Account for Pin::new(_) and Pin::new(Box::new(_)) when Box::pin(_) would be applicable

src/libcore/marker.rs

@@ -727,6 +727,13 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 /// [`Pin<P>`]: ../pin/struct.Pin.html
 /// [`pin module`]: ../../std/pin/index.html
 #[stable(feature = "pin", since = "1.33.0")]
+#[rustc_on_unimplemented(
+    on(
+        _Self = "dyn std::future::Future<Output = i32> + std::marker::Send",
+        note = "consider using `Box::pin`",
+    ),
+    message = "`{Self}` cannot be unpinned"
+)]
 #[lang = "unpin"]
 pub auto trait Unpin {}
 

src/test/ui/generator/static-not-unpin.rs

@@ -11,5 +11,5 @@ fn main() {
     let mut generator = static || {
         yield;
     };
-    assert_unpin(generator); //~ ERROR std::marker::Unpin` is not satisfied
+    assert_unpin(generator); //~ ERROR E0277
 }

src/test/ui/generator/static-not-unpin.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `[static generator@$DIR/static-not-unpin.rs:11:25: 13:6 _]: std::marker::Unpin` is not satisfied
+error[E0277]: `[static generator@$DIR/static-not-unpin.rs:11:25: 13:6 _]` cannot be unpinned
   --> $DIR/static-not-unpin.rs:14:18
    |
 LL | fn assert_unpin<T: Unpin>(_: T) {

src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs

@@ -10,17 +10,18 @@ fn foo<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32>
     // We could instead use an `async` block, but this way we have no std spans.
     x //~ ERROR mismatched types
 }
+
 fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
     Box::new(x) //~ ERROR mismatched types
-    //~^ HELP use `Box::pin`
 }
+
 fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
     Pin::new(x) //~ ERROR mismatched types
-    //~^ ERROR the trait bound
+    //~^ ERROR E0277
 }
+
 fn qux<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
-    Pin::new(Box::new(x)) //~ ERROR mismatched types
-    //~^ ERROR the trait bound
+    Pin::new(Box::new(x)) //~ ERROR E0277
 }
 
 fn main() {}

src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr

@@ -16,7 +16,7 @@ LL |     x
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error[E0308]: mismatched types
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:14:5
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:15:5
    |
 LL | fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
    |                                                         ----------------------- expected `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>` because of return type
@@ -28,7 +28,7 @@ LL |     Box::new(x)
    = help: use `Box::pin`
 
 error[E0308]: mismatched types
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:18:14
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:19:14
    |
 LL | fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
    |        - this type parameter
@@ -44,20 +44,22 @@ LL |     Pin::new(x)
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
    = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
 
-error[E0277]: the trait bound `dyn std::future::Future<Output = i32> + std::marker::Send: std::marker::Unpin` is not satisfied
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:18:5
+error[E0277]: `dyn std::future::Future<Output = i32> + std::marker::Send` cannot be unpinned
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:19:5
    |
 LL |     Pin::new(x)
    |     ^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `dyn std::future::Future<Output = i32> + std::marker::Send`
    |
+   = note: consider using `Box::pin`
    = note: required by `std::pin::Pin::<P>::new`
 
-error[E0277]: the trait bound `dyn std::future::Future<Output = i32> + std::marker::Send: std::marker::Unpin` is not satisfied
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:22:5
+error[E0277]: `dyn std::future::Future<Output = i32> + std::marker::Send` cannot be unpinned
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:24:5
    |
 LL |     Pin::new(Box::new(x))
    |     ^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `dyn std::future::Future<Output = i32> + std::marker::Send`
    |
+   = note: consider using `Box::pin`
    = note: required by `std::pin::Pin::<P>::new`
 
 error: aborting due to 5 previous errors