do not constrain infer vars in find_best_leaf_obligation

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -346,12 +346,21 @@ fn find_best_leaf_obligation<'tcx>(
     consider_ambiguities: bool,
 ) -> PredicateObligation<'tcx> {
     let obligation = infcx.resolve_vars_if_possible(obligation.clone());
+    // FIXME: we use a probe here as the `BestObligation` visitor does not
+    // check whether it uses candidates which get shadowed by where-bounds.
+    //
+    // We should probably fix the visitor to not do so instead, as this also
+    // means the leaf obligation may be incorrect.
     infcx
-        .visit_proof_tree(obligation.clone().into(), &mut BestObligation {
-            obligation: obligation.clone(),
-            consider_ambiguities,
+        .fudge_inference_if_ok(|| {
+            infcx
+                .visit_proof_tree(obligation.clone().into(), &mut BestObligation {
+                    obligation: obligation.clone(),
+                    consider_ambiguities,
+                })
+                .break_value()
+                .ok_or(())
         })
-        .break_value()
         .unwrap_or(obligation)
 }
 

tests/ui/impl-trait/unsized_coercion.next.stderr

@@ -1,35 +1,11 @@
 error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
-  --> $DIR/unsized_coercion.rs:15:17
+  --> $DIR/unsized_coercion.rs:14:17
    |
 LL |         let x = hello();
    |                 ^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `dyn Trait`
 
-error[E0308]: mismatched types
-  --> $DIR/unsized_coercion.rs:19:5
-   |
-LL | fn hello() -> Box<impl Trait> {
-   |               ---------------
-   |               |   |
-   |               |   the expected opaque type
-   |               expected `Box<impl Trait>` because of return type
-...
-LL |     Box::new(1u32)
-   |     ^^^^^^^^^^^^^^ types differ
-   |
-   = note: expected struct `Box<impl Trait>`
-              found struct `Box<u32>`
+error: aborting due to 1 previous error
 
-error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
-  --> $DIR/unsized_coercion.rs:12:1
-   |
-LL | fn hello() -> Box<impl Trait> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
-   |
-   = help: the trait `Sized` is not implemented for `dyn Trait`
-
-error: aborting due to 3 previous errors
-
-Some errors have detailed explanations: E0277, E0308.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion.rs

@@ -10,13 +10,12 @@ trait Trait {}
 impl Trait for u32 {}
 
 fn hello() -> Box<impl Trait> {
-    //[next]~^ ERROR the size for values of type `dyn Trait` cannot be known at compilation time
     if true {
         let x = hello();
         //[next]~^ ERROR: the size for values of type `dyn Trait` cannot be known at compilation time
         let y: Box<dyn Trait> = x;
     }
-    Box::new(1u32) //[next]~ ERROR: mismatched types
+    Box::new(1u32)
 }
 
 fn main() {}

tests/ui/impl-trait/unsized_coercion3.next.stderr

@@ -1,35 +1,11 @@
 error[E0277]: the trait bound `dyn Send: Trait` is not satisfied
-  --> $DIR/unsized_coercion3.rs:14:17
+  --> $DIR/unsized_coercion3.rs:13:17
    |
 LL |         let x = hello();
    |                 ^^^^^^^ the trait `Trait` is not implemented for `dyn Send`
    |
    = help: the trait `Trait` is implemented for `u32`
 
-error[E0308]: mismatched types
-  --> $DIR/unsized_coercion3.rs:19:5
-   |
-LL | fn hello() -> Box<impl Trait + ?Sized> {
-   |               ------------------------
-   |               |   |
-   |               |   the expected opaque type
-   |               expected `Box<impl Trait + ?Sized>` because of return type
-...
-LL |     Box::new(1u32)
-   |     ^^^^^^^^^^^^^^ types differ
-   |
-   = note: expected struct `Box<impl Trait + ?Sized>`
-              found struct `Box<u32>`
+error: aborting due to 1 previous error
 
-error[E0277]: the trait bound `dyn Send: Trait` is not satisfied
-  --> $DIR/unsized_coercion3.rs:11:1
-   |
-LL | fn hello() -> Box<impl Trait + ?Sized> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `dyn Send`
-   |
-   = help: the trait `Trait` is implemented for `u32`
-
-error: aborting due to 3 previous errors
-
-Some errors have detailed explanations: E0277, E0308.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion3.old.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the size for values of type `impl Trait + ?Sized` cannot be known at compilation time
-  --> $DIR/unsized_coercion3.rs:16:32
+  --> $DIR/unsized_coercion3.rs:15:32
    |
 LL |         let y: Box<dyn Send> = x;
    |                                ^ doesn't have a size known at compile-time

tests/ui/impl-trait/unsized_coercion3.rs

@@ -9,7 +9,6 @@ trait Trait {}
 impl Trait for u32 {}
 
 fn hello() -> Box<impl Trait + ?Sized> {
-    //[next]~^ ERROR: the trait bound `dyn Send: Trait` is not satisfied
     if true {
         let x = hello();
         //[next]~^ ERROR: the trait bound `dyn Send: Trait` is not satisfied
@@ -17,7 +16,6 @@ fn hello() -> Box<impl Trait + ?Sized> {
         //[old]~^ ERROR: the size for values of type `impl Trait + ?Sized` cannot be know
     }
     Box::new(1u32)
-    //[next]~^ ERROR: mismatched types
 }
 
 fn main() {}

tests/ui/traits/negative-bounds/opaque-type-unsatisfied-bound.rs

@@ -14,13 +14,7 @@ fn main() {
 
 fn weird0() -> impl Sized + !Sized {}
 //~^ ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied
 fn weird1() -> impl !Sized + Sized {}
 //~^ ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied
 fn weird2() -> impl !Sized {}
 //~^ ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied
-//~| ERROR the trait bound `(): !Sized` is not satisfied

tests/ui/traits/negative-bounds/opaque-type-unsatisfied-bound.stderr

@@ -5,53 +5,17 @@ LL | fn weird0() -> impl Sized + !Sized {}
    |                ^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
 
 error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:15:36
-   |
-LL | fn weird0() -> impl Sized + !Sized {}
-   |                                    ^^ the trait bound `(): !Sized` is not satisfied
-
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:15:1
-   |
-LL | fn weird0() -> impl Sized + !Sized {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
-
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:19:16
+  --> $DIR/opaque-type-unsatisfied-bound.rs:17:16
    |
 LL | fn weird1() -> impl !Sized + Sized {}
    |                ^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
 
 error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:19:36
-   |
-LL | fn weird1() -> impl !Sized + Sized {}
-   |                                    ^^ the trait bound `(): !Sized` is not satisfied
-
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:19:1
-   |
-LL | fn weird1() -> impl !Sized + Sized {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
-
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:23:16
+  --> $DIR/opaque-type-unsatisfied-bound.rs:19:16
    |
 LL | fn weird2() -> impl !Sized {}
    |                ^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
 
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:23:28
-   |
-LL | fn weird2() -> impl !Sized {}
-   |                            ^^ the trait bound `(): !Sized` is not satisfied
-
-error[E0277]: the trait bound `(): !Sized` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-bound.rs:23:1
-   |
-LL | fn weird2() -> impl !Sized {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Sized` is not satisfied
-
 error[E0277]: the trait bound `impl !Trait: Trait` is not satisfied
   --> $DIR/opaque-type-unsatisfied-bound.rs:12:13
    |
@@ -66,6 +30,6 @@ note: required by a bound in `consume`
 LL | fn consume(_: impl Trait) {}
    |                    ^^^^^ required by this bound in `consume`
 
-error: aborting due to 10 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/negative-bounds/opaque-type-unsatisfied-fn-bound.rs

@@ -4,7 +4,5 @@
 
 fn produce() -> impl !Fn<(u32,)> {}
 //~^ ERROR the trait bound `(): !Fn(u32)` is not satisfied
-//~| ERROR the trait bound `(): !Fn(u32)` is not satisfied
-//~| ERROR the trait bound `(): !Fn(u32)` is not satisfied
 
 fn main() {}

tests/ui/traits/negative-bounds/opaque-type-unsatisfied-fn-bound.stderr

@@ -4,18 +4,6 @@ error[E0277]: the trait bound `(): !Fn(u32)` is not satisfied
 LL | fn produce() -> impl !Fn<(u32,)> {}
    |                 ^^^^^^^^^^^^^^^^ the trait bound `(): !Fn(u32)` is not satisfied
 
-error[E0277]: the trait bound `(): !Fn(u32)` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-fn-bound.rs:5:34
-   |
-LL | fn produce() -> impl !Fn<(u32,)> {}
-   |                                  ^^ the trait bound `(): !Fn(u32)` is not satisfied
-
-error[E0277]: the trait bound `(): !Fn(u32)` is not satisfied
-  --> $DIR/opaque-type-unsatisfied-fn-bound.rs:5:1
-   |
-LL | fn produce() -> impl !Fn<(u32,)> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait bound `(): !Fn(u32)` is not satisfied
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.