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Only check for errors in predicate when skipping impl assembly
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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.
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Aaron1011 committed Nov 26, 2021
1 parent 1e79d79 commit 6daa9f1
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Showing 5 changed files with 20 additions and 25 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -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;
}

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Original file line number Diff line number Diff line change
@@ -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`
Expand All @@ -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`
Expand All @@ -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`
Expand All @@ -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`
Expand All @@ -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`.
Original file line number Diff line number Diff line change
Expand Up @@ -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
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2 changes: 1 addition & 1 deletion src/test/ui/issues/issue-77919.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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`
15 changes: 8 additions & 7 deletions src/test/ui/issues/issue-77919.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -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`.

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