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Iterate generics_def_id_map in reverse order to fix P-critical issue #100340

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merged 3 commits into from
Aug 10, 2022
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Iterate generics_def_id_map in reverse order to fix P-critical issue #100340

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e6478a3
Add regression test for issue 100187
spastorino Aug 9, 2022
457ff7c
Iterate def_ids map backwards to try first the latest mappings (it's …
spastorino Aug 9, 2022
750a04e
Add docs for get_remapped_def_id
spastorino Aug 9, 2022
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15 changes: 14 additions & 1 deletion compiler/rustc_ast_lowering/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -220,7 +220,20 @@ impl ResolverAstLoweringExt for ResolverAstLowering {
}

fn get_remapped_def_id(&self, mut local_def_id: LocalDefId) -> LocalDefId {
for map in &self.generics_def_id_map {
// `generics_def_id_map` is a stack of mappings. As we go deeper in impl traits nesting we
// push new mappings so we need to try first the latest mappings, hence `iter().rev()`.
//
// Consider:
//
// `fn test<'a, 'b>() -> impl Trait<&'a u8, Ty = impl Sized + 'b> {}`
//
// We would end with a generics_def_id_map like:
//
// `[[fn#'b -> impl_trait#'b], [fn#'b -> impl_sized#'b]]`
//
// for the opaque type generated on `impl Sized + 'b`, We want the result to be:
// impl_sized#'b, so iterating forward is the wrong thing to do.
for map in self.generics_def_id_map.iter().rev() {
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It would be nice (in a separate fix) to avoid exposing this iter() altogether by wrapping this in an API called something like "StackMap" that does the iteration internally and only exposes a .get

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Yes, was thinking about the same thing. Didn't want to complicate this PR so it's easier to review for beta backport.

if let Some(r) = map.get(&local_def_id) {
debug!("def_id_remapper: remapping from `{local_def_id:?}` to `{r:?}`");
local_def_id = *r;
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12 changes: 12 additions & 0 deletions src/test/ui/impl-trait/issue-100187.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
// check-pass

trait Trait<T> {
type Ty;
}
impl Trait<&u8> for () {
type Ty = ();
}

fn test<'a, 'b>() -> impl Trait<&'a u8, Ty = impl Sized + 'b> {}

fn main() {}