Generic parameter capture

stable

When you use an impl trait in return position, the hidden type may make use of any of the type parameters, and hence the following function is legal:


#![allow(unused)]
fn main() {
// Hidden type: Option<T>, which references T
fn foo<T: Clone>(t: T) -> impl Clone {
    Some(t)
}
}

However, it may not reference lifetime parameters unless those lifetime parameters appear in the impl trait bounds. The following


#![allow(unused)]
fn main() {
// Error: hidden type `Option<&'a u32>` references `'a`
fn foo<'a>(t: &'a u32) -> impl Clone {
    Some(t)
}
}

XXX document:

  • you can capture type parameters
  • but not lifetimes, unless they appear in the bounds