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Rollup merge of #104427 - compiler-errors:rematch-impl-may-fail-actually, r=lcnr

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Explain why `rematch_impl` fails to be infallible

Fixes #104395

r? `@lcnr`
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Matthias Krüger 2022-11-29 22:43:16 +01:00 committed by GitHub
commit d88699f9f5
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7
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -2140,6 +2140,13 @@ fn rematch_impl(
match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
Ok(substs) => substs,
Err(()) => {
// FIXME: A rematch may fail when a candidate cache hit occurs
// on thefreshened form of the trait predicate, but the match
// fails for some reason that is not captured in the freshened
// cache key. For example, equating an impl trait ref against
// the placeholder trait ref may fail due the Generalizer relation
// raising a CyclicalTy error due to a sub_root_var relation
// for a variable being generalized...
self.infcx.tcx.sess.delay_span_bug(
obligation.cause.span,
&format!(