regression 1.50: Borrow checker reports function param as not living long enough

@rustbot

This might not actually be a regression an instead the code may have previously erroneously compiled. I cannot find an an open issue obviously related to this though.

As you can see here this code does not compile with a "borrowed value does not live long enough" where it did previously. This comes from a call to diff::diff which has this signature.

The error message states that n may be dropped while the borrow in the next param state is still active. This fails to compile on both 1.50 and 1.49 if the param state is move to a local variable.

@rustbot modify labels: +regression-from-stable-to-beta

@rustbot

Let's try to reduce it @rustbot ping cleanup

@AminArria

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
instructions for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @camelid @chrissimpkins @contrun @DutchGhost @elshize @ethanboxx @h-michael @HallerPatrick @hdhoang @hellow554 @imtsuki @JamesPatrickGill @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @shekohex @sinato @smmalis37 @steffahn @Stupremee @tamuhey @turboladen @woshilapin @yerke

Still with dependency, but simplified code

use euca::diff;
use euca::dom::Dom;
use euca::vdom::DomIter;

#[test]
fn from_empty() {
    let new: Dom<(), (), &()> = Dom::elem("div");

    let n = new.dom_iter();
    let mut storage = vec![];
    let patch_set = diff::diff(std::iter::empty(), n, &mut storage);
}

let's see what I can get from there

@matthewjasper

searched nightlies: from nightly-2020-12-01 to nightly-2021-01-06
regressed nightly: nightly-2020-12-06
searched commits: from 3ff10e7 to e792288
regressed commit: 9122b76

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc 2021-01-06 --without-cargo --access github --regress error

The code I tested:

struct Dom;
impl Dom {
    fn dom_iter(&self) -> Box<dyn Iterator<Item = &()>> {
        todo!()
    }
}

fn diff<'a, M, O, N, S>(_: O, _: N, _: S)
where
    M: 'a,
    O: IntoIterator<Item = M>,
    N: IntoIterator<Item = &'a M>,
    S: IntoIterator<Item = &'a M>,
{
    todo!()
}

fn main() {
    let n = Dom.dom_iter();
    let storage = vec![];

    diff(std::iter::empty(), n, &storage);
}

cc #78373 @matthewjasper

Error: Label ICEBreaker-Cleanup-Crew can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

So #78373 resulted in MIR having one fewer block, which is making borrowck be less precise with how long the borrow lasts. There are a few possible fixes here for me to look at.

Assigning P-high as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

Hmm, that example compiles on beta though.

I believe that's an unrelated issue. Opened #81460 to track it. Thanks for reporting it!

Fix backported to 1.50 and landed in 1.51, closing this.

Only remove unreachable blocks

Auto merge of rust-lang#125923 - matthewjasper:no-return-leak, r=<try>

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regression 1.50: Borrow checker reports function param as not living long enough #80949

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regression 1.50: Borrow checker reports function param as not living long enough #80949

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apiraino commented on Jan 13, 2021

rustbot commented on Jan 13, 2021

hellow554 commented on Jan 13, 2021

hellow554 commented on Jan 13, 2021

rustbot commented on Jan 13, 2021

matthewjasper commented on Jan 14, 2021

apiraino commented on Jan 14, 2021

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camelid commented on Jan 28, 2021

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emilyalbini commented on Feb 8, 2021

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