NLL: Implicit reborrow hides error "Cannot move out of Struct with destructor"

[lqd]

Found while triaging the NLL crater run, in Servo's url crate.

Playground

There's a Drop impl preventing , so the error should at least mention it. But the implicit reborrow self.url hides this error with a '*self.url' does not live long enough one.

Changing the impl to

impl<'a> S<'a> {
    fn finish(self) -> &'a mut String {
        let p = self.url;
        p
    }
}

gives a better error message: self.url cannot move out of type "S<'_>", which implements the "Drop" trait.

[pnkfelix]

assigning to self, hoping to investigate this week

[pnkfelix]

(to be clear, even an explicit reborrow would produce the error message that is being deemed inferior here...)

[pnkfelix]

To be even more clear: the error here isn't about the illegality of moving out of a struct with a destructor, because the code is not doing any such move.

What the code is doing is borrowing into the interior of a struct with a destructor, and that borrow outlives the lifetime of the struct. Thus the struct's Drop impl might mutably access the borrowed (i.e. aliased) state, violating the rule that all such &mut-accesses are unaliased.

---

That's not to say that the given error message cannot be improved.

In particular, it might be better if it said something like:

15 |         self.url 
   |         ^^^^^^^^ borrow extends past lifetime of value 
16 |     }
   |     - `self` dropped here while `self.url` still borrowed
   |
   = note: `self` is of type `S<'_>`, which implements `Drop`; thus 
           dropping `self` requires exclusive access to `self.url`.

[pnkfelix]

Okay I think I have a patch to fix this. And it seems like it improves some of our other diagnostics elsewhere. Will post PR soonish.