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Fix fallback note

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This commit is contained in:
Andrew Cann 2018-03-14 12:03:33 +08:00
parent 5b32211e62
commit 00a52a2be3
1 changed files with 10 additions and 9 deletions
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19
src/librustc/traits/error_reporting.rs
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@ -539,7 +539,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
fallback_has_occurred: bool)
{
let span = obligation.cause.span;
let _ = fallback_has_occurred;
let mut err = match *error {
SelectionError::Unimplemented => {
@ -623,9 +622,13 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
self.report_similar_impl_candidates(impl_candidates, &mut err);
}
// If this error is due to `!: !Trait` but `(): Trait` then add a note
// about the fallback behaviour change.
if trait_predicate.skip_binder().self_ty().is_never() {
// If this error is due to `!: Trait` not implemented but `(): Trait` is
// implemented, and fallback has occured, then it could be due to a
// variable that used to fallback to `()` now falling back to `!`. Issue a
// note informing about the change in behaviour.
if trait_predicate.skip_binder().self_ty().is_never()
&& fallback_has_occurred
{
let predicate = trait_predicate.map_bound(|mut trait_pred| {
{
let trait_ref = &mut trait_pred.trait_ref;
@ -638,13 +641,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
trait_pred
});
let unit_obligation = Obligation {
cause: obligation.cause.clone(),
param_env: obligation.param_env,
recursion_depth: obligation.recursion_depth,
predicate,
predicate: ty::Predicate::Trait(predicate),
.. obligation.clone()
};
let mut selcx = SelectionContext::new(self);
if let Ok(Some(..)) = selcx.select(&unit_obligation) {
if selcx.evaluate_obligation(&unit_obligation) {
err.note("the trait is implemented for `()`. \
Possibly this error has been caused by changes to \
Rust's type-inference algorithm \