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On type mismatch involving associated type, suggest constraint

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When an associated type is found when a specific type was expected, if
possible provide a structured suggestion constraining the associated
type in a bound.

```
error[E0271]: type mismatch resolving `<T as Foo>::Y == i32`
  --> $DIR/associated-types-multiple-types-one-trait.rs:13:5
   |
LL |     want_y(t);
   |     ^^^^^^ expected `i32`, found associated type
...
LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
   |                 ----- required by this bound in `want_y`
   |
   = note:         expected type `i32`
           found associated type `<T as Foo>::Y`
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[E0308]: mismatched types
  --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9
   |
LL |     qux(x.func())
   |         ^^^^^^^^ expected `usize`, found associated type
   |
   = note:         expected type `usize`
           found associated type `<impl Trait as Trait>::A`
help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
   |
LL | fn foo(x: impl Trait<A = usize>) {
   |                     ^^^^^^^^^^
```
This commit is contained in:
Esteban Küber 2020-04-13 13:36:35 -07:00
parent f05a524044
commit 3a795fba03
22 changed files with 469 additions and 66 deletions
Download patch file Download diff file Expand all files Collapse all files

12
src/test/ui/impl-trait/bound-normalization-fail.stderr
View file

@ -14,9 +14,11 @@ LL | fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
|
= note: expected type `()`
found associated type `<T as impl_trait::Trait>::Assoc`
= note: consider constraining the associated type `<T as impl_trait::Trait>::Assoc` to `()`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
= note: the return type of a function must have a statically known size
help: consider constraining the associated type `<T as impl_trait::Trait>::Assoc` to `()`
|
LL | fn foo_fail<T: Trait<Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
| ^^^^^^^^^^^^
error: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
--> $DIR/bound-normalization-fail.rs:43:41
@ -32,9 +34,11 @@ LL | fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
|
= note: expected type `()`
found associated type `<T as lifetimes::Trait<'static>>::Assoc`
= note: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
= note: the return type of a function must have a statically known size
help: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
|
LL | fn foo2_fail<'a, T: Trait<'a, Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
| ^^^^^^^^^^^^
error: aborting due to 3 previous errors; 1 warning emitted

2
src/test/ui/impl-trait/equality2.stderr
View file

@ -25,7 +25,7 @@ LL | let _: i32 = Leak::leak(hide(0_i32));
|
= note: expected type `i32`
found associated type `<impl Foo as Leak>::T`
= note: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
= help: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
error[E0308]: mismatched types

2
src/test/ui/impl-trait/universal-mismatched-type.stderr
View file

@ -10,8 +10,6 @@ LL | x
|
= note: expected struct `std::string::String`
found type parameter `impl Debug`
= 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