Abstract return types (written impl Trait for some trait Trait) are only
allowed as function and inherent impl return types.
Erroneous code example:
fn main() {
let count_to_ten: impl Iterator<Item=usize> = 0..10;
// error: `impl Trait` not allowed outside of function and inherent method
// return types
for i in count_to_ten {
println!("{}", i);
}
}Make sure impl Trait only appears in return-type position.
fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
0..n
}
fn main() {
for i in count_to_n(10) { // ok!
println!("{}", i);
}
}See RFC 1522 for more details.