考虑下面的玩具例子:
use std::cmp::Ordering;
pub trait SimpleOrder {
fn key(&self) -> u32;
}
impl PartialOrd for dyn SimpleOrder {
fn partial_cmp(&self, other: &dyn SimpleOrder) -> Option<Ordering> {
Some(self.cmp(other))
}
}
impl Ord for dyn SimpleOrder {
fn cmp(&self, other: &dyn SimpleOrder) -> Ordering {
self.key().cmp(&other.key())
}
}
impl PartialEq for dyn SimpleOrder {
fn eq(&self, other: &dyn SimpleOrder) -> bool {
self.key() == other.key()
}
}
impl Eq for SimpleOrder {}
这不能编译.它声称在partial_cmp年的实施中存在一个终生问题:
error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
--> src/main.rs:9:23
|
9 | Some(self.cmp(other))
| ^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the method body at 8:5...
--> src/main.rs:8:5
|
8 | / fn partial_cmp(&self, other: &dyn SimpleOrder) -> Option<Ordering> {
9 | | Some(self.cmp(other))
10| | }
| |_____^
note: ...so that the declared lifetime parameter bounds are satisfied
--> src/main.rs:9:23
|
9 | Some(self.cmp(other))
| ^^^^^
= note: but, the lifetime must be valid for the static lifetime...
= note: ...so that the types are compatible:
expected std::cmp::Eq
found std::cmp::Eq
我真的不明白这个错误.尤其令人费解.
如果我手动内联调用，它可以很好地编译:
fn partial_cmp(&self, other: &dyn SimpleOrder) -> Option<Ordering> {
Some(self.key().cmp(&other.key()))
}
这是怎么回事？