impl-trait return type is bounded by all input type parameters, even when unnecessary #42940
Description
rustc 1.20.0-nightly (f590a44ce 2017-06-27) binary: rustc commit-hash: f590a44ce61888c78b9044817d8b798db5cd2ffd commit-date: 2017-06-27 host: x86_64-pc-windows-msvc release: 1.20.0-nightly LLVM version: 4.0 trait Future {} impl<F> Future for Box<F> where F: Future + ?Sized {} struct SomeFuture<'a>(&'a Client); impl<'a> Future for SomeFuture<'a> {} struct Client; impl Client { fn post<'a, B>(&'a self, _body: &B) -> impl Future + 'a /* (1) */ { SomeFuture(self) } } fn login<'a>(client: &'a Client, username: &str) -> impl Future + 'a { client.post(&[username]) } fn main() { let client = Client; let _f = { let username = "foo".to_string(); login(&client, &username) }; }Since SomeFuture borrows 'a Client, I'd expect impl Future + 'a to be the correct return type, but it gives this error:
error[E0700]: hidden type for `impl Future + 'a` captures lifetime that does not appear in bounds --> src/main.rs:16:5 | 15 | fn login<'a>(client: &'a Client, username: &str) -> impl Future + 'a { | ---- ---------------- opaque type defined here | | | hidden type `impl Future + 'a` captures the anonymous lifetime defined here 16 | client.post(&[username]) | ^^^^^^^^^^^^^^^^^^^^^^^^ | help: add a `use<...>` bound to explicitly capture `'_` | 15 | fn login<'a>(client: &'a Client, username: &str) -> impl Future + 'a + use<'a, '_> { | +++++++++++++ -
Changing
_bodyto have an explicit lifetime like_body: &'b Bwhere'bis independent of'aor where'a: 'bdoes not change the error. This and the original error make it seem that returning animpl traitis somehow causing the_bodyparameter to get the'alifetime, even though it's clearly unused, let alone used in a way that it would require the'alifetime. -
Changing
(1)fromimpl Future + 'atoSomeFuture<'a>fixes it. -
Changing
(1)fromimpl Future + 'atoBox<Future + 'a>and returningBox::new(SomeFuture(self))fixes it.