diff options
| -rw-r--r-- | compiler/rustc_resolve/src/late/lifetimes.rs | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/compiler/rustc_resolve/src/late/lifetimes.rs b/compiler/rustc_resolve/src/late/lifetimes.rs index a47a574218f..698e5048f7b 100644 --- a/compiler/rustc_resolve/src/late/lifetimes.rs +++ b/compiler/rustc_resolve/src/late/lifetimes.rs @@ -341,6 +341,34 @@ pub fn provide(providers: &mut ty::query::Providers) { /// Like `resolve_lifetimes`, but does not resolve lifetimes for trait items. /// Also does not generate any diagnostics. +/// +/// This is ultimately a subset of the `resolve_lifetimes` work. It effectively +/// resolves lifetimes only within the trait "header" -- that is, the trait +/// and supertrait list. In contrast, `resolve_lifetimes` resolves all the +/// lifetimes within the trait and its items. There is room to refactor this, +/// for example to resolve lifetimes for each trait item in separate queries, +/// but it's convenient to do the entire trait at once because the lifetimes +/// from the trait definition are in scope within the trait items as well. +/// +/// The reason for this separate call is to resolve what would otherwise +/// be a cycle. Consider this example: +/// +/// ```rust +/// trait Base<'a> { +/// type BaseItem; +/// } +/// trait Sub<'b>: for<'a> Base<'a> { +/// type SubItem: Sub<BaseItem = &'b u32>; +/// } +/// ``` +/// +/// When we resolve `Sub` and all its items, we also have to resolve `Sub<BaseItem = &'b u32>`. +/// To figure out the index of `'b`, we have to know about the supertraits +/// of `Sub` so that we can determine that the `for<'a>` will be in scope. +/// (This is because we -- currently at least -- flatten all the late-bound +/// lifetimes into a single binder.) This requires us to resolve the +/// *trait definition* of `Sub`; basically just enough lifetime information +/// to look at the supertraits. #[tracing::instrument(level = "debug", skip(tcx))] fn resolve_lifetimes_trait_definition( tcx: TyCtxt<'_>, |