Issue #1133: ADR-views-2 §6, left for later: mut on one name of a tuple pattern, and a map’s values written in place (NK1307)

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note · opened 2026-11-01 · updated 2026-11-01 · 0 comments

Moved here from docs/selfhosting-gaps.md, which is being deleted. Tracks two follow-ups that ADR-views-2 §6 leaves open. The owner deferred them on 2026-10-29 to keep the pace; they are not refused on purpose.

Probes (develop, 2026-11-01)

fn main() {
    let (a, mut b) = (1, 2)
    b += a
    println(f"{b}")
}

Result: NK1307, mut in front of one name of a tuple pattern isn’t supported yet.

use std::collections
fn main() {
    let mut m: collections::BTreeMap[String, i64] = collections::BTreeMap()
    m.insert("a", 1)
    for mut v in m.values_mut() { v += 1 }
    println(f"{m["a"] ?? 0}")
}

Result: NK1171, collections::BTreeMap has no method called values_mut.

What §6 already says

• When mut is allowed on one name of a tuple pattern, it means the same as let mut on that name alone (D1). The grammar already keeps the place for it.
• Map values written in place would be for (k, mut v) in m, lowered as m.iter_mut(), and for (_, mut v) in m, lowered as m.values_mut(). Both need the first point.
• Until then, a map’s value is changed by key, m[k], outside a loop over m.

When it comes back

The status changes to ready once a move needs it, or when the owner takes up the follow-up.

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