Account for Box::new(impl Future) and emit help use Box::pin

Author: estebank

src/librustc_typeck/check/demand.rs

@@ -24,8 +24,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.annotate_expected_due_to_let_ty(err, expr);
         self.suggest_compatible_variants(err, expr, expected, expr_ty);
         self.suggest_ref_or_into(err, expr, expected, expr_ty);
+        if self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty) {
+            return;
+        }
         self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty);
-        self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty);
         self.suggest_missing_await(err, expr, expected, expr_ty);
     }
 

src/librustc_typeck/check/mod.rs

@@ -5053,18 +5053,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &hir::Expr<'_>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
-    ) {
+    ) -> bool {
         // Handle #68197.
 
         if self.tcx.hir().is_const_context(expr.hir_id) {
             // Do not suggest `Box::new` in const context.
-            return;
+            return false;
         }
         let pin_did = self.tcx.lang_items().pin_type();
         match expected.kind {
-            ty::Adt(def, _) if Some(def.did) != pin_did => return,
+            ty::Adt(def, _) if Some(def.did) != pin_did => return false,
             // This guards the `unwrap` and `mk_box` below.
-            _ if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() => return,
+            _ if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() => return false,
             _ => {}
         }
         let boxed_found = self.tcx.mk_box(found);
@@ -5073,12 +5073,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.can_coerce(new_found, expected),
             self.sess().source_map().span_to_snippet(expr.span),
         ) {
-            err.span_suggestion(
-                expr.span,
-                "you need to pin and box this expression",
-                format!("Box::pin({})", snippet),
-                Applicability::MachineApplicable,
-            );
+            match found.kind {
+                ty::Adt(def, _) if def.is_box() => {
+                    err.help("use `Box::pin`");
+                }
+                _ => {
+                    err.span_suggestion(
+                        expr.span,
+                        "you need to pin and box this expression",
+                        format!("Box::pin({})", snippet),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+            true
+        } else {
+            false
         }
     }
 

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

@@ -1,5 +1,4 @@
 // edition:2018
-// run-rustfix
 #![allow(dead_code)]
 use std::future::Future;
 use std::pin::Pin;
@@ -11,5 +10,17 @@ 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
+}
+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
+}
 
 fn main() {}

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

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/expected-boxed-future-isnt-pinned.rs:12:5
+  --> $DIR/expected-boxed-future-isnt-pinned.rs:11:5
    |
 LL | fn foo<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
    |        - this type parameter                            ----------------------- expected `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>` because of return type
@@ -15,6 +15,52 @@ 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:14:5
+   |
+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:18:14
+   |
+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]: 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
+   |
+LL |     Pin::new(x)
+   |     ^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `dyn std::future::Future<Output = i32> + std::marker::Send`
+   |
+   = 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
+   |
+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: required by `std::pin::Pin::<P>::new`
+
+error: aborting due to 5 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`.