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avoid using `skip_binder` and instead look for bound vars properly

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nikomatsakis committed Oct 2, 2019
1 parent 3f277e1 commit a96bce7c060d9c5215ecdc10d02b34fbc3f8ad35
Showing with 25 additions and 12 deletions.
  1. +25 −12 src/librustc_typeck/check/closure.rs
@@ -698,39 +698,52 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {

/// Given a projection like
///
/// `<_ as Future>::Output = T`
/// `<X as Future>::Output = T`
///
/// returns `Some(T)`. If the projection is for some other trait,
/// returns `None`.
/// where `X` is some type that has no late-bound regions, returns
/// `Some(T)`. If the projection is for some other trait, returns
/// `None`.
fn deduce_future_output_from_projection(
&self,
cause_span: Span,
projection: &ty::PolyProjectionPredicate<'tcx>,
predicate: &ty::PolyProjectionPredicate<'tcx>,
) -> Option<Ty<'tcx>> {
debug!("deduce_future_output_from_projection(projection={:?})", projection);
debug!("deduce_future_output_from_projection(predicate={:?})", predicate);

// We do not expect any bound regions in our predicate, so
// skip past the bound vars.
let predicate = match predicate.no_bound_vars() {
Some(p) => p,
None => {
debug!("deduce_future_output_from_projection: has late-bound regions");
return None;
}
};

let trait_ref = projection.to_poly_trait_ref(self.tcx);
// Check that this is a projection from the `Future` trait.
let trait_ref = predicate.projection_ty.trait_ref(self.tcx);
let future_trait = self.tcx.lang_items().future_trait().unwrap();
if trait_ref.def_id() != future_trait {
if trait_ref.def_id != future_trait {
debug!("deduce_future_output_from_projection: not a future");
return None;
}

// The `Future` trait has only one associted item, `Output`,
// so check that this is what we see.
let output_assoc_item = self.tcx.associated_items(future_trait).nth(0).unwrap().def_id;
if output_assoc_item != projection.projection_def_id() {
if output_assoc_item != predicate.projection_ty.item_def_id {
span_bug!(
cause_span,
"projecting associated item `{:?}` from future, which is not Output `{:?}`",
projection.projection_def_id(),
predicate.projection_ty.item_def_id,
output_assoc_item,
);
}

// Extract the type from the projection.
let output_ty = projection.skip_binder().ty;
let output_ty = self.resolve_vars_if_possible(&output_ty);
// Extract the type from the projection. Note that there can
// be no bound variables in this type because the "self type"
// does not have any regions in it.
let output_ty = self.resolve_vars_if_possible(&predicate.ty);
debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
Some(output_ty)
}

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