Description
Given
use std::fmt::Debug;
fn foobar<'a, 'b>(foo: &'a str, bar: &'b str) -> impl Debug + 'a + 'b {
(foo, bar)
}
fn main() {
let foo = "hello".to_owned();
let bar = "world".to_owned();
println!("{:?}", foobar(&foo, &bar));
}you currently (rustc 1.26.0-nightly (188e693b3 2018-03-26)) get the errors
error[E0623]: lifetime mismatch
--> foo.rs:3:50
|
3 | fn foobar<'a, 'b>(foo: &'a str, bar: &'b str) -> impl Debug + 'a + 'b {
| ------- ^^^^^^^^^^^^^^^^^^^^
| | |
| | ...but data from `foo` is returned here
| this parameter and the return type are declared with different lifetimes...
error[E0623]: lifetime mismatch
--> foo.rs:3:50
|
3 | fn foobar<'a, 'b>(foo: &'a str, bar: &'b str) -> impl Debug + 'a + 'b {
| ------- ^^^^^^^^^^^^^^^^^^^^
| | |
| | ...but data from `bar` is returned here
| this parameter and the return type are declared with different lifetimes...
This not being an allowed syntax was briefly mentioned by @nikomatsakis in #34511, quoting the relevant part of the comment:
This is kind of annoying to do; we can't use
'cx + 'gcx. We can I suppose make a dummy trait:trait Captures<'a> { } impl<T: ?Sized> Captures<'a> for T { }and then return something like this
impl Iterator<Item = &'tcx Foo> + Captures<'gcx> + Captures<'cx>.
As far as I recall/can discover this limitation was never followed up on in that thread.
After some RFC spelunking I cannot find anything restricting a type bound to having at most a single lifetime bound, RFC 192 Appendix B may be relevant, but I don't believe that the Trait part of impl Trait is an "object type", it seems to me to be a "type parameter bound". Re-reading the impl Trait RFC series doesn't appear to clear this up, exactly what sort of syntax Trait is is never explicitly named, the original RFC simply says
The proposed syntax is
impl Traitin return type position, composing like trait objects to forms likeimpl Foo+Send+'a.
It seems to me that supporting multiple lifetime bounds on impl Trait should be fine, conceptually it seems to be the equivalent of:
use std::fmt::Debug;
fn foobar<'a, 'b, T: Debug + 'a + 'b>(foo: &'a &'b T) {
println!("{:?}", foo);
}
fn main() {
let foo = "hello world";
{
let bar = &foo;
{
let baz = &bar;
foobar(baz);
}
}
}Whether or not impl Trait can be extended to support multiple lifetimes, it seems like there should be an error message that the form impl Foo + 'a + 'b is not valid currently, rather than some weird lifetime mismatch errors.