Broken MIR on HRTB

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c0293ace3f56fd3d1d3c79ed971331f3

UPDATE: MCVE is here: #71546 (comment)

Code:

use serde::Serializer;
use itertools::Itertools;

pub fn serialize_as_csv<V, S>(value: &V, serializer: S) -> Result<S::Ok, S::Error>
where
    V: 'static,
    for<'a> &'a V: IntoIterator,
    for<'a> <&'a V as IntoIterator>::Item: ToString + 'static,
    S: Serializer,
{
    let csv_str = value.into_iter().map(|elem| elem.to_string()).join(",");
    serializer.serialize_str(&csv_str)
}

Crashes compiler with

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: broken MIR in DefId(0:5 ~ playground[59fe]::serialize_as_csv[0]) (NoSolution): could not prove Binder(OutlivesPredicate(<&'a V as std::iter::IntoIterator>::Item, ReStatic))
  --> src/lib.rs:11:41
   |
11 |     let csv_str = value.into_iter().map(|elem| elem.to_string()).join(",");
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:355:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0 (4fb7144ed 2020-04-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: could not compile `playground`.

To learn more, run the command again with --verbose.

@rustbot

Would be nice to figure out if this is a regression or not and if it is when has this regressed.

@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 @chrissimpkins @contrun @DutchGhost @elshize @ethanboxx @h-michael @HallerPatrick @hdhoang @hellow554 @imtsuki @jakevossen5 @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

It has been present since at least 2019-08-01 according to bisect-rustc

Simplified code to remove dependencies

pub fn serialize_as_csv<V>(value: &V) -> Result<String, &str>
where
    V: 'static,
    for<'a> &'a V: IntoIterator,
    for<'a> <&'a V as IntoIterator>::Item: ToString + 'static
{
    let csv_str: String = value.into_iter().map(|elem| elem.to_string()).collect::<String>();
    Ok(csv_str)
}

This compiles for 1.20.0 and not for 1.30.0 (setting edition to 2015)

I'll try and pinpoint at least the version where it causes the ICE

From what I can tell the first time this ICE would happen is in 1.24.0

~/r/i71546> cargo +1.24.0 build
   Compiling i71546 v0.1.0 (file:///Users/aminarria/rust-cleanup/i71546)
error: internal compiler error: broken MIR in NodeId(4) (""): errors selecting obligation: [FulfillmentError(Obligation(predicate=Binder(OutlivesPredicate(<&'a V as std::iter::IntoIterator>::Item, ReStatic)),depth=0),Unimplemented)]
 --> src/lib.rs:7:49
  |
7 |     let csv_str: String = value.into_iter().map(|elem| elem.to_string()).collect::<String>();
  |                                                 ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: Could not compile `i71546`.

Assigning P-medium as discussed as part of the Prioritization WG procedure

Re-add rust-lang/rust#71546 with another MCVE

Merge pull request #452 from JohnTitor/revive-71546

Rollup merge of rust-lang#74889 - JohnTitor:hrtb-tests, r=nikomatsakis

Auto merge of rust-lang#74889 - JohnTitor:hrtb-tests, r=nikomatsakis

No longer ICEs since ff23ad3, likely from #91321

Add regression test for rust-lang#71546

Rollup merge of rust-lang#97580 - JohnTitor:issue-71546, r=compiler-e…

Auto merge of rust-lang#97622 - JohnTitor:rollup-4qoxrjn, r=JohnTitor

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Broken MIR on HRTB #71546

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spastorino commented on Apr 25, 2020

rustbot commented on Apr 25, 2020

chrissimpkins commented on Apr 25, 2020

AminArria commented on Apr 25, 2020

AminArria commented on Apr 25, 2020

LeSeulArtichaut commented on Apr 26, 2020

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Alexendoo commented on Dec 4, 2021

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