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#![feature(specialization)]
pub trait TryDefault<V> {
fn try_default() -> Option<V>;
}
impl<V> TryDefault<V> for V {
default fn try_default() -> Option<V> {
None
}
}
impl<V: Default> TryDefault<V> for V {
fn try_default() -> Option<V> {
Some(V::default())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[derive(Default, PartialEq, Debug)]
struct IsDefault {
a: u32,
}
#[derive(PartialEq, Debug)]
struct NonDefault {
a: u32,
}
#[test]
fn it_should_default_when_used_on_core_type() {
let n = <u32>::try_default();
assert_eq!(n, Some(u32::default()));
}
#[test]
fn it_should_default_when_used_on_core_type_with_different_sytax() {
let n = <u32 as TryDefault<u32>>::try_default();
assert_eq!(n, Some(u32::default()));
}
#[test]
fn it_should_return_none_when_used_on_core_without_default() {
let f = <::std::fs::File>::try_default();
assert!(f.is_none());
}
#[test]
fn it_returns_default_on_own_types() {
let n = <IsDefault>::try_default();
assert_eq!(n, Some(IsDefault::default()));
}
#[test]
fn it_returns_none_on_non_default_types() {
let n = <NonDefault>::try_default();
assert_eq!(n, None);
}
}