Rollup merge of #89390 - tmandry:issue-72117, r=estebank

Fix incorrect Box::pin suggestion

The suggestion checked if `Pin<Box<T>>` could be coeerced to the expected
type, but did not check predicates created by the coercion. We now
look for predicates that definitely cannot be satisfied before giving
the suggestion.

The suggestion is still marked MaybeIncorrect because we allow predicates that
are still ambiguous and can't be proven.

Fixes #72117.

6 files changed, 46 insertions, 16 deletions

diff --git a/src/test/ui/cross/cross-borrow-trait.stderr b/src/test/ui/cross/cross-borrow-trait.stderr
index f693a3149b2..81f309eae08 100644
--- a/src/test/ui/cross/cross-borrow-trait.stderr
+++ b/src/test/ui/cross/cross-borrow-trait.stderr
@@ -2,10 +2,8 @@ error[E0308]: mismatched types
   --> $DIR/cross-borrow-trait.rs:10:26
    |
 LL |     let _y: &dyn Trait = x;
-   |             ----------   ^
-   |             |            |
-   |             |            expected `&dyn Trait`, found struct `Box`
-   |             |            help: consider borrowing here: `&x`
+   |             ----------   ^ expected `&dyn Trait`, found struct `Box`
+   |             |
    |             expected due to this
    |
    = note: expected reference `&dyn Trait`
diff --git a/src/test/ui/dst/dst-bad-coercions.stderr b/src/test/ui/dst/dst-bad-coercions.stderr
index 3e23c5f5c74..01f862ed516 100644
--- a/src/test/ui/dst/dst-bad-coercions.stderr
+++ b/src/test/ui/dst/dst-bad-coercions.stderr
@@ -13,10 +13,8 @@ error[E0308]: mismatched types
   --> $DIR/dst-bad-coercions.rs:15:21
    |
 LL |     let y: &dyn T = x;
-   |            ------   ^
-   |            |        |
-   |            |        expected `&dyn T`, found *-ptr
-   |            |        help: consider borrowing here: `&x`
+   |            ------   ^ expected `&dyn T`, found *-ptr
+   |            |
    |            expected due to this
    |
    = note: expected reference `&dyn T`
@@ -37,10 +35,8 @@ error[E0308]: mismatched types
   --> $DIR/dst-bad-coercions.rs:20:21
    |
 LL |     let y: &dyn T = x;
-   |            ------   ^
-   |            |        |
-   |            |        expected `&dyn T`, found *-ptr
-   |            |        help: consider borrowing here: `&x`
+   |            ------   ^ expected `&dyn T`, found *-ptr
+   |            |
    |            expected due to this
    |
    = note: expected reference `&dyn T`
diff --git a/src/test/ui/suggestions/box-future-wrong-output.rs b/src/test/ui/suggestions/box-future-wrong-output.rs
new file mode 100644
index 00000000000..d49819fcb14
--- /dev/null
+++ b/src/test/ui/suggestions/box-future-wrong-output.rs
@@ -0,0 +1,22 @@
+// Issue #72117
+// edition:2018
+
+use core::future::Future;
+use core::pin::Pin;
+
+pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+
+impl<T: ?Sized> FutureExt for T where T: Future {}
+trait FutureExt: Future {
+    fn boxed<'a>(self) -> BoxFuture<'a, Self::Output>
+    where
+        Self: Sized + Send + 'a,
+    {
+        Box::pin(self)
+    }
+}
+
+fn main() {
+    let _: BoxFuture<'static, bool> = async {}.boxed();
+    //~^ ERROR: mismatched types
+}
diff --git a/src/test/ui/suggestions/box-future-wrong-output.stderr b/src/test/ui/suggestions/box-future-wrong-output.stderr
new file mode 100644
index 00000000000..e0c57af25b3
--- /dev/null
+++ b/src/test/ui/suggestions/box-future-wrong-output.stderr
@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/box-future-wrong-output.rs:20:39
+   |
+LL |     let _: BoxFuture<'static, bool> = async {}.boxed();
+   |            ------------------------   ^^^^^^^^^^^^^^^^ expected `bool`, found `()`
+   |            |
+   |            expected due to this
+   |
+   = note: expected struct `Pin<Box<(dyn Future<Output = bool> + Send + 'static)>>`
+              found struct `Pin<Box<dyn Future<Output = ()> + Send>>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs
index 5dee0f5dae0..89a36e89b0a 100644
--- a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs
+++ b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs
@@ -11,13 +11,13 @@ fn foo<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32>
     x //~ ERROR mismatched types
 }
 
-// This case is still subpar:
-// `Pin::new(x)`: store this in the heap by calling `Box::new`: `Box::new(x)`
-// Should suggest changing the code from `Pin::new` to `Box::pin`.
 fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
     Box::new(x) //~ ERROR mismatched types
 }
 
+// This case is still subpar:
+// `Pin::new(x)`: store this in the heap by calling `Box::new`: `Box::new(x)`
+// Should suggest changing the code from `Pin::new` to `Box::pin`.
 fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
     Pin::new(x) //~ ERROR mismatched types
     //~^ ERROR E0277
diff --git a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
index ff08178cb74..f0af37e0cbe 100644
--- a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
+++ b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
@@ -15,7 +15,7 @@ LL |     Box::pin(x)
    |     +++++++++ +
 
 error[E0308]: mismatched types
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:18:5
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:15:5
    |
 LL | fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
    |                                                         ----------------------- expected `Pin<Box<(dyn Future<Output = i32> + Send + 'static)>>` because of return type