Why is rustc's suggestion here wrong? - rust

Question

I'm trying to implement the following trait, where the <'baz> is required for a later impl<'baz> Baz<'baz> for T where T: OtherTrait†.
trait Baz<'baz>: Clone {
fn baz(&'baz mut self);
fn bazzed(&self) -> Self {
let mut x = self.clone();
x.baz();
x
}
}
Rustc understandably fails:
Compiling playground v0.0.1 (/playground)
error[E0309]: the parameter type `Self` may not live long enough
--> src/lib.rs:6:9
|
6 | x.baz();
| ^
|
= help: consider adding an explicit lifetime bound `Self: 'baz`...
= note: ...so that the type `Self` is not borrowed for too long
error[E0309]: the parameter type `Self` may not live long enough
--> src/lib.rs:6:11
|
6 | x.baz();
| ^^^
|
= help: consider adding an explicit lifetime bound `Self: 'baz`...
= note: ...so that the reference type `&'baz mut Self` does not outlive the data it points at
For more information about this error, try `rustc --explain E0309`.
error: could not compile `playground` due to 2 previous errors
So I follow its suggestion:
trait Baz<'baz>: Clone {
fn baz(&'baz mut self);
fn bazzed(&self) -> Self
where
Self: 'baz
{
let mut x = self.clone();
x.baz();
x
}
}
And I don't quite understand what it's not liking now...
Compiling playground v0.0.1 (/playground)
error[E0597]: `x` does not live long enough
--> src/lib.rs:9:9
|
1 | trait Baz<'baz>: Clone {
| ---- lifetime `'baz` defined here
...
9 | x.baz();
| ^^^^^^^
| |
| borrowed value does not live long enough
| argument requires that `x` is borrowed for `'baz`
10 | x
11 | }
| - `x` dropped here while still borrowed
error[E0505]: cannot move out of `x` because it is borrowed
--> src/lib.rs:10:9
|
1 | trait Baz<'baz>: Clone {
| ---- lifetime `'baz` defined here
...
9 | x.baz();
| -------
| |
| borrow of `x` occurs here
| argument requires that `x` is borrowed for `'baz`
10 | x
| ^ move out of `x` occurs here
Some errors have detailed explanations: E0505, E0597.
For more information about an error, try `rustc --explain E0505`.
error: could not compile `playground` due to 2 previous errors
I tried making Self: 'static too but that did not help either. What am I missing here?
† The <'baz> I need for another trait, that looks roughly like:
trait ReturnIteratorMut<'item> {
type IterType: Iterator<Item = &'item mut Thing>;
fn iterator<'slf: 'item>(&'slf mut self) -> Self::IterType;
}
I want to implement Baz for all of these by applying baz() to every item in the iterator, so I need 'baz: 'item. This works fine with just the baz() method, it's only the bazzed() that it starts to complain at.

Answer 1

Let's first try to understand what's wrong with this code, without the compiler's help.
x.baz() requires x to live for 'baz. Since 'baz can be any lifetime this means x has to live for 'static. It does not - it is a local variable, and this is an error (you might want HRTB, but you said you want to focus on the compiler errors so I don't deal with the solution).
The compiler's second errors are just that - x does not live long enough, or, if we look at the opposite angle, x is borrowed for 'static and so we cannot move it (the compiler shows both errors in order to maximize displayed errors - sometimes it works, sometimes, like in this case, it leads to confusing errors being shown).
The first error(s) is a negligible result of this constraint, but it hides the actual error (for what it's worth, on nightly they're all shown together): x needs to live for 'static, but not just it does not, sometimes it is impossible for any variable of its type to be. Suppose Self is &'some_short_lifetime i32 - it cannot live for 'static, since it contains a shorter lifetime. This is the reason "Self may not live long enough`.