NLL Regression: Conditional control flow returning from functions no longer works

[Osspial]

If a match expression has one arm that returns a borrowed matched value from its pattern, and another arm that doesn't borrow directly from a matched value but instead borrows the variable the matched value borrows and returns it, compilation fails if the match statement is returning from a function, but not if it's assigning to a variable. This used to work (and iirc one of the goals of NLLs was to make this pattern work), but it broke between nightly-2018-05-17 and nightly-2018-05-19. A nightly for May 18th doesn't appear to exist.

Examples:

This doesn't work now:

// Doesn't compile on nightly-2018-05-19 and later, but does on nightly-2018-05-17.
fn borrow(o: &mut Option<i32>) -> Option<&mut i32> {
    match o.as_mut() {
        Some(i) => Some(i),
        None => o.as_mut()
    }
}

This is similar to the other code, but does work both before and after nightly-2018-05-19

fn main() {
    let mut o: Option<i32> = Some(1i32);

    // Compiles everywhere!
    let x = match o.as_mut() {
        Some(i) => Some(i),
        None => o.as_mut()
    };
}

[Osspial]

Playground link: https://play.rust-lang.org/?gist=fed5dbd5df01cfd8448614f1b34e0cb0&version=nightly&mode=release

[Osspial]

More generally, this means that Problem Case 3 from the NLL RFC no longer works.

https://play.rust-lang.org/?gist=49970ae2c8f31d747f3dbbee90975c70&version=nightly&mode=debug

[shepmaster]

Oh, is this what we were talking about? Yes, this is intentional. Check out Niko's blog post:

We originally intended for NLL to accept examples like this: in the RFC, this was called Problem Case # 3. However, we had to remove that support because it was simply killing compilation times, and there were also cases where it wasn’t as precise as we wanted.

I’m happy to report that this problem is basically solved. Despite the increased precision, the Polonius analysis is now easily as fast as the existing Nightly analysis, thanks some smarter encoding of the rules as well as the move to use datafrog. We’ve not done detailed comparisons, but I consider this problem essentially solved.

If you’d like, you can try Polonius today using the -Zpolonius switch to Nightly. However, keep in mind that this would be a ‘pre-alpha’ state: there are still some known bugs that we have not prioritized fixing and so forth.

[Havvy]

This looks like a duplicate of #21906.