Something funky crashes typeck (stable and nightly)

[mental32]

Code

type A = ();

pub struct B(());

enum Thing {}

type FnAlias<R = B> = fn(A, &[B], R);

impl From<FnAlias<()>> for Thing {
    fn from(_: FnAlias) -> Self {}
}

Meta

rustc --version --verbose:

rustc 1.58.0-nightly (e249ce6b2 2021-10-30)
binary: rustc
commit-hash: e249ce6b2345587d6e11052779c86adbad626dff
commit-date: 2021-10-30
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

Also get this using stable:

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-unknown-linux-gnu
release: 1.56.0
LLVM version: 13.0.0

You can see it on the playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3249dfb3850d684ed13b10623e3dfa60

Error output

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', compiler/rustc_typeck/src/check/compare_method.rs:485:31
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.56.0 (09c42c458 2021-10-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [check_mod_item_types] checking item types in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack

Backtrace

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', compiler/rustc_typeck/src/check/compare_method.rs:485:31
stack backtrace:
   0: rust_begin_unwind
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/panicking.rs:101:14
   2: core::panicking::panic
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/panicking.rs:50:5
   3: rustc_typeck::check::compare_method::extract_spans_for_error_reporting
   4: rustc_infer::infer::InferCtxtBuilder::enter
   5: rustc_typeck::check::compare_method::compare_impl_method
   6: rustc_typeck::check::check::check_item_type
   7: rustc_middle::hir::map::Map::visit_item_likes_in_module
   8: rustc_typeck::check::check::check_mod_item_types
   9: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  10: rustc_data_structures::stack::ensure_sufficient_stack
  11: rustc_query_system::query::plumbing::force_query_with_job
  12: rustc_query_system::query::plumbing::get_query_impl
  13: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
  14: rustc_session::utils::<impl rustc_session::session::Session>::time
  15: rustc_typeck::check_crate
  16: rustc_interface::passes::analysis
  17: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  18: rustc_data_structures::stack::ensure_sufficient_stack
  19: rustc_query_system::query::plumbing::force_query_with_job
  20: rustc_query_system::query::plumbing::get_query_impl
  21: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  22: rustc_interface::passes::QueryContext::enter
  23: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  24: rustc_span::with_source_map
  25: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

[BlaCoiso]

Can reproduce even without the type aliases:

pub struct B(());

enum Thing {}

impl From<fn((), &[B], ())> for Thing {
    fn from(_: fn((), &[B], ()) -> Self {}
}

Edit: even smaller reproducer:

enum Thing {}

impl From<fn((), ())> for Thing {
    fn from(_: fn((), ()) -> Self {}
}

Produces this output: (rustc rustc_crash.rs)

error: mismatched closing delimiter: `}`
 --> rustc_crash.rs:6:12
  |
5 | impl From<fn((), &[B], ())> for Thing {
  |                                       - closing delimiter possibly meant for this
6 |     fn from(_: fn((), &[B], ()) -> Self {}
  |            ^ unclosed delimiter
7 | }
  | ^ mismatched closing delimiter

error: expected one of `!`, `(`, `)`, `,`, `::`, or `<`, found `{`
 --> rustc_crash.rs:6:12
  |
6 |     fn from(_: fn((), &[B], ()) -> Self {}
  |            ^                           -^
  |            |                           |
  |            unclosed delimiter          help: `)` may belong here

error: non-item in item list
 --> rustc_crash.rs:7:1
  |
5 | impl From<fn((), &[B], ())> for Thing {
  |                                       - item list starts here
6 |     fn from(_: fn((), &[B], ()) -> Self {}
7 | }
  | ^
  | |
  | non-item starts here
  | item list ends here

error[E0601]: `main` function not found in crate `rustc_crash`
 --> rustc_crash.rs:1:1
  |
1 | / pub struct B(());
2 | |
3 | | enum Thing {}
4 | |
5 | | impl From<fn((), &[B], ())> for Thing {
6 | |     fn from(_: fn((), &[B], ()) -> Self {}
7 | | }
  | |_^ consider adding a `main` function to `rustc_crash.rs`

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', compiler\rustc_typeck\src\check\compare_method.rs:485:31
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.56.0 (09c42c458 2021-10-18) running on x86_64-pc-windows-msvc

query stack during panic:
#0 [check_mod_item_types] checking item types in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0601`.

rustc --version --verbose:

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-pc-windows-msvc
release: 1.56.0
LLVM version: 13.0.0

[BlaCoiso]

After some testing, this can be reproduced since 1.53, might have been introduced in d43ede1

rust/compiler/rustc_typeck/src/check/compare_method.rs

Lines 410 to 418 in d43ede1

	match *terr {
	TypeError::ArgumentMutability(i) => {
	(impl_args.nth(i).unwrap(), trait_args.and_then(|mut args| args.nth(i)))
	}
	TypeError::ArgumentSorts(ExpectedFound { .. }, i) => {
	(impl_args.nth(i).unwrap(), trait_args.and_then(|mut args| args.nth(i)))
	}
	_ => (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)),
	}