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improve comment about instantiating anon types

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This commit is contained in:
Niko Matsakis 2017-12-20 16:48:41 -05:00
parent 3f490ca4bd
commit 4f549fe4fe
1 changed files with 34 additions and 6 deletions
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40
src/librustc/infer/anon_types/mod.rs
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@ -509,12 +509,40 @@ impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> {
tcx,
fldop: |ty| {
if let ty::TyAnon(def_id, substs) = ty.sty {
// Check that this is `impl Trait` type is declared by
// `parent_def_id`. During the first phase of type-check, this
// is true, but during NLL type-check, we sometimes encounter
// `impl Trait` types in e.g. inferred closure signatures that
// are not 'local' to the current function and hence which
// ought not to be instantiated.
// Check that this is `impl Trait` type is
// declared by `parent_def_id` -- i.e., one whose
// value we are inferring. At present, this is
// always true during the first phase of
// type-check, but not always true later on during
// NLL. Once we support named abstract types more fully,
// this same scenario will be able to arise during all phases.
//
// Here is an example using `abstract type` that indicates
// the distinction we are checking for:
//
// ```rust
// mod a {
// pub abstract type Foo: Iterator;
// pub fn make_foo() -> Foo { .. }
// }
//
// mod b {
// fn foo() -> a::Foo { a::make_foo() }
// }
// ```
//
// Here, the return type of `foo` references a
// `TyAnon` indeed, but not one whose value is
// presently being inferred. You can get into a
// similar situation with closure return types
// today:
//
// ```rust
// fn foo() -> impl Iterator { .. }
// fn bar() {
// let x = || foo(); // returns the Anon assoc with `foo`
// }
// ```
if let Some(anon_node_id) = tcx.hir.as_local_node_id(def_id) {
let anon_parent_node_id = tcx.hir.get_parent(anon_node_id);
let anon_parent_def_id = tcx.hir.local_def_id(anon_parent_node_id);