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rewrite the comments

This commit is contained in:
Ellen 2021-02-10 13:11:12 +00:00
parent 0ffa2da186
commit 7ca96ed2af
1 changed files with 21 additions and 12 deletions
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33
compiler/rustc_typeck/src/collect/type_of.rs
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@ -29,20 +29,28 @@ pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<
let parent_node = tcx.hir().get(parent_node_id);
match parent_node {
// This matches on types who's paths couldn't be resolved without typeck'ing e.g.
// This match arm is for when the def_id appears in a GAT whose
// path can't be resolved without typechecking e.g.
//
// trait Foo {
// type Assoc<const N1: usize>;
// type Assoc<const N: usize>;
// fn foo() -> Self::Assoc<3>;
// // note: if the def_id argument is the 3 then in this example
// // parent_node would be the node for Self::Assoc<_>
// }
// We didnt write <Self as Foo>::Assoc so the Self::Assoc<_> is lowered to QPath::TypeRelative.
//
// In the above code we would call this query with the def_id of 3 and
// the parent_node we match on would be the hir node for Self::Assoc<3>
//
// `Self::Assoc<3>` cant be resolved without typchecking here as we
// didnt write <Self as Foo>::Assoc<3>. If we did then another match
// arm would handle this.
//
// I believe this match arm is only needed for GAT but I am not 100% sure - BoxyUwU
Node::Ty(hir_ty @ Ty { kind: TyKind::Path(QPath::TypeRelative(_, segment)), .. }) => {
// Walk up from the parent_node to find an item so that
// we can resolve the relative path to an actual associated type.
// For the code example above, this item would be the Foo trait.
// Find the Item containing the associated type so we can create an ItemCtxt.
// Using the ItemCtxt convert the HIR for the unresolved assoc type into a
// ty which is a fully resolved projection.
// For the code example above, this would mean converting Self::Assoc<3>
// into a ty::Projection(<Self as Foo>::Assoc<3>)
let item_hir_id = tcx
.hir()
.parent_iter(hir_id)
@ -52,11 +60,12 @@ pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<
.unwrap();
let item_did = tcx.hir().local_def_id(item_hir_id).to_def_id();
let item_ctxt = &ItemCtxt::new(tcx, item_did) as &dyn crate::astconv::AstConv<'_>;
// This ty will be the actual associated type so that we can
// go through its generics to find which param our def_id corresponds to.
// For the code example above, this ty would be the Assoc<const N1: usize>.
let ty = item_ctxt.ast_ty_to_ty(hir_ty);
// Iterate through the generics of the projection to find the one that corresponds to
// the def_id that this query was called with. We filter to only const args here as a
// precaution for if it's ever allowed to elide lifetimes in GAT's. It currently isn't
// but it can't hurt to be safe ^^
if let ty::Projection(projection) = ty.kind() {
let generics = tcx.generics_of(projection.item_def_id);