Rollup merge of #71781 - estebank:box-pin-test, r=tmandry

Uncomment test code for failure to use `Box::pin`

Close #69083.

Author: RalfJung

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

@@ -11,19 +11,26 @@ fn foo<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32>
     x //~ ERROR mismatched types
 }
 
-// FIXME: uncomment these once this commit is in Beta and we can rely on `rustc_on_unimplemented`
-//        having filtering for `Self` being a trait.
-//
-// fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
-//     Box::new(x)
-// }
-//
-// fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
-//     Pin::new(x)
-// }
-//
-// fn qux<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
-//     Pin::new(Box::new(x))
-// }
+// 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
+}
+
+fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
+    Pin::new(x) //~ ERROR mismatched types
+    //~^ ERROR E0277
+}
+
+fn qux<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
+    Pin::new(Box::new(x)) //~ ERROR E0277
+}
+
+fn zap() -> BoxFuture<'static, i32> {
+    async { //~ ERROR mismatched types
+        42
+    }
+}
 
 fn main() {}

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

@@ -15,6 +15,78 @@ LL |     x
    = help: type parameters must be constrained to match other types
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
-error: aborting due to previous error
+error[E0308]: mismatched types
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:18:5
+   |
+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
+LL |     Box::new(x)
+   |     ^^^^^^^^^^^ expected struct `std::pin::Pin`, found struct `std::boxed::Box`
+   |
+   = note: expected struct `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>`
+              found struct `std::boxed::Box<F>`
+   = help: use `Box::pin`
+
+error[E0308]: mismatched types
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:22:14
+   |
+LL | fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
+   |        - this type parameter
+LL |     Pin::new(x)
+   |              ^
+   |              |
+   |              expected struct `std::boxed::Box`, found type parameter `F`
+   |              help: store this in the heap by calling `Box::new`: `Box::new(x)`
+   |
+   = note:      expected struct `std::boxed::Box<dyn std::future::Future<Output = i32> + std::marker::Send>`
+           found type parameter `F`
+   = help: type parameters must be constrained to match other types
+   = 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]: `dyn std::future::Future<Output = i32> + std::marker::Send` cannot be unpinned
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:22:5
+   |
+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]: `dyn std::future::Future<Output = i32> + std::marker::Send` cannot be unpinned
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:27:5
+   |
+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[E0308]: mismatched types
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:31:5
+   |
+LL |   fn zap() -> BoxFuture<'static, i32> {
+   |               ----------------------- expected `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>` because of return type
+LL | /     async {
+LL | |         42
+LL | |     }
+   | |_____^ expected struct `std::pin::Pin`, found opaque type
+   | 
+  ::: $SRC_DIR/libcore/future/mod.rs:LL:COL
+   |
+LL |   pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
+   |                                             ------------------------------- the found opaque type
+   |
+   = note:   expected struct `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>`
+           found opaque type `impl std::future::Future`
+help: you need to pin and box this expression
+   |
+LL |     Box::pin(async {
+LL |         42
+LL |     })
+   |
+
+error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.