Trying to pattern-match field 1 of Box causes an ICE

[CAD97]

Code

fn main() {
    let Box { 1: _, .. }: Box<()>;
}

[playground]

Meta

rustc --version --verbose:

1.52.0-nightly

(2021-03-01 4f20caa6258d4c74ce6b)

Error output

thread 'rustc' panicked at 'index out of bounds: the len is 1 but the index is 1', compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs:1346:21

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/4f20caa6258d4c74ce6b316fd347e3efe81cf557/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/4f20caa6258d4c74ce6b316fd347e3efe81cf557/library/core/src/panicking.rs:92:14
   2: core::panicking::panic_bounds_check
             at /rustc/4f20caa6258d4c74ce6b316fd347e3efe81cf557/library/core/src/panicking.rs:69:5
   3: rustc_mir_build::thir::pattern::deconstruct_pat::Fields::replace_with_pattern_arguments
   4: rustc_mir_build::thir::pattern::usefulness::PatStack::pop_head_constructor
   5: rustc_mir_build::thir::pattern::usefulness::is_useful
   6: <core::iter::adapters::copied::Copied<I> as core::iter::traits::iterator::Iterator>::fold
   7: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
   8: rustc_mir_build::thir::pattern::usefulness::compute_match_usefulness
   9: rustc_mir_build::thir::pattern::check_match::MatchVisitor::check_irrefutable
  10: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_local
  11: rustc_hir::intravisit::walk_block
  12: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  13: rustc_mir_build::thir::pattern::check_match::check_match
  14: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  15: rustc_data_structures::stack::ensure_sufficient_stack
  16: rustc_query_system::query::plumbing::force_query_with_job
  17: rustc_query_system::query::plumbing::get_query_impl
  18: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_match
  19: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  20: rustc_session::utils::<impl rustc_session::session::Session>::time
  21: rustc_interface::passes::analysis
  22: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  23: rustc_data_structures::stack::ensure_sufficient_stack
  24: rustc_query_system::query::plumbing::force_query_with_job
  25: rustc_query_system::query::plumbing::get_query_impl
  26: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  27: rustc_interface::passes::QueryContext::enter
  28: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  29: rustc_span::with_source_map
  30: rustc_interface::interface::create_compiler_and_run
  31: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

query stack during panic:
#0 [check_match] match-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack

[CAD97]

Seems to be the indicated function:

rust/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

Lines 1329 to 1359 in ec7f258

	/// Overrides some of the fields with the provided patterns. This is used when a pattern
	/// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start
	/// with a `Fields` that is just one wildcard per field of the `Foo` struct, and override the
	/// entry corresponding to `field1` with the pattern `Some(_)`. This is also used for slice
	/// patterns for the same reason.
	fn replace_fields_indexed(
	&self,
	new_pats: impl IntoIterator<Item = (usize, &'p Pat<'tcx>)>,
	) -> Self {
	let mut fields = self.clone();
	if let Fields::Slice(pats) = fields {
	fields = Fields::Vec(pats.iter().collect());
	}
	match &mut fields {
	Fields::Vec(pats) => {
	for (i, pat) in new_pats {
	pats[i] = pat
	}
	}
	Fields::Filtered { fields, .. } => {
	for (i, pat) in new_pats {
	if let FilteredField::Kept(p) = &mut fields[i] {
	*p = pat
	}
	}
	}
	Fields::Slice(_) => unreachable!(),
	}
	fields
	}

[estebank]

1.49 didn't ICE.

[apiraino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

@rustbot label -I-prioritize +P-medium

[Nadrieril]

Oh this is ugly x) Reading the PR, the problem is that there are two ways to match a Box: via a box-pattern or via Box { .. }. But we can't know which will be used based on the type only. Current code assumes box-patterns, hence the error. The PR works because luckily all the fields of Box are private.
If you tried pattern-matching within the module that defines Box, you'd get crazy results like (actually wouldn't even typecheck):

match Box::new(true) {
    Box(true, _) => {}
    box true => {} //~ ERROR unreachable pattern
	_ => {}
}

match Box::new(true) {
    box true => {}
	box false => {}
	Box(_, _) => {} //~ ERROR unreachable pattern
}

We'll need to figure out a real solution when we implement generalized Deref patterns...

[estebank]

@Nadrieril in that case we should open a new ticket for that work, tagged something like F-deref_patterns or something.