Lifetime error when indexing with borrowed index in beta but not in stable

[rodrimati1992]

I tried to compile this code:

use std::mem;
use std::ops::{Index, IndexMut};

pub struct FieldMap;

pub fn insert(this: &mut FieldMap, field: &Field<'_>, value: ()) -> () {
    this[field] = value;
}

pub struct Field<'a> {
    x: &'a (),
}

impl<'a> Index<&'a Field<'a>> for FieldMap {
    type Output = ();

    fn index(&self, _: &'a Field<'a>) -> &() {
        unimplemented!()
    }
}

impl<'a> IndexMut<&'a Field<'a>> for FieldMap {
    fn index_mut(&mut self, _: &'a Field<'a>) -> &mut () {
        unimplemented!()
    }
}

I expected the code to compile

Instead, it failed to compile with this lifetime error:

error[E0623]: lifetime mismatch
 --> src/lib.rs:7:5
  |
6 | pub fn insert(this: &mut FieldMap, field: &Field<'_>, value: ()) -> () {
  |                                           ----------
  |                                           |
  |                                           these two types are declared with different lifetimes...
7 |     this[field] = value;
  |     ^^^^^^^^^^^ ...but data from `field` flows into `field` here

Workarounds

If I change the function in these ways it compiles.

https://play.rust-lang.org/?version=beta&mode=debug&edition=2018&gist=ca6b8a2243d7116cb8180bb4300f6edc

pub fn insert<'a,'b,'c>(this: &'a mut FieldMap, field: &'b Field<'c>, value: ()) -> () {
    this[field as &'b Field<'b>] = value;
}

https://play.rust-lang.org/?version=beta&mode=debug&edition=2018&gist=21326b3dcd6c017d0e644ef7eab89769

pub fn insert<'a,'b,'c>(this: &'a mut FieldMap, field: &'b Field<'c>, value: ()) -> () {
    *this.index_mut(field) = value;
}

Meta

This compiles successfully on Stable Rust 1.45.0 back to 1.26.0 (when '_ lifetimes were stabilized)

This fails to compile on these versions(haven't tried previous ones in the same channels):

• 1.46.0-beta.2 (2020-07-23 6f95990)
• 1.47.0-nightly (2020-07-28 a7eff79)

This issue has been assigned to @nbdd0121 via this comment.

[LeSeulArtichaut]

Let’s find the culprit for this. @rustbot ping cleanup

[rustbot]

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 @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @senden9 @shekohex @sinato @spastorino @turboladen @woshilapin @yerke

[spastorino]

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

[spastorino]

searched nightlies: from nightly-2020-06-19 to nightly-2020-07-11
regressed nightly: nightly-2020-06-20
searched commits: from https://github.com/rust-lang/rust/commit/e55d3f9c5213fe1a25366450127bdff67ad1eca2 to https://github.com/rust-lang/rust/commit/2d8bd9b74dc0cf06d881bac645698ccbcf9d9c5e
regressed commit: https://github.com/rust-lang/rust/commit/72417d84fb51495a4f1d007fb2397a0b2609ab63

Regressed in #73504 which is a roll-up, likely #72280