`index out of bounds` error

[ehuss]

Tried to run cargo semver on the toml-rs crate and got an error:

~/Proj/rust/toml-rs> RUST_BACKTRACE=1 rustup run nightly-2020-12-20-x86_64-apple-darwin cargo semver
    Updating crates.io index
    Finished dev [unoptimized + debuginfo] target(s) in 0.82s
    Finished dev [unoptimized + debuginfo] target(s) in 0.83s
thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /rustc/1f5bc176b0e54a8e464704adcd7e571700207fe9/compiler/rustc_middle/src/ty/relate.rs:144:60
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_bounds_check
   3: <core::result::Result<T,E> as rustc_middle::ty::context::InternIteratorElement<T,R>>::intern_with
   4: <semverver::mismatch::MismatchRelation as rustc_middle::ty::relate::TypeRelation>::tys
   5: semverver::mismatch::MismatchRelation::process
   6: semverver::traverse::run_analysis
   7: rustc_interface::passes::QueryContext::enter
   8: <rust_semverver::main::SemverCallbacks as rustc_driver::Callbacks>::after_analysis
   9: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  10: rustc_span::with_source_map
  11: rustc_interface::interface::create_compiler_and_run
  12: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: rustc-semverver errored

Details:

• rust-semverver built from 0bdee60 using nightly-2020-12-20
• Test against a5dc85d0a38cc55033a367b0f24c7a8c23f314e6 of toml-rs (compared against 0.5.8)
• Running on x86_64-apple-darwin

[JohnTitor]

The relevant part is:

rust-semverver/src/mismatch.rs

Lines 214 to 226 in b494170

	(&TyKind::FnDef(a_def_id, a_substs), &TyKind::FnDef(b_def_id, b_substs)) => {
	if self.check_substs(a_substs, b_substs) {
	let a_sig = a.fn_sig(self.tcx);
	let b_sig = b.fn_sig(self.tcx);
	let _ = self.relate_item_substs(a_def_id, a_substs, b_substs)?;
	let _ = self.relate(a_sig.skip_binder(), b_sig.skip_binder())?;
	}
	let a = Res::Def(DefKind::Fn, a_def_id);
	let b = Res::Def(DefKind::Fn, b_def_id);
	Some((a, b))
	}

We should filter them somehow not to relate.

[dario23]

so adding && !self.tcx.variances_of(a_def_id).is_empty() to the check in line 215 seems to work around this, but i'm not sure of the wider context, i.e. are we hiding differences by ignoring this? it also doesn't make cargo run --bin cargo-semver -- -S toml:0.5.8 -C toml:0.5.9 succeed, but it runs until an error like #216 / #296 with that.

@JohnTitor do you have any input/thoughts on what/how to filter? in my tests it failed on token::Token enum variants with fields with lifetimes, e.g. Token::Whitespace. i noticed that the debug output had entries for the variant/field, which has an empty variances list and made us fail previously, and a separate one for the variant constructor, with variance like a fn(&'a str) -> Token<'a> function.

[JohnTitor]

Thanks for investigating but no, I don't. So, that is a deep problem I think, the code here is not actively developed but passive-maintained, so (I assume) some of the code doesn't cooperate with the current rustc code nicely. We have to take a closer look at the code here, and it might be required to re-think the structure itself. It'll need some dedicated time but I don't think I can make it right now, sorry.

[dario23]

would you be willing to merge a workaround and leave the issue open?

[JohnTitor]

So, currently things don't work on that condition anyway, right? Then it seems fine to me :)