Non-lexical lifetimes has different behavior for Vec<&i32> and Vec<&String>

[shepmaster]

With 1.24.0-nightly (2017-12-21 250b492):

This compiles:

#![feature(nll)]

fn main() {
    let mut vec = Vec::new();

    let a = 1;
    vec.push(&a);

    let b = 2;
    vec.push(&b);

    println!("{:?}", vec);
}

But this does not:

#![feature(nll)]

fn f() {
    let mut vec = Vec::new();

    let a = String::from("");
    vec.push(&a);

    let b = String::from("");
    vec.push(&b);

    println!("{:?}", vec);
}

fn main() {}

error[E0597]: `a` does not live long enough
  --> src/main.rs:7:14
   |
7  |     vec.push(&a);
   |              ^^ borrowed value does not live long enough
...
13 | }
   |  - borrowed value only lives until here
   |
   = note: borrowed value must be valid for lifetime '_#11r...

/cc @arielb1 @nikomatsakis

[Ryman]

The first example has constant propagation (or whatever that's called these days) occurring, you can add let _: Vec<&'static i32> = vec; to the end and it still compiles which shows they're getting promoted to statics.

In the second, if you bring a and b to be above vec then the example works. Isn't this necessary due to destructor ordering? I'm not up to date on NLL enough to know if that's intended to work.

[CryZe]

This might be a duplicate of #46935 where I also noticed that drop and non-drop types behave very differently.

[nikomatsakis]

I have a suspicion what this might be but will investigate. On Dec 22, 2017 4:39 PM, Jake Goulding <notifications@github.com> wrote:With 1.24.0-nightly (2017-12-21 250b492): This compiles: #![feature(nll)] fn main() { let mut vec = Vec::new(); let a = 1; vec.push(&a); let b = 2; vec.push(&b); println!("{:?}", vec); } But this does not: #![feature(nll)] fn f() { let mut vec = Vec::new(); let a = String::from(""); vec.push(&a); let b = String::from(""); vec.push(&b); println!("{:?}", vec); } fn main() {} error[E0597]: `a` does not live long enough --> src/main.rs:7:14 | 7 | vec.push(&a); | ^^ borrowed value does not live long enough ... 13 | } | - borrowed value only lives until here | = note: borrowed value must be valid for lifetime '_#11r... /cc @arielb1 @nikomatsakis —You are receiving this because you were mentioned.Reply to this email directly, view it on GitHub, or mute the thread.

[nikomatsakis]

Both versions compile now: https://play.rust-lang.org/?gist=79c56c0a2310df3880212d7124be65cd&version=nightly

Fixed by #46984