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Auto merge of #119947 - compiler-errors:old-solver-instantiate-response, r=lcnr

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Make sure to instantiate placeholders correctly in old solver

When creating the query substitution guess for an input placeholder type like `!1_T` (in universe 1), we were guessing the response substitution with something like `!0_T`. This failed to unify with `!1_T`, causing an ICE.

This PR reworks the query substitution guess code to work a bit more like the new solver. I'm *pretty* sure this is correct, though I'd really appreciate some scrutiny from someone (*cough* lcnr) who knows a bit more about query instantiation :)

Fixes #119941

r? lcnr
This commit is contained in:
bors 2024-01-16 13:33:04 +00:00
commit 533cfde67c
7 changed files with 106 additions and 4 deletions
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16
compiler/rustc_infer/src/infer/canonical/query_response.rs
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@ -483,7 +483,13 @@ impl<'tcx> InferCtxt<'tcx> {
let result_subst = CanonicalVarValues {
var_values: self.tcx.mk_args_from_iter(
query_response.variables.iter().enumerate().map(|(index, info)| {
if info.is_existential() {
if info.universe() != ty::UniverseIndex::ROOT {
// A variable from inside a binder of the query. While ideally these shouldn't
// exist at all, we have to deal with them for now.
self.instantiate_canonical_var(cause.span, info, |u| {
universe_map[u.as_usize()]
})
} else if info.is_existential() {
match opt_values[BoundVar::new(index)] {
Some(k) => k,
None => self.instantiate_canonical_var(cause.span, info, |u| {
@ -491,9 +497,11 @@ impl<'tcx> InferCtxt<'tcx> {
}),
}
} else {
self.instantiate_canonical_var(cause.span, info, |u| {
universe_map[u.as_usize()]
})
// For placeholders which were already part of the input, we simply map this
// universal bound variable back the placeholder of the input.
opt_values[BoundVar::new(index)].expect(
"expected placeholder to be unified with itself during response",
)
}
}),
),