Span is too large for unused rest @ .. pattern warning

[camelid]

The compiler suggests replacing rest @ .. with _rest, when in fact it should suggest _rest @ ..:

fn main() {
    let [rest @ ..] = [1, 2, 3];
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `rest`
 --> src/main.rs:2:10
  |
2 |     let [rest @ ..] = [1, 2, 3];
  |          ^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_rest`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: 1 warning emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.73s
     Running `target/debug/playground`

The span should point to just rest.

[camelid]

This is where the suggestion is emitted:

rust/compiler/rustc_passes/src/liveness.rs

Lines 1536 to 1583 in 4d0dd02

	self.ir.tcx.struct_span_lint_hir(
	lint::builtin::UNUSED_VARIABLES,
	first_hir_id,
	hir_ids_and_spans.iter().map(|(_, sp)| *sp).collect::<Vec<_>>(),
	|lint| {
	let mut err = lint.build(&format!("unused variable: `{}`", name));
	let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
	hir_ids_and_spans.into_iter().partition(|(hir_id, span)| {
	let var = self.variable(*hir_id, *span);
	self.ir.variable_is_shorthand(var)
	});
	let mut shorthands = shorthands
	.into_iter()
	.map(|(_, span)| (span, format!("{}: _", name)))
	.collect::<Vec<_>>();
	// If we have both shorthand and non-shorthand, prefer the "try ignoring
	// the field" message, and suggest `_` for the non-shorthands. If we only
	// have non-shorthand, then prefix with an underscore instead.
	if !shorthands.is_empty() {
	shorthands.extend(
	non_shorthands
	.into_iter()
	.map(|(_, span)| (span, "_".to_string()))
	.collect::<Vec<_>>(),
	);
	err.multipart_suggestion(
	"try ignoring the field",
	shorthands,
	Applicability::MachineApplicable,
	);
	} else {
	err.multipart_suggestion(
	"if this is intentional, prefix it with an underscore",
	non_shorthands
	.into_iter()
	.map(|(_, span)| (span, format!("_{}", name)))
	.collect::<Vec<_>>(),
	Applicability::MachineApplicable,
	);
	}
	err.emit()
	},
	);

However, I think the issue is in the hir_ids_and_spans argument. That argument is passed in here:

rust/compiler/rustc_passes/src/liveness.rs

Lines 1503 to 1511 in 4d0dd02

	for (_, (ln, var, hir_ids_and_spans)) in vars {
	if self.used_on_entry(ln, var) {
	let id = hir_ids_and_spans[0].0;
	let spans = hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect();
	on_used_on_entry(spans, id, ln, var);
	} else {
	self.report_unused(hir_ids_and_spans, ln, var);
	}
	}

You might have to hunt around a bit to find the spot where the spans are created.

[camelid]

Feel free to ask in #t-compiler/help on Zulip if you have any questions! You may also want to look through the rustc-dev-guide.

[estebank]

For working on this you won't need stage 2, stage 1 will suffice.

[sladyn98]

I guess I can give this issue a shot I have the setup ready and stage one works fine. I am new to rust so forgive me for asking too many questions

[SkiFire13]

Since it has been more than 2 weeks since the last update I guess this is free to pick? In case someone is still working on this let me know

@rustbot claim