Closed
Description
@Kimundi showed me this example program which fails to compile. Here is a reduced version:
fn main() {
let s: &'static str = "foo";
let b: B<()> = B::new(s, ());
b.get_short();
}
trait IntoRef<'a> {
type T: Clone;
fn into_ref(self, &'a str) -> Self::T;
}
impl<'a> IntoRef<'a> for () {
type T = &'a str;
fn into_ref(self, s: &'a str) -> &'a str {
s
}
}
struct B<'a, P: IntoRef<'a>>(P::T);
impl<'a, P: IntoRef<'a>> B<'a, P> {
fn new(s: &'a str, i: P) -> B<'a, P> {
B(i.into_ref(s))
}
fn get_short(&self) -> P::T {
self.0.clone()
}
}This yields:
lunch-box. rustc ~/tmp/kimundi.rs
/home/nmatsakis/tmp/kimundi.rs:3:20: 3:26 error: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
/home/nmatsakis/tmp/kimundi.rs:3 let b: B<()> = B::new(s, ());
^~~~~~
/home/nmatsakis/tmp/kimundi.rs:4:5: 4:18 note: first, the lifetime must be contained by the method call at 4:4...
/home/nmatsakis/tmp/kimundi.rs:4 b.get_short();
^~~~~~~~~~~~~
/home/nmatsakis/tmp/kimundi.rs:4:5: 4:18 note: ...so type `&str` of expression is valid during the expression
/home/nmatsakis/tmp/kimundi.rs:4 b.get_short();
^~~~~~~~~~~~~
/home/nmatsakis/tmp/kimundi.rs:3:27: 3:28 note: but, the lifetime must also be contained by the expression at 3:26...
/home/nmatsakis/tmp/kimundi.rs:3 let b: B<()> = B::new(s, ());
^
/home/nmatsakis/tmp/kimundi.rs:3:27: 3:28 note: ...so that auto-reference is valid at the time of borrow
/home/nmatsakis/tmp/kimundi.rs:3 let b: B<()> = B::new(s, ());
^
error: aborting due to previous error
However, if you change the type of b to B<'static, ()>, it builds. Seems like there is a missing constraint somewhere in the region inference graph.
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