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Rollup merge of rust-lang#65235 - nikomatsakis:issue-65159-async-fn-r…
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…eturn-ice, r=cramertj

don't assume we can *always* find a return type hint in async fn

In particular, we sometimes cannot if there is an earlier error.

Fixes rust-lang#65159

r? @cramertj, who reviewed the original PR
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Centril authored Oct 15, 2019
2 parents 07ccdcf + 48fff6f commit 2e686ef
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Showing 3 changed files with 31 additions and 3 deletions.
15 changes: 12 additions & 3 deletions src/librustc_typeck/check/closure.rs
Original file line number Diff line number Diff line change
Expand Up @@ -611,6 +611,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
debug!("supplied_sig_of_closure: closure is async fn body");
self.deduce_future_output_from_obligations(expr_def_id)
.unwrap_or_else(|| {
// AFAIK, deducing the future output
// always succeeds *except* in error cases
// like #65159. I'd like to return Error
// here, but I can't because I can't
// easily (and locally) prove that we
// *have* reported an
// error. --nikomatsakis
astconv.ty_infer(None, decl.output.span())
})
}

_ => astconv.ty_infer(None, decl.output.span()),
Expand Down Expand Up @@ -645,7 +655,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
fn deduce_future_output_from_obligations(
&self,
expr_def_id: DefId,
) -> Ty<'tcx> {
) -> Option<Ty<'tcx>> {
debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);

let ret_coercion =
Expand Down Expand Up @@ -688,8 +698,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
} else {
None
}
})
.unwrap();
});

debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
output_ty
Expand Down
10 changes: 10 additions & 0 deletions src/test/ui/async-await/issues/issue-65159.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// Regression test for #65159. We used to ICE.
//
// edition:2018

async fn copy() -> Result<()> //~ ERROR wrong number of type arguments
{
Ok(())
}

fn main() { }
9 changes: 9 additions & 0 deletions src/test/ui/async-await/issues/issue-65159.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
error[E0107]: wrong number of type arguments: expected 2, found 1
--> $DIR/issue-65159.rs:5:20
|
LL | async fn copy() -> Result<()>
| ^^^^^^^^^^ expected 2 type arguments

error: aborting due to previous error

For more information about this error, try `rustc --explain E0107`.

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