Working on my crate to provide Lua generic sandboxes, I ran into the following problem. I would like to create to type aliases M<'a> and D<'a, 'b>, which refer to a particular machine type (which has a lifetime argument) and its associated type Datum (which is a GAT with another lifetime argument).
While I can define M<'a> as alias for LuaMachine<'a> and use <M as Machine>::Datum<'_> as type in my code, I cannot define an alias D<'a, 'b> = <M<'a> as Machine>::Datum<'b>.
Does anyone know why?
#![feature(generic_associated_types)]
use std::marker::PhantomData;
trait Machine<'a> {
type Datum<'b>
where
Self: 'b;
}
#[derive(Default)]
struct LuaMachine<'a> {
_phantom: PhantomData<fn(&'a ()) -> &'a ()>,
}
struct LuaDatum<'a, 'b> {
_machine: &'b LuaMachine<'a>,
}
impl<'a> Machine<'a> for LuaMachine<'a> {
type Datum<'b>
where
Self: 'b,
= LuaDatum<'a, 'b>;
}
type M<'a> = LuaMachine<'a>;
// This won't work:
type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;
fn main() {
let _m: M = Default::default();
// But this works:
let _d: <M as Machine>::Datum<'_> = LuaDatum { _machine: &_m };
}
Errors:
Compiling playground v0.0.1 (/playground)
error[E0106]: missing lifetime specifier
--> src/main.rs:29:18
|
29 | type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected named lifetime parameter
|
note: these named lifetimes are available to use
--> src/main.rs:29:8
|
29 | type D<'a, 'b> = <M<'a> as Machine>::Datum<'b>;
| ^^ ^^
For more information about this error, try `rustc --explain E0106`.
error: could not compile `playground` due to previous error
