Only check for errors in predicate when skipping impl assembly

Prior to PR rust-lang#91205, checking for errors in the overall obligation
would check checking the `ParamEnv`, due to an incorrect
`super_visit_with` impl. With this bug fixed, we will now
bail out of impl candidate assembly if the `ParamEnv` contains
any error types.

In practice, this appears to be overly conservative - when an error
occurs early in compilation, we end up giving up early for some
predicates that we could have successfully evaluated without overflow.
By only checking for errors in the predicate itself, we avoid causing
additional spurious 'type annotations needed' errors after a 'real'
error has already occurred.

With this PR, the diagnostic changes caused by PR rust-lang#91205 are reverted.

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -536,7 +536,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // This helps us avoid overflow: see issue #72839
         // Since compilation is already guaranteed to fail, this is just
         // to try to show the 'nicest' possible errors to the user.
-        if obligation.references_error() {
+        // We don't check for errors in the `ParamEnv` - in practice,
+        // it seems to cause us to be overly aggressive in deciding
+        // to give up searching for candidates, leading to spurious errors.
+        if obligation.predicate.references_error() {
             return;
         }
 

src/test/ui/const-generics/generic_const_exprs/issue-72787.min.stderr

@@ -1,5 +1,5 @@
 error: generic parameters may not be used in const operations
-  --> $DIR/issue-72787.rs:12:17
+  --> $DIR/issue-72787.rs:11:17
    |
 LL |     Condition<{ LHS <= RHS }>: True
    |                 ^^^ cannot perform const operation using `LHS`
@@ -8,7 +8,7 @@ LL |     Condition<{ LHS <= RHS }>: True
    = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
-  --> $DIR/issue-72787.rs:12:24
+  --> $DIR/issue-72787.rs:11:24
    |
 LL |     Condition<{ LHS <= RHS }>: True
    |                        ^^^ cannot perform const operation using `RHS`
@@ -17,7 +17,7 @@ LL |     Condition<{ LHS <= RHS }>: True
    = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
-  --> $DIR/issue-72787.rs:26:25
+  --> $DIR/issue-72787.rs:25:25
    |
 LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    |                         ^ cannot perform const operation using `I`
@@ -26,7 +26,7 @@ LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
-  --> $DIR/issue-72787.rs:26:36
+  --> $DIR/issue-72787.rs:25:36
    |
 LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    |                                    ^ cannot perform const operation using `J`
@@ -35,29 +35,21 @@ LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
 
 error[E0283]: type annotations needed
-  --> $DIR/issue-72787.rs:10:38
-   |
-LL | impl<const LHS: u32, const RHS: u32> True for IsLessOrEqual<LHS, RHS> where
-   |                                      ^^^^ cannot infer type for struct `IsLessOrEqual<LHS, RHS>`
-   |
-   = note: cannot satisfy `IsLessOrEqual<LHS, RHS>: True`
-
-error[E0283]: type annotations needed
-  --> $DIR/issue-72787.rs:22:26
+  --> $DIR/issue-72787.rs:21:26
    |
 LL |     IsLessOrEqual<I, 8>: True,
    |                          ^^^^ cannot infer type for struct `IsLessOrEqual<I, 8_u32>`
    |
    = note: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
 
 error[E0283]: type annotations needed
-  --> $DIR/issue-72787.rs:22:26
+  --> $DIR/issue-72787.rs:21:26
    |
 LL |     IsLessOrEqual<I, 8>: True,
    |                          ^^^^ cannot infer type for struct `IsLessOrEqual<I, 8_u32>`
    |
    = note: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
 
-error: aborting due to 7 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0283`.

src/test/ui/const-generics/generic_const_exprs/issue-72787.rs

@@ -8,7 +8,6 @@ pub struct Condition<const CONDITION: bool>;
 pub trait True {}
 
 impl<const LHS: u32, const RHS: u32> True for IsLessOrEqual<LHS, RHS> where
-//[min]~^ ERROR type annotations needed
     Condition<{ LHS <= RHS }>: True
 //[min]~^ Error generic parameters may not be used in const operations
 //[min]~| Error generic parameters may not be used in const operations

src/test/ui/issues/issue-77919.rs

@@ -10,4 +10,4 @@ struct Multiply<N, M> {
 }
 impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
 //~^ ERROR cannot find type `VAL` in this scope
-//~| ERROR type annotations needed
+//~| ERROR not all trait items implemented, missing: `VAL`

src/test/ui/issues/issue-77919.stderr

@@ -17,15 +17,16 @@ LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
    |          |
    |          help: you might be missing a type parameter: `, VAL`
 
-error[E0283]: type annotations needed
-  --> $DIR/issue-77919.rs:11:12
+error[E0046]: not all trait items implemented, missing: `VAL`
+  --> $DIR/issue-77919.rs:11:1
    |
+LL |     const VAL: T;
+   |     ------------- `VAL` from trait
+...
 LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
-   |            ^^^^^^^^^^^^^^ cannot infer type for struct `Multiply<N, M>`
-   |
-   = note: cannot satisfy `Multiply<N, M>: TypeVal<usize>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0283, E0412.
-For more information about an error, try `rustc --explain E0283`.
+Some errors have detailed explanations: E0046, E0412.
+For more information about an error, try `rustc --explain E0046`.