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Explain RPITs in the way they actually work

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This commit is contained in:
Oli Scherer 2023-07-24 15:34:36 +00:00
parent 5b4549dd13
commit 30f787800a
1 changed files with 16 additions and 10 deletions
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26
compiler/rustc_hir_analysis/src/check/check.rs
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@ -545,17 +545,23 @@ fn sanity_check_found_hidden_type<'tcx>(
/// }
/// fn func<'a>(x: &'a ()) -> impl Id<Assoc = impl Sized + 'a> { x }
/// // desugared to
/// fn func<'a>(x: &'a () -> Outer<'a, Assoc = Inner<'a>> {
/// // Note that in contrast to other nested items, RPIT type aliases can
/// // access their parents' generics.
///
/// // hidden type is `&'bDup ()`
/// // During wfcheck the hidden type of `Inner` is `&'a ()`, but
/// // `typeof(Inner<'b, 'bDup>) = &'bDup ()`.
/// // So we walk the signature of `func` to find the use of `Inner<'static, 'a>`
/// // and then use that to replace the lifetimes in the hidden type, obtaining
/// // `&'a ()`.
/// type Outer<'b, 'bDup> = impl Id<Assoc = Inner<'b, 'bDup>>;
/// // hidden type is `&'cDup ()`
/// type Inner<'c, 'cDup> = impl Sized + 'cDup;
/// fn func<'a>(x: &'a () -> Outer<'static, 'a> { x }
/// // hidden type is `&'aDupOuter ()`
/// // During wfcheck the hidden type of `Inner<'aDupOuter>` is `&'a ()`, but
/// // `typeof(Inner<'aDupOuter>) = &'aDupOuter ()`.
/// // So we walk the signature of `func` to find the use of `Inner<'a>`
/// // and then use that to replace the lifetimes in the hidden type, obtaining
/// // `&'a ()`.
/// type Outer<'aDupOuter> = impl Id<Assoc = Inner<'aDupOuter>>;
///
/// // hidden type is `&'aDupInner ()`
/// type Inner<'aDupInner> = impl Sized + 'aDupInner;
///
/// x
/// }
/// ```
fn find_and_apply_rpit_args<'tcx>(
tcx: TyCtxt<'tcx>,