ICE: OutputTypeParameterMismatch

[dwhjames]

Meta

We have a weekly CI job with a build matrix of Rust versions

• 1.65.0
• stable
• beta
• nightly

The last passing build was 2023-10-24. The following is a chronology.

---

2023-10-24
All rust versions PASS ✅

'1.65.0-x86_64-unknown-linux-gnu'
latest update on 2022-11-03, rust version 1.65.0 (897e375 2022-11-02)

'stable-x86_64-unknown-linux-gnu'
latest update on 2023-10-05, rust version 1.73.0 (cc66ad4 2023-10-03)

'beta-x86_64-unknown-linux-gnu'
latest update on 2023-10-21, rust version 1.74.0-beta.4 (489647f 2023-10-21)

'nightly-x86_64-unknown-linux-gnu'
latest update on 2023-10-24, rust version 1.75.0-nightly (cd674d6 2023-10-24)

---

2023-10-31

PASS for 1.65.0, stable, and beta ✅

'nightly-x86_64-unknown-linux-gnu'
(exact version already GC’ed from CI systems but, reproduced with 2023-10-30 nightly)
FAIL ❌

---

2023-11-07

PASS for 1.65.0 and stable ✅

'beta-x86_64-unknown-linux-gnu'
latest update on 2023-10-21, rust version 1.74.0-beta.4 (489647f 2023-10-21)
PASS ✅

'nightly-x86_64-unknown-linux-gnu'
latest update on 2023-11-07, rust version 1.75.0-nightly (189d6c7 2023-11-06)
FAIL ❌

---

2023-11-14

PASS for 1.65.0 and stable ✅

'nightly-x86_64-unknown-linux-gnu'
latest update on 2023-11-14, rust version 1.76.0-nightly (ba7c7a3 2023-11-13)
PASS ✅

'beta-x86_64-unknown-linux-gnu'
latest update on 2023-11-13, rust version 1.75.0-beta.1 (782883f 2023-11-12)
FAIL ❌

---

Summary:

• 1.75.0-nightly did succeed on 2023-10-24
• 1.75.0-nightly was first observed to fail on 2023-10-31
• 1.76.0-nightly now succeeds (2023-11-14)
• 1.75.0-beta.1 was first observed to fail on 2023-11-14

Error output

Backtrace

error: internal compiler error: compiler/rustc_traits/src/codegen.rs:43:13: Encountered error `
    OutputTypeParameterMismatch(
      SelectionOutputTypeParameterMismatch {
        found_trait_ref: Binder { value: <fn(<round_digest::RoundDigestTag<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2> as mpc_signing::utils::VerifyDeserialize>::VerifyError) -> ds::VerifyError<Dps, Ds> {ds::VerifyError::<Dps, Ds>::Round2} as std::ops::FnOnce<(<round_digest::RoundDigestTag<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2> as mpc_signing::utils::VerifyDeserialize>::VerifyError,)>>, bound_vars: [] },
        expected_trait_ref: Binder { value: <fn(<round_digest::RoundDigestTag<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2> as mpc_signing::utils::VerifyDeserialize>::VerifyError) -> ds::VerifyError<Dps, Ds> {ds::VerifyError::<Dps, Ds>::Round2} as std::ops::FnOnce<(round_digest::VerifyError<<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2 as mpc_signing::utils::VerifyDeserialize>::VerifyError>,)>>, bound_vars: [] },
        terr: Sorts(ExpectedFound {
          expected: round_digest::VerifyError<Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Dps/#0], def_id: DefId(22:4767 ~ mpc_signing[4ddb]::protocols::cggmp21::dps_protocol::traits::DpsProtocol::SendAllReport2) })], def_id: DefId(22:3108 ~ mpc_signing[4ddb]::utils::zkp_stream::VerifyDeserialize::VerifyError) })>,
          found: Alias(Projection, AliasTy { args: [round_digest::RoundDigestTag<Alias(Projection, AliasTy { args: [Dps/#0], def_id: DefId(22:4767 ~ mpc_signing[4ddb]::protocols::cggmp21::dps_protocol::traits::DpsProtocol::SendAllReport2) })>], def_id: DefId(22:3108 ~ mpc_signing[4ddb]::utils::zkp_stream::VerifyDeserialize::VerifyError) }) }) }
    )` selecting `
      <fn(<round_digest::RoundDigestTag<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2> as mpc_signing::utils::VerifyDeserialize>::VerifyError)
        -> ds::VerifyError<Dps, Ds> {ds::VerifyError::<Dps, Ds>::Round2} as std::ops::FnOnce<(round_digest::VerifyError<<<Dps as mpc_signing::protocols::cggmp21::DpsProtocol>::SendAllReport2 as mpc_signing::utils::VerifyDeserialize>::VerifyError>,)>>
  ` during codegen

Capture of Nightly failure (2023-10-31) ❌
rustc-ice-2023-10-31T16_49_05-19815.txt

Capture of Beta failure (2023-11-15) ❌
rustc-ice-2023-11-15.txt

Code

From 2023-10-24 to today the source code has not changed. The code is not public, but could potentially be shared. Unfortunately, the code is complex due to the use of a declarative macro, so we do not currently have a minimization.

We would welcome some hints/suggestions about what code/context we could capture to improve this issue.

[lqd]

A bisection using cargo-bisect-rustc (documentation and tutorials are available here; also: use the --access github flag, otherwise it will clone this repo) would at least show the PRs that:

• caused the error on nightly, and that PR should now be on beta
• fixed the issue on nightly only

It's likely the PR you will find fixing the issue on nightly will also need to be backported to beta.

[dwhjames]

@lqd thanks for the directions. I will take a crack at that.

Could you help me understand the release process… will there be any further promotions of changes in nightly to beta? Or is beta ‘locked in’ and explicit action is required to address this issue in beta?
I am fine waiting for stabilization, but I’m obviously concerned about this issue eventually showing up in stable.

[lqd]

AFAICT beta has branched already and all further promotions will require explicit decisions and actions. But that’s extremely common and business as usual, there’s an entire section of the weekly compiler meeting dedicated to that.

I could be wrong but I don’t believe crater runs have been done for the next beta yet, and that issue may also show up there (it would if your project was on crates.io for example), but even if not: once more details are known about the severity of the issue, how common it can be, how simple the fix is to backport, and so on, all will be taken into account when these decisions are made.

This will all follow naturally once the PRs are identified.

[dwhjames]

searched nightlies: from nightly-2023-10-17 to nightly-2023-10-22
regressed nightly: nightly-2023-10-21
searched commit range: 4578435...249624b
regressed commit: 5cee4f3

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --start=2023-10-17 --end=2023-10-22 --access github --regress=ice --preserve --prompt -- build --release

[dwhjames]

Had a few false starts here until I figured out the … -- build --release to build in release mode (no ICE in debug mode).

[dwhjames]

run again with --regress=non-ice (as this failure PASSED in our weekly build on 2023-10-17 and 2023-10-24, so the failure flip flopped during that period)

---

searched nightlies: from nightly-2023-10-21 to nightly-2023-10-24
fixed nightly: nightly-2023-10-23
searched commit range: 1c05d50...54b0434
fixed commit: 54b0434

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --start=2023-10-21 --end=2023-10-24 --access github --regress=non-ice --preserve --prompt -- build --release

[dwhjames]

Further searching 2023-10-24 to 2023-10-31

…
looking for regression commit between 2023-10-25 and 2023-10-26
fetching (via remote github) commits from max(df871fbf053de3a855398964cd05fadbe91cf4fd, 2023-10-23) to ab5c841a1f3c09edc5ea07722519627c960aed17
ending github query because we found starting sha: df871fbf053de3a855398964cd05fadbe91cf4fd
get_commits_between returning commits, len: 13
…

checking the start range to verify it passes
installing df871fbf053de3a855398964cd05fadbe91cf4fd
…

ERROR: the commit at the start of the range (df871fbf053de3a855398964cd05fadbe91cf4fd) includes the regression

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --start=2023-10-24 --end=2023-10-31 --access github --regress=ice --preserve -- build --release--release

[lqd]

Kind of unexpected, the PR that fixed the issue is about cargo and shouldn’t change how the compiler is called or behaves. Try double checking maybe, preferably with a script looking for the exact ICE message here rather than with --prompt.

[dwhjames]

❯ rustup override set nightly-2023-11-14
❯ cargo build --release -p mpc-fsm
💥 

❯ rustup override set nightly-2023-11-15
❯ cargo build --release -p mpc-fsm
✅ 

❯ rustup override unset

Yet

cargo bisect-rustc --start=2023-11-14 --end=2023-11-15 --access github --regress=non-ice --preserve -- build --release

ICE at both start and end 🤔