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

2 files changed, 36 insertions, 0 deletions

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`.