Suggest changing impl parameter types to match trait

This is particularly useful for cases where arbitrary self types are
used, like in custom `Future`s.

3 files changed, 57 insertions, 2 deletions

diff --git a/src/test/ui/compare-method/bad-self-type.rs b/src/test/ui/compare-method/bad-self-type.rs
new file mode 100644
index 00000000000..9eb978664bf
--- /dev/null
+++ b/src/test/ui/compare-method/bad-self-type.rs
@@ -0,0 +1,23 @@
+use std::future::Future;
+use std::task::{Context, Poll};
+
+fn main() {}
+
+struct MyFuture {}
+
+impl Future for MyFuture {
+    type Output = ();
+    fn poll(self, _: &mut Context<'_>) -> Poll<()> {
+    //~^ ERROR method `poll` has an incompatible type for trait
+        todo!()
+    }
+}
+
+trait T {
+    fn foo(self);
+}
+
+impl T for MyFuture {
+    fn foo(self: Box<Self>) {}
+    //~^ ERROR method `foo` has an incompatible type for trait
+}
diff --git a/src/test/ui/compare-method/bad-self-type.stderr b/src/test/ui/compare-method/bad-self-type.stderr
new file mode 100644
index 00000000000..4d85ff86df5
--- /dev/null
+++ b/src/test/ui/compare-method/bad-self-type.stderr
@@ -0,0 +1,30 @@
+error[E0053]: method `poll` has an incompatible type for trait
+  --> $DIR/bad-self-type.rs:10:13
+   |
+LL |     fn poll(self, _: &mut Context<'_>) -> Poll<()> {
+   |             ^^^^
+   |             |
+   |             expected struct `Pin`, found struct `MyFuture`
+   |             help: change the self-receiver type to match the trait: `self: Pin<&mut MyFuture>`
+   |
+   = note: expected fn pointer `fn(Pin<&mut MyFuture>, &mut Context<'_>) -> Poll<_>`
+              found fn pointer `fn(MyFuture, &mut Context<'_>) -> Poll<_>`
+
+error[E0053]: method `foo` has an incompatible type for trait
+  --> $DIR/bad-self-type.rs:21:18
+   |
+LL |     fn foo(self);
+   |            ---- type in trait
+...
+LL |     fn foo(self: Box<Self>) {}
+   |            ------^^^^^^^^^
+   |            |     |
+   |            |     expected struct `MyFuture`, found struct `Box`
+   |            help: change the self-receiver type to match the trait: `self`
+   |
+   = note: expected fn pointer `fn(MyFuture)`
+              found fn pointer `fn(Box<MyFuture>)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0053`.
diff --git a/src/test/ui/compare-method/reordered-type-param.stderr b/src/test/ui/compare-method/reordered-type-param.stderr
index f1f8a663f21..d581628ea48 100644
--- a/src/test/ui/compare-method/reordered-type-param.stderr
+++ b/src/test/ui/compare-method/reordered-type-param.stderr
@@ -5,8 +5,10 @@ LL |   fn b<C:Clone,D>(&self, x: C) -> C;
    |                             - type in trait
 ...
 LL |   fn b<F:Clone,G>(&self, _x: G) -> G { panic!() }
-   |        -       -             ^ expected type parameter `F`, found type parameter `G`
-   |        |       |
+   |        -       -             ^
+   |        |       |             |
+   |        |       |             expected type parameter `F`, found type parameter `G`
+   |        |       |             help: change the parameter type to match the trait: `F`
    |        |       found type parameter
    |        expected type parameter
    |