Avoid side-effects from try_coerce when suggesting borrowing LHS of cast

compiler/rustc_hir_typeck/src/cast.rs

@@ -389,34 +389,26 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 if let ty::Ref(reg, cast_ty, mutbl) = *self.cast_ty.kind() {
                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind()
                         && fcx
-                            .try_coerce(
+                            .can_coerce(
-                                self.expr,
                                 Ty::new_ref(fcx.tcx,
                                     fcx.tcx.lifetimes.re_erased,
                                     TypeAndMut { ty: expr_ty, mutbl },
                                 ),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg = Some((format!("&{}*", mutbl.prefix_str()), cast_ty == expr_ty));
                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind()
                         && expr_mutbl == Mutability::Not
                         && mutbl == Mutability::Mut
                         && fcx
-                            .try_coerce(
+                            .can_coerce(
-                                self.expr,
                                 Ty::new_ref(fcx.tcx,
                                     expr_reg,
                                     TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
                                 ),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg_mutref = true;
                     }
@@ -424,30 +416,22 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     if !sugg_mutref
                         && sugg == None
                         && fcx
-                            .try_coerce(
+                            .can_coerce(
-                                self.expr,
                                 Ty::new_ref(fcx.tcx,reg, TypeAndMut { ty: self.expr_ty, mutbl }),
                                 self.cast_ty,
-                                AllowTwoPhase::No,
-                                None,
                             )
-                            .is_ok()
                     {
                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
                     }
                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind()
                     && fcx
-                        .try_coerce(
+                        .can_coerce(
-                            self.expr,
                             Ty::new_ref(fcx.tcx,
                                 fcx.tcx.lifetimes.re_erased,
                                 TypeAndMut { ty: self.expr_ty, mutbl },
                             ),
                             self.cast_ty,
-                            AllowTwoPhase::No,
-                            None,
                         )
-                        .is_ok()
                 {
                     sugg = Some((format!("&{}", mutbl.prefix_str()), false));
                 }

tests/ui/traits/trait-upcasting/type-checking-test-1.current.stderr

@@ -2,22 +2,8 @@ error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<_>`
   --> $DIR/type-checking-test-1.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
-   |             ^^^^^^^^^^^^^^^^ invalid cast
+   |             ^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<_>; // Ambiguous
-   |             +
 
-error[E0277]: the trait bound `&dyn Foo: Bar<_>` is not satisfied
+error: aborting due to previous error
-  --> $DIR/type-checking-test-1.rs:19:13
-   |
-LL |     let _ = x as &dyn Bar<_>; // Ambiguous
-   |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo`
-   |
-   = note: required for the cast from `&&dyn Foo` to `&dyn Bar<_>`
 
-error: aborting due to 2 previous errors
+For more information about this error, try `rustc --explain E0605`.
-
-Some errors have detailed explanations: E0277, E0605.
-For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/trait-upcasting/type-checking-test-1.rs

@@ -18,7 +18,6 @@ fn test_specific(x: &dyn Foo) {
 fn test_unknown_version(x: &dyn Foo) {
     let _ = x as &dyn Bar<_>; // Ambiguous
                               //~^ ERROR non-primitive cast
-                              //[current]~^^ ERROR the trait bound `&dyn Foo: Bar<_>` is not satisfied
 }
 
 fn test_infer_version(x: &dyn Foo) {

tests/ui/traits/trait-upcasting/type-checking-test-2.rs

@@ -18,13 +18,11 @@ fn test_specific2(x: &dyn Foo<u32>) {
 fn test_specific3(x: &dyn Foo<i32>) {
     let _ = x as &dyn Bar<u32>; // Error
                                 //~^ ERROR non-primitive cast
-                                //~^^ ERROR the trait bound `&dyn Foo<i32>: Bar<u32>` is not satisfied
 }
 
 fn test_infer_arg(x: &dyn Foo<u32>) {
     let a = x as &dyn Bar<_>; // Ambiguous
                               //~^ ERROR non-primitive cast
-                              //~^^ ERROR the trait bound `&dyn Foo<u32>: Bar<_>` is not satisfied
     let _ = a.bar();
 }
 

tests/ui/traits/trait-upcasting/type-checking-test-2.stderr

@@ -2,41 +2,14 @@ error[E0605]: non-primitive cast: `&dyn Foo<i32>` as `&dyn Bar<u32>`
   --> $DIR/type-checking-test-2.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<u32>; // Error
-   |             ^^^^^^^^^^^^^^^^^^ invalid cast
+   |             ^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<u32>; // Error
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo<i32>: Bar<u32>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:19:13
-   |
-LL |     let _ = x as &dyn Bar<u32>; // Error
-   |             ^ the trait `Bar<u32>` is not implemented for `&dyn Foo<i32>`
-   |
-   = note: required for the cast from `&&dyn Foo<i32>` to `&dyn Bar<u32>`
 
 error[E0605]: non-primitive cast: `&dyn Foo<u32>` as `&dyn Bar<_>`
-  --> $DIR/type-checking-test-2.rs:25:13
+  --> $DIR/type-checking-test-2.rs:24:13
    |
 LL |     let a = x as &dyn Bar<_>; // Ambiguous
-   |             ^^^^^^^^^^^^^^^^ invalid cast
+   |             ^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
-   |
-help: consider borrowing the value
-   |
-LL |     let a = &x as &dyn Bar<_>; // Ambiguous
-   |             +
 
-error[E0277]: the trait bound `&dyn Foo<u32>: Bar<_>` is not satisfied
+error: aborting due to 2 previous errors
-  --> $DIR/type-checking-test-2.rs:25:13
-   |
-LL |     let a = x as &dyn Bar<_>; // Ambiguous
-   |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo<u32>`
-   |
-   = note: required for the cast from `&&dyn Foo<u32>` to `&dyn Bar<_>`
 
-error: aborting due to 4 previous errors
+For more information about this error, try `rustc --explain E0605`.
-
-Some errors have detailed explanations: E0277, E0605.
-For more information about an error, try `rustc --explain E0277`.