[kixelated]

Problem

rust2-ts refused BTreeMap::extract_if and BTreeSet::extract_if (std 1.91, with a range). #547 landed the hash map's and set's; this finishes the B-tree's, part of the m3 "all of std" plan. No consumer calls them yet.

Approach

• rt.BTreeExtractIf keeps std's cursor: the leaf and index of the entry it reads next. A removal that rebalances only merges the leaf with one neighbor or takes from it. So the cursor follows without comparing keys: it moves into the previous leaf if this one merged into it, or shifts by what that leaf gave to this one's front.
• Matches std's ExtractIfInner:
  • It reads start_bound once, only for a map with a root (m.r).
  • It reads end_bound at each key, before the predicate, and compares by PartialOrd::le/lt. A key of the crate's own passes its own tests, as range passes eq/gt.
  • It never panics on a backwards range or one excluded at both ends.
  • It stops for good past the range's end, at the tree's end, or on a panic in the predicate or the bound, since the cursor is cleared before each call.
  • size_hint is (0, Some(len)) of the live map.
• The map's &mut V predicate gets a place where JS copies the value (place_call), so writes land on kept entries too.
• Debug writes ExtractIf { peek: .., .. }: the next entry, or a set's key, even past the range's end until it stops there, and None once stopped.
• Drop: the closure drops, then the range, as std's field order does. A range that drops is handed to the iterator through the new Ranged::held. rt.boundsOf now returns { startBound, endBound } so each bound can be read on its own. Before, it was one function that read both.
• Tests:
  • collections::btree_extracts (differential): maps and sets of up to 510 keys, every range form, partial reads, Debug ({:?} and {:#?}), and maps with no root.
  • Keys and values that log their drops and their le/lt, plus a closure and range that drop with the iterator.
  • A RangeBounds of the crate's own that logs each bound read.
  • tests/btree.ts against std (runtime.rs): extract_if over random ranges of a ~20k-entry tree, read part way, with the tree's shape checked after.
  • Mutation-checked: dropping either half of the cursor's move after a rebalance, ignoring le, swapping the drop order, reading the start of a rootless map, and peeking after a stop each fail the differential, and the first also fails btree.ts.

Impact

• New: rt.btreeExtractIf, rt.btreeExtractIfMut, rt.btreeExtractKeysIf, rt.fmtBTreeExtractIf.
• Breaking: rt.boundsOf returns a { startBound, endBound } record instead of a function (ts/MIGRATE.md). Generated code is unchanged.

Alternatives

• Flatten and rebuild, as retain does: O(n) per next, and wrong for a read-part-way iterator unless it rebuilds on drop. Rejected.
• Defer rebalancing until the iterator drops: leaves a malformed tree if it is forgotten (mem::forget). Rejected.
• Keep rt.boundsOf a function and call it at every key: that would read start_bound again at each key, which std doesn't do.

Follow-ups

• removeAt, which pop_first, pop_last, remove, and now extract_if share, still calls Ord::cmp to rebalance where std's removals call none. A B-tree's searches call Ord as std's do now names these removals in its Goal (5ca5a9b).
• The fixtures' nightly std (1.101) drops a whole multi-level B-tree in postorder (drop_tree), while the runtime drops in key order (stable 1.98.1's order). This was already the maintainer's decision of 2026-10-30, recorded in ts/README.md, so the new fixture drains its logged map in order. No new quest.

Review

quest review: Codex, gpt-6.1-sol, medium effort. Round 3, head d724cc2: no findings.

• Round 1: removal calls Ord::cmp, unlike std's. Refuted as the existing removeAt behavior (pop_first has it too), which the B-tree search-order quest now covers.
• Round 2: the cursor's re-find by key added calls of its own. Fixed in 7504ae8: the cursor follows a rebalance structurally, with no comparisons.

🤖 Generated with Claude Code

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Commits

Reviewing files that changed from the base of the PR and between 16de89b and d724cc2.

Files selected for processing: 18

• quest/m3/README.md
• quest/m3/ts-btree-extract-if.md
• quest/m3/ts-btree-search-order.md
• ts/MIGRATE.md
• ts/README.md
• ts/fixtures/lang/collections/expected/collections.Logged.ts
• ts/fixtures/lang/collections/expected/collections.Window.ts
• ts/fixtures/lang/collections/expected/collections.ts
• ts/fixtures/lang/collections/expected/harness.ts
• ts/fixtures/lang/collections/expected/lib.ts
• ts/fixtures/lang/collections/src/collections.rs
• ts/fixtures/lang/collections/src/lib.rs
• ts/runtime/rt/collections.ts
• ts/src/lower/collections.rs
• ts/src/lower/fmt.rs
• ts/src/lower/iter.rs
• ts/tests/btree.ts
• ts/tests/runtime.rs

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