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Improve cause information for NLL higher-ranked errors

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This PR has several interconnected pieces:

1. In some of the NLL region error code, we now pass
   around an `ObligationCause`, instead of just a plain `Span`.
   This gets forwarded into `fulfill_cx.register_predicate_obligation`
   during error reporting.
2. The general InferCtxt error reporting code is extended to
   handle `ObligationCauseCode::BindingObligation`
3. A new enum variant `ConstraintCategory::Predicate` is added.
   We try to avoid using this as the 'best blame constraint' - instead,
   we use it to enhance the `ObligationCause` of the `BlameConstraint`
   that we do end up choosing.

As a result, several NLL error messages now contain the same
"the lifetime requirement is introduced here" message as non-NLL
errors.

Having an `ObligationCause` available will likely prove useful
for future improvements to NLL error messages.
This commit is contained in:
Aaron Hill 2021-08-28 18:45:37 -05:00
parent 3e8f32e1c5
commit 93ab12eeab
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GPG key ID: B4087E510E98B164
16 changed files with 194 additions and 82 deletions
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13
compiler/rustc_infer/src/infer/error_reporting/mod.rs
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@ -609,6 +609,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
err: &mut DiagnosticBuilder<'tcx>,
cause: &ObligationCause<'tcx>,
exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
terr: &TypeError<'tcx>,
) {
match cause.code {
ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
@ -785,7 +786,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
err.help("try adding a diverging expression, such as `return` or `panic!(..)`");
err.help("...or use `match` instead of `let...else`");
}
_ => (),
_ => {
if let ObligationCauseCode::BindingObligation(_, binding_span) =
cause.code.peel_derives()
{
if matches!(terr, TypeError::RegionsPlaceholderMismatch) {
err.span_note(*binding_span, "the lifetime requirement is introduced here");
}
}
}
}
}
@ -1724,7 +1733,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
// It reads better to have the error origin as the final
// thing.
self.note_error_origin(diag, cause, exp_found);
self.note_error_origin(diag, cause, exp_found, terr);
}
pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {