Auto merge of #71108 - estebank:suggest-proj-type-mismatch-constraint, r=oli-obk

On type mismatch involving associated type, suggest constraint

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>) {
   |                     ^^^^^^^^^^
```

Fix #71035. Related to #70908.

4 files changed, 4 insertions, 22 deletions

diff --git a/src/test/ui/issues/issue-13853.stderr b/src/test/ui/issues/issue-13853.stderr
index 2f31636f8ad..3f1b955dddb 100644
--- a/src/test/ui/issues/issue-13853.stderr
+++ b/src/test/ui/issues/issue-13853.stderr
@@ -9,8 +9,6 @@ LL |         self.iter()
    |
    = note: expected type parameter `I`
                       found struct `std::slice::Iter<'_, N>`
-   = 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[E0599]: no method named `iter` found for reference `&G` in the current scope
   --> $DIR/issue-13853.rs:27:23
diff --git a/src/test/ui/issues/issue-20225.stderr b/src/test/ui/issues/issue-20225.stderr
index 1c5911e05f7..3bcc50ded84 100644
--- a/src/test/ui/issues/issue-20225.stderr
+++ b/src/test/ui/issues/issue-20225.stderr
@@ -8,8 +8,6 @@ LL |   extern "rust-call" fn call(&self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(&Foo, (&'a T,))`
               found fn pointer `extern "rust-call" fn(&Foo, (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[E0053]: method `call_mut` has an incompatible type for trait
   --> $DIR/issue-20225.rs:11:3
@@ -21,8 +19,6 @@ LL |   extern "rust-call" fn call_mut(&mut self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(&mut Foo, (&'a T,))`
               found fn pointer `extern "rust-call" fn(&mut Foo, (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[E0053]: method `call_once` has an incompatible type for trait
   --> $DIR/issue-20225.rs:18:3
@@ -35,8 +31,6 @@ LL |   extern "rust-call" fn call_once(self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(Foo, (&'a T,))`
               found fn pointer `extern "rust-call" fn(Foo, (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 3 previous errors
 
diff --git a/src/test/ui/issues/issue-32323.stderr b/src/test/ui/issues/issue-32323.stderr
index 7c0928b1924..369f56b9869 100644
--- a/src/test/ui/issues/issue-32323.stderr
+++ b/src/test/ui/issues/issue-32323.stderr
@@ -8,8 +8,10 @@ LL | pub fn f<'a, T: Tr<'a>>() -> <T as Tr<'a>>::Out {}
    |
    = note: expected associated type `<T as Tr<'a>>::Out`
                     found unit type `()`
-   = note: consider constraining the associated type `<T as Tr<'a>>::Out` to `()` or calling a method that returns `<T as Tr<'a>>::Out`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<T as Tr<'a>>::Out` to `()`
+   |
+LL | pub fn f<'a, T: Tr<'a, Out = ()>>() -> <T as Tr<'a>>::Out {}
+   |                      ^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-69306.stderr b/src/test/ui/issues/issue-69306.stderr
index a2a42739ca8..58e85ec700d 100644
--- a/src/test/ui/issues/issue-69306.stderr
+++ b/src/test/ui/issues/issue-69306.stderr
@@ -8,8 +8,6 @@ LL |     const C: S0<u8> = Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:5:23
@@ -21,8 +19,6 @@ LL |     const C: S0<u8> = Self(0);
    |
    = note: expected struct `S0<u8>`
               found struct `S0<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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:10:14
@@ -35,8 +31,6 @@ LL |         Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:27:14
@@ -49,8 +43,6 @@ LL |         Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:33:32
@@ -62,8 +54,6 @@ LL |     const C: S1<u8, u8> = Self(0, 1);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:33:27
@@ -75,8 +65,6 @@ LL |     const C: S1<u8, u8> = Self(0, 1);
    |
    = note: expected struct `S1<u8, _>`
               found struct `S1<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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:41:14