Rollup merge of #118146 - compiler-errors:deref-into-dyn-regions, r=lcnr

Rework supertrait lint once again

I accidentally pushed the wrong commits because I totally didn't check I was on the right computer when updating #118026.
Sorry, this should address all the nits in #118026.

r? lcnr

tests/ui/traits/trait-upcasting/deref-lint-regions.stderr

@@ -2,7 +2,7 @@ warning: this `Deref` implementation is covered by an implicit supertrait coerci
   --> $DIR/deref-lint-regions.rs:8:1
    |
 LL | impl<'a> Deref for dyn Foo<'a> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Foo<'_>` implements `Deref<Target = dyn Bar<'_>>` which conflicts with supertrait `Bar<'_>`
 LL |
 LL |     type Target = dyn Bar<'a>;
    |     -------------------------- target type is a supertrait of `dyn Foo<'_>`

tests/ui/traits/trait-upcasting/deref-lint.rs

@@ -8,6 +8,7 @@ trait B: A {}
 
 impl<'a> Deref for dyn 'a + B {
     //~^ WARN this `Deref` implementation is covered by an implicit supertrait coercion
+
     type Target = dyn A;
     fn deref(&self) -> &Self::Target {
         todo!()

tests/ui/traits/trait-upcasting/deref-lint.stderr

@@ -2,8 +2,8 @@ warning: this `Deref` implementation is covered by an implicit supertrait coerci
   --> $DIR/deref-lint.rs:9:1
    |
 LL | impl<'a> Deref for dyn 'a + B {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-LL |
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn B` implements `Deref<Target = dyn A>` which conflicts with supertrait `A`
+...
 LL |     type Target = dyn A;
    |     -------------------- target type is a supertrait of `dyn B`
    |

tests/ui/traits/trait-upcasting/deref-upcast-behavioral-change.rs

@@ -0,0 +1,34 @@
+#![deny(deref_into_dyn_supertrait)]
+use std::ops::Deref;
+
+trait Bar<T> {}
+impl<T, U> Bar<U> for T {}
+
+trait Foo: Bar<i32> {
+    fn as_dyn_bar_u32<'a>(&self) -> &(dyn Bar<u32> + 'a);
+}
+
+impl Foo for () {
+    fn as_dyn_bar_u32<'a>(&self) -> &(dyn Bar<u32> + 'a) {
+        self
+    }
+}
+
+impl<'a> Deref for dyn Foo + 'a {
+    type Target = dyn Bar<u32> + 'a;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_dyn_bar_u32()
+    }
+}
+
+fn take_dyn<T>(x: &dyn Bar<T>) -> T {
+    todo!()
+}
+
+fn main() {
+    let x: &dyn Foo = &();
+    let y = take_dyn(x);
+    let z: u32 = y;
+    //~^ ERROR mismatched types
+}

tests/ui/traits/trait-upcasting/deref-upcast-behavioral-change.stderr

@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/deref-upcast-behavioral-change.rs:32:18
+   |
+LL |     let z: u32 = y;
+   |            ---   ^ expected `u32`, found `i32`
+   |            |
+   |            expected due to this
+   |
+help: you can convert an `i32` to a `u32` and panic if the converted value doesn't fit
+   |
+LL |     let z: u32 = y.try_into().unwrap();
+   |                   ++++++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/trait-upcasting/migrate-lint-different-substs.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+use std::ops::Deref;
+
+trait Bar<T> {}
+
+trait Foo: Bar<i32> {
+    fn as_dyn_bar_u32<'a>(&self) -> &(dyn Bar<u32> + 'a);
+}
+
+impl<'a> Deref for dyn Foo + 'a {
+    //~^ WARN this `Deref` implementation is covered by an implicit supertrait coercion
+    type Target = dyn Bar<u32> + 'a;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_dyn_bar_u32()
+    }
+}
+
+fn main() {}

tests/ui/traits/trait-upcasting/migrate-lint-different-substs.stderr

@@ -0,0 +1,14 @@
+warning: this `Deref` implementation is covered by an implicit supertrait coercion
+  --> $DIR/migrate-lint-different-substs.rs:11:1
+   |
+LL | impl<'a> Deref for dyn Foo + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Foo` implements `Deref<Target = dyn Bar<u32>>` which conflicts with supertrait `Bar<i32>`
+LL |
+LL |     type Target = dyn Bar<u32> + 'a;
+   |     -------------------------------- target type is a supertrait of `dyn Foo`
+   |
+   = help: consider removing this implementation or replacing it with a method instead
+   = note: `#[warn(deref_into_dyn_supertrait)]` on by default
+
+warning: 1 warning emitted
+