Non-Lexical Lifetimes only work with some types

[CryZe]

This one is really weird. It's the same code, but one is a Vec and one is a i32. The Vec one compiles, the i32 one doesn't:

fn vec() {
    let mut _x = vec!['c'];
    let _y = &_x;
    _x = Vec::new();
}

fn int()  {
    let mut _x = 5;
    let _y = &_x;
    _x = 7;
}

Error:

error[E0506]: cannot assign to `_x` because it is borrowed
  --> src\lib.rs:49:5
   |
48 |     let _y = &_x;
   |              --- borrow of `_x` occurs here
49 |     _x = 7;
   |     ^^^^^^ assignment to borrowed `_x` occurs here

[CryZe]

It looks like if the type of _x is either &T, &mut T, or T: Drop (or rather T that contains a Drop type somewhere) then NLL behaves just fine, but if you have a type that is not a reference and doesn't cause any calls to Drop, then it breaks.

[matthewjasper]

I think this is #46875. Delaying the assignment to _x makes this compile:

fn int1()  {
    let mut _x = 5;
    let _y = &_x;
    ();
    _x = 7;
}

fn int2()  {
    let mut _x = 5;
    let _y = &_x;
    _x = 1 + 2 + 4;
}