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.

8 files changed, 85 insertions, 28 deletions

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 07e542b70b9..a87318ff34e 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -42,7 +42,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{Coercion, InferOk, InferResult};
-use rustc_infer::traits::Obligation;
+use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
@@ -146,6 +146,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
     }
 
+    #[instrument(skip(self))]
     fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
         // First, remove any resolved type variables (at the top level, at least):
         let a = self.shallow_resolve(a);
@@ -933,14 +934,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     /// Same as `try_coerce()`, but without side-effects.
+    ///
+    /// Returns false if the coercion creates any obligations that result in
+    /// errors.
     pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
         let source = self.resolve_vars_with_obligations(expr_ty);
-        debug!("coercion::can({:?} -> {:?})", source, target);
+        debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
 
         let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
         // We don't ever need two-phase here since we throw out the result of the coercion
         let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
-        self.probe(|_| coerce.coerce(source, target)).is_ok()
+        self.probe(|_| {
+            let ok = match coerce.coerce(source, target) {
+                Ok(ok) => ok,
+                _ => return false,
+            };
+            let mut fcx = traits::FulfillmentContext::new_in_snapshot();
+            fcx.register_predicate_obligations(self, ok.obligations);
+            fcx.select_where_possible(&self).is_ok()
+        })
     }
 
     /// Given a type and a target type, this function will calculate and return
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs b/compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs
index da8b863e2db..339c46616a5 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs
@@ -341,7 +341,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 for (sp, label) in spans_and_labels {
                     multi_span.push_span_label(sp, label);
                 }
-                err.span_note(multi_span, "closures can only be coerced to `fn` types if they do not capture any variables");
+                err.span_note(
+                    multi_span,
+                    "closures can only be coerced to `fn` types if they do not capture any variables"
+                );
             }
         }
     }
@@ -361,15 +364,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         }
         let pin_did = self.tcx.lang_items().pin_type();
+        // This guards the `unwrap` and `mk_box` below.
+        if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() {
+            return false;
+        }
         match expected.kind() {
-            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 false,
-            _ => {}
+            ty::Adt(def, _) if Some(def.did) == pin_did => (),
+            _ => return false,
         }
-        let boxed_found = self.tcx.mk_box(found);
-        let new_found = self.tcx.mk_lang_item(boxed_found, LangItem::Pin).unwrap();
-        if self.can_coerce(new_found, expected) {
+        let box_found = self.tcx.mk_box(found);
+        let pin_box_found = self.tcx.mk_lang_item(box_found, LangItem::Pin).unwrap();
+        let pin_found = self.tcx.mk_lang_item(found, LangItem::Pin).unwrap();
+        if self.can_coerce(pin_box_found, expected) {
+            debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
             match found.kind() {
                 ty::Adt(def, _) if def.is_box() => {
                     err.help("use `Box::pin`");
@@ -381,11 +388,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             (expr.span.shrink_to_lo(), "Box::pin(".to_string()),
                             (expr.span.shrink_to_hi(), ")".to_string()),
                         ],
-                        Applicability::MachineApplicable,
+                        Applicability::MaybeIncorrect,
                     );
                 }
             }
             true
+        } else if self.can_coerce(pin_found, expected) {
+            match found.kind() {
+                ty::Adt(def, _) if def.is_box() => {
+                    err.help("use `Box::pin`");
+                    true
+                }
+                _ => false,
+            }
         } else {
             false
         }
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