Encountering the type length limit ICEs Miri

[saethlin]

The following program ICEs Miri when invoked with -Zenforce-type-length-limit:

#![type_length_limit = "8"]
fn main() {}

Miri reports:

Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
note: the place in the program where the ICE was triggered
   --> /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:140:5
    |
140 |     panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) }).map_err(rt_abort)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: BACKTRACE:
    = note: inside `std::rt::lang_start_internal` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:140:5: 140:70
    = note: inside `std::rt::lang_start::<()>` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:158:17: 163:6

error: internal compiler error: src/tools/miri/src/diagnostics.rs:324:17: This error should be impossible in Miri: encountered overly generic constant

thread 'rustc' panicked at src/tools/miri/src/diagnostics.rs:324:17:
<backtrace to Miri's error reporting code, not relevant>

I'm filing this here because I'm pretty sure the problem is that the core interpreter logic does not handle the new failure path out of Instance::try_resolve correctly.

rustc 1.81.0-nightly (c6727fc9b 2024-07-12)
binary: rustc
commit-hash: c6727fc9b5c64cefa7263486497ee95e529bd0f8
commit-date: 2024-07-12
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

[RalfJung]

Seems like this leads to a TooGeneric?

error: internal compiler error: src/tools/miri/src/diagnostics.rs:324:17: This error should be impossible in Miri: encountered overly generic constant

Miri bails out on TooGeneric since it knows everything is monomorphized. (codegen does the same.) Does the type length limit cause TooGeneric errors? That would IMO be a wrong use of that error code, precisely because we have users that know that things are not generic any more so we should be able to distinguish errors they can see (like this type length limit) and errors they cannot see (like failing to compute the layout of a type due to generics).

[tiif]

I could reproduce this on:

rustc 1.81.0-nightly (5315cbe15 2024-07-11)
binary: rustc
commit-hash: 5315cbe15b79533f380bbb6685aa5480d5ff4ef5
commit-date: 2024-07-11
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

but could no longer reproduce on:

rustc 1.85.0-nightly (dff3e7ccd 2024-11-26)
binary: rustc
commit-hash: dff3e7ccd4a18958c938136c4ccdc853fcc86194
commit-date: 2024-11-26
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.4

Is this resolved?

[saethlin]

Sounds resolved to me. If I manage to trip this again I'll file a new issue. Thanks for the gardening!

[theemathas]

I can still reproduce the issue with the command RUSTFLAGS="-Zenforce-type-length-limit" cargo +nightly miri run

See #127670

Maybe this should be reopened?

[tiif]

Ah makes sense. I didn't use -Zenforce-type-length-limit.

[RalfJung]

Cc @compiler-errors