NLL allows creating variables that are immediately unusable.

[matthewjasper]

The following code compiles with NLL currently

fn main() {
    let x = {
        let z = 0;
        &z
    };
}

because it's equivalent to:

fn main() {
    let x;
    {
        let z = 0;
        x = &z
    };
}

However, any further use of x will result in a "z does not live long enough" warning. Should there be some read of x after the rest of the let is done to ensure that this doesn't happen?

[pnkfelix]

I suppose since we already have ReadForMatch (which is essentially an over-approximation of what a given pattern in a match may do, and thus I think of it as a "fake read"), that provides some precedent for trying to add some sort of fake read in this scenario too... I guess the idea is that we'd put it immediately after each let statement...?

[pnkfelix]

(nominating in the hopes that the WG-compiler-nll team will discuss this. Though its possible that the decision needs to be with T-compiler. ((or even T-lang??)))

[pnkfelix]

(I also wonder if adding a fake read for every local will fix some of the issues we've seen with the compiler accepting "surprising" cases where the issue was that a closure was constructed but never called.)

[matthewjasper]

Weren't those issues with AST borrowck?

[pnkfelix]

@matthewjasper well, I was thinking of the cases where I had to add explicit references to a local variable in order to observe a desired unit test failure. Let me find the link...

[pnkfelix]

@matthewjasper i.e I was thinking of things like this: https://github.com/rust-lang/rust/pull/53407/files#diff-11c4698b7f13fd531c6a5a7b78f27d27R57

Update: oops, sorry I meant https://github.com/rust-lang/rust/pull/53407/files?utf8=%E2%9C%93&diff=split#diff-11c4698b7f13fd531c6a5a7b78f27d27R58