Use of uninitialized variable after pattern error

[matthewjasper]

The following code reports a warning that x is uninitialized as well as the expected pattern error.

fn destructure_slice(s: &[i32]) {
    let [x] = *s;
    drop(x);
}

playground

[Pulkit07]

I will like to work on this bug.

On initial findings, the initial pattern error is coming from https://github.com/rust-lang/rust/blob/master/src/librustc_mir/hair/pattern/check_match.rs#L261.

My hypothesis is that because of the pattern error, we don't add x to a list of initialized variables and hence we get the uninitialized error.

Does that sounds like a good start?

[matthewjasper]

KindThe place to start looking would be here:

rust/src/librustc_mir/build/matches/mod.rs

Lines 417 to 470 in 472211a

	pub fn place_into_pattern(
	&mut self,
	block: BasicBlock,
	irrefutable_pat: Pattern<'tcx>,
	initializer: &Place<'tcx>,
	set_match_place: bool,
	) -> BlockAnd<()> {
	// create a dummy candidate
	let mut candidate = Candidate {
	span: irrefutable_pat.span,
	match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
	bindings: vec![],
	ascriptions: vec![],
	// since we don't call `match_candidates`, next fields are unused
	otherwise_block: None,
	pre_binding_block: block,
	next_candidate_pre_binding_block: block,
	};
	// Simplify the candidate. Since the pattern is irrefutable, this should
	// always convert all match-pairs into bindings.
	self.simplify_candidate(&mut candidate);
	if !candidate.match_pairs.is_empty() {
	// ICE if no other errors have been emitted. This used to be a hard error that wouldn't
	// be reached because `hair::pattern::check_match::check_match` wouldn't have let the
	// compiler continue. In our tests this is only ever hit by
	// `ui/consts/const-match-check.rs` with `--cfg eval1`, and that file already generates
	// a different error before hand.
	self.hir.tcx().sess.delay_span_bug(
	candidate.match_pairs[0].pattern.span,
	&format!(
	"match pairs {:?} remaining after simplifying irrefutable pattern",
	candidate.match_pairs,
	),
	);
	}
	// for matches and function arguments, the place that is being matched
	// can be set when creating the variables. But the place for
	// let PATTERN = ... might not even exist until we do the assignment.
	// so we set it here instead
	if set_match_place {
	for binding in &candidate.bindings {
	let local = self.var_local_id(binding.var_id, OutsideGuard);
	if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
	opt_match_place: Some((ref mut match_place, _)),
	..
	}))) = self.local_decls[local].is_user_variable
	{
	*match_place = Some(initializer.clone());
	} else {

You'll need to use the functions in rustc_mir::build::matches::test to extract any bindings from the match_pairs.

[Centril]

@matthewjasper We now have two errors instead... is there anything more to do here, e.g. improving diagnostics?