
+   |             -------- the expected opaque type
+...
+LL | fn baz() -> impl Bar<Item = i32> {
+   |             ^^^^^^^^^^^^^^^^^^^^ expected associated type, found `i32`
+   |
+   = note: expected associated type `<impl Bar as Foo>::Item`
+                         found type `i32`
+   = note: the return type of a function must have a statically known size
+help: consider constraining the associated type `<impl Bar as Foo>::Item` to `i32`
+   |
+LL | fn bar() -> impl Bar<Item = i32> {
+   |                     ^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.

src/test/ui/associated-types/issue-26681.stderr

@@ -6,7 +6,7 @@ LL |     const C: <Self::Fv as Foo>::Bar = 6665;
    |
    = note: expected associated type `<<Self as Baz>::Fv as Foo>::Bar`
                          found type `{integer}`
-   = note: consider constraining the associated type `<<Self as Baz>::Fv as Foo>::Bar` to `{integer}` or calling a method that returns `<<Self as Baz>::Fv as Foo>::Bar`
+   = help: consider constraining the associated type `<<Self as Baz>::Fv as Foo>::Bar` to `{integer}`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to previous error