这只是当前的限制还是技术原因？随着泛型函数被编译成专门的代码，我看不出什么应该阻止它工作.它在main函数中也可以正常工作.
示例(playground):
#![feature(associated_consts)]
trait HasNumber<T> {
const Number: usize;
}
enum One {}
enum Two {}
enum Foo {}
impl<T> HasNumber<One> for T {
const Number: usize = 1;
}
impl<T> HasNumber<Two> for T {
const Number: usize = 2;
}
fn use_number<T, H: HasNumber<T>>() {
let a: [u8; H::Number] = unsafe { ::std::mem::uninitialized() };
}
fn main() {
let a: [u8; <Foo as HasNumber<One>>::Number] = unsafe { ::std::mem::uninitialized() };
println!("{}", <Foo as HasNumber<One>>::Number);
println!("{}", <Foo as HasNumber<Two>>::Number);
}