Auto merge of #105888 - skyzh:skyzh/suggest-lifetime-closure, r=compiler-errors

suggest lifetime for closure parameter type when mismatch

This is a draft PR, will add test cases later and be ready for review.

This PR fixes https://github.com/rust-lang/rust/issues/105675 by adding a diagnostics suggestion. Also a partial fix to https://github.com/rust-lang/rust/issues/105528.

The following code will have a compile error now:

```
fn const_if_unit(input: bool) -> impl for<'a> FnOnce(&'a ()) -> usize {
    let x = |_| 1;
    x
}
```

Before this PR:

```
error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ one type is more general than the other
  |
  = note: expected trait `for<'a> FnOnce<(&'a (),)>`
             found trait `FnOnce<(&(),)>`
note: this closure does not fulfill the lifetime requirements
 --> src/lib.rs:2:13
  |
2 |     let x = |_| 1;
  |             ^^^

error: implementation of `FnOnce` is not general enough
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ implementation of `FnOnce` is not general enough
  |
  = note: closure with signature `fn(&'2 ()) -> usize` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rust-test` due to 2 previous errors
```

After this PR:

```
error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ one type is more general than the other
  |
  = note: expected trait `for<'a> FnOnce<(&'a (),)>`
             found trait `FnOnce<(&(),)>`
note: this closure does not fulfill the lifetime requirements
 --> src/lib.rs:2:13
  |
2 |     let x = |_| 1;
  |             ^^^
help: consider changing the type of the closure parameters
  |
2 |     let x = |_: &_| 1;
  |             ~~~~~~~

error: implementation of `FnOnce` is not general enough
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ implementation of `FnOnce` is not general enough
  |
  = note: closure with signature `fn(&'2 ()) -> usize` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rust-test` due to 2 previous errors
```

After applying the suggestion, it compiles. The suggestion might not always be correct as the generation procedure of that suggestion is quite simple...

6 files changed, 171 insertions, 1 deletions

diff --git a/tests/ui/lifetimes/issue-105675.rs b/tests/ui/lifetimes/issue-105675.rs
new file mode 100644
index 00000000000..58d8be8b65f
--- /dev/null
+++ b/tests/ui/lifetimes/issue-105675.rs
@@ -0,0 +1,14 @@
+fn thing(x: impl FnOnce(&u32, &u32, u32)) {}
+
+fn main() {
+    let f = | _ , y: &u32 , z | ();
+    thing(f);
+    //~^ ERROR mismatched types
+    //~^^ ERROR mismatched types
+    let f = | x, y: _  , z: u32 | ();
+    thing(f);
+    //~^ ERROR mismatched types
+    //~^^ ERROR mismatched types
+    //~^^^ ERROR implementation of `FnOnce` is not general enough
+    //~^^^^ ERROR implementation of `FnOnce` is not general enough
+}
diff --git a/tests/ui/lifetimes/issue-105675.stderr b/tests/ui/lifetimes/issue-105675.stderr
new file mode 100644
index 00000000000..66415f72bcb
--- /dev/null
+++ b/tests/ui/lifetimes/issue-105675.stderr
@@ -0,0 +1,109 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-105675.rs:5:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a, 'b> FnOnce<(&'a u32, &'b u32, u32)>`
+              found trait `for<'a> FnOnce<(&u32, &'a u32, u32)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-105675.rs:4:13
+   |
+LL |     let f = | _ , y: &u32 , z | ();
+   |             ^^^^^^^^^^^^^^^^^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/issue-105675.rs:1:18
+   |
+LL | fn thing(x: impl FnOnce(&u32, &u32, u32)) {}
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     let f = |_: &_, y: &u32, z| ();
+   |             ~~~~~~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-105675.rs:5:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a, 'b> FnOnce<(&'a u32, &'b u32, u32)>`
+              found trait `for<'a> FnOnce<(&u32, &'a u32, u32)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-105675.rs:4:13
+   |
+LL |     let f = | _ , y: &u32 , z | ();
+   |             ^^^^^^^^^^^^^^^^^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/issue-105675.rs:1:18
+   |
+LL | fn thing(x: impl FnOnce(&u32, &u32, u32)) {}
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/issue-105675.rs:9:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a, 'b> FnOnce<(&'a u32, &'b u32, u32)>`
+              found trait `FnOnce<(&u32, &u32, u32)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-105675.rs:8:13
+   |
+LL |     let f = | x, y: _  , z: u32 | ();
+   |             ^^^^^^^^^^^^^^^^^^^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/issue-105675.rs:1:18
+   |
+LL | fn thing(x: impl FnOnce(&u32, &u32, u32)) {}
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     let f = |x: &_, y: &_, z: u32| ();
+   |             ~~~~~~~~~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/issue-105675.rs:9:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a, 'b> FnOnce<(&'a u32, &'b u32, u32)>`
+              found trait `FnOnce<(&u32, &u32, u32)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-105675.rs:8:13
+   |
+LL |     let f = | x, y: _  , z: u32 | ();
+   |             ^^^^^^^^^^^^^^^^^^^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/issue-105675.rs:1:18
+   |
+LL | fn thing(x: impl FnOnce(&u32, &u32, u32)) {}
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     let f = |x: &_, y: &_, z: u32| ();
+   |             ~~~~~~~~~~~~~~~~~~~~~~
+
+error: implementation of `FnOnce` is not general enough
+  --> $DIR/issue-105675.rs:9:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 u32, &u32, u32)` must implement `FnOnce<(&'1 u32, &u32, u32)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 u32, &u32, u32)>`, for some specific lifetime `'2`
+
+error: implementation of `FnOnce` is not general enough
+  --> $DIR/issue-105675.rs:9:5
+   |
+LL |     thing(f);
+   |     ^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&u32, &'2 u32, u32)` must implement `FnOnce<(&u32, &'1 u32, u32)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&u32, &'2 u32, u32)>`, for some specific lifetime `'2`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/lifetimes/issue-79187-2.stderr b/tests/ui/lifetimes/issue-79187-2.stderr
index c5f654b37bf..75fd87b3fe9 100644
--- a/tests/ui/lifetimes/issue-79187-2.stderr
+++ b/tests/ui/lifetimes/issue-79187-2.stderr
@@ -43,6 +43,10 @@ note: the lifetime requirement is introduced here
    |
 LL | fn take_foo(_: impl Foo) {}
    |                     ^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     take_foo(|a: &_| a);
+   |              ~~~~~~~
 
 error[E0308]: mismatched types
   --> $DIR/issue-79187-2.rs:11:5
diff --git a/tests/ui/lifetimes/issue-79187.stderr b/tests/ui/lifetimes/issue-79187.stderr
index ee6e7b89d5f..209f2b7b739 100644
--- a/tests/ui/lifetimes/issue-79187.stderr
+++ b/tests/ui/lifetimes/issue-79187.stderr
@@ -16,6 +16,10 @@ note: the lifetime requirement is introduced here
    |
 LL | fn thing(x: impl FnOnce(&u32)) {}
    |                  ^^^^^^^^^^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     let f = |_: &_| ();
+   |             ~~~~~~~
 
 error: implementation of `FnOnce` is not general enough
   --> $DIR/issue-79187.rs:5:5
diff --git a/tests/ui/mismatched_types/closure-mismatch.rs b/tests/ui/mismatched_types/closure-mismatch.rs
index b0644e79611..4eb33497c39 100644
--- a/tests/ui/mismatched_types/closure-mismatch.rs
+++ b/tests/ui/mismatched_types/closure-mismatch.rs
@@ -8,4 +8,7 @@ fn main() {
     baz(|_| ());
     //~^ ERROR implementation of `FnOnce` is not general enough
     //~| ERROR mismatched types
+    baz(|x| ());
+    //~^ ERROR implementation of `FnOnce` is not general enough
+    //~| ERROR mismatched types
 }
diff --git a/tests/ui/mismatched_types/closure-mismatch.stderr b/tests/ui/mismatched_types/closure-mismatch.stderr
index a7ef8fa0892..c5b8270ba84 100644
--- a/tests/ui/mismatched_types/closure-mismatch.stderr
+++ b/tests/ui/mismatched_types/closure-mismatch.stderr
@@ -25,7 +25,43 @@ note: the lifetime requirement is introduced here
    |
 LL | fn baz<T: Foo>(_: T) {}
    |           ^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     baz(|_: &_| ());
+   |         ~~~~~~~
+
+error: implementation of `FnOnce` is not general enough
+  --> $DIR/closure-mismatch.rs:11:5
+   |
+LL |     baz(|x| ());
+   |     ^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 ())` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`
+
+error[E0308]: mismatched types
+  --> $DIR/closure-mismatch.rs:11:5
+   |
+LL |     baz(|x| ());
+   |     ^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a> Fn<(&'a (),)>`
+              found trait `Fn<(&(),)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/closure-mismatch.rs:11:9
+   |
+LL |     baz(|x| ());
+   |         ^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/closure-mismatch.rs:5:11
+   |
+LL | fn baz<T: Foo>(_: T) {}
+   |           ^^^
+help: consider specifying the type of the closure parameters
+   |
+LL |     baz(|x: &_| ());
+   |         ~~~~~~~
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.