[kixelated]

Problem

rust2-ts refused std functions that write what a &mut borrows when passed as a value, such as v.iter_mut().map(std::mem::take): "unsupported call to std::mem::take::<Vec<u8>>". mem::take, replace, swap, Option::replace, String::push/push_str, Clone::clone_from, u8::make_ascii_uppercase, and the rest of intercept's calls lower only from a MIR call, which has the place. A function value instead got a hand-written arrow over the library mappings, which lack them. The same gap hit f(place, value) inside a generic instantiated with such a function.

Approach

• A std function value is now an arrow over rustc's reify shim (_0 = f(move _1, ...)), lowered with Fx as any body. Its call goes through the same path as a written call, intercept and the &mut pointee handling included. The shim's spans are moved to the use site, and its arguments are named a0 on, as before.
• FnOnce::call_once of a mapped std FnDef that takes a &mut calls that value. Other std calls keep the inline call_def, so Option::map(x, From::from) stays x.
• This deletes the duplicated pointee logic from Body::fn_value. Constructors keep their arrow.
• A next keeps its call's Option: a shim of std's next no longer takes the item-or-rt.DONE protocol of next's own body, and the crate's own next as a value goes through the shim too instead of being its bare symbol. That bare symbol leaked rt.DONE before this PR.
• Differential test edits::values: take/replace/swap as values through map, a generic helper, and fn pointers, on bytes (places), objects written in place (Vec), and Strings. It also checks drop order of replaced values against native Rust, plus the other place-writing std calls above as values, and std's and the crate's own next as fn pointers called past the end.

Impact

• Translator output: these function values now translate. Arrow params and $hash suffixes in snapshots are renumbered where std instances' text changed. No public Rust API change.

Alternatives

• Expression-level duplicates of each intercepted call for the value path: rejected, as they would duplicate intercept and drift from it.
• Always calling the shim value from call_once: rejected, because it turns identity conversions in translated std generics into IIFEs.

Follow-ups

• Operator trait methods called by path, planned in moq-dev/rust2#481: <u8 as AddAssign>::add_assign and the other std::ops methods called by path, written or as a value.

Review

• Codex gpt-6.1-sol, medium effort, via quest review. Latest round 4 on head a4a2935b: no findings.
• Round 1, [P2] a reify shim of next returned rt.DONE: fixed in 422dc352, along with the same leak for the crate's own next as a value. Both are covered by the differential.
• Rounds 2 and 3: no findings. Round 4 covers the merges of main, including moq-dev/rust2#474's fixture split. edits::values now lives in ts/fixtures/lang/edits, and the only conflicts were the case registration and regenerated snapshots.

🤖 Generated with Claude Code

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Commits

Reviewing files that changed from the base of the PR and between 3d37327 and a4a2935.

Files selected for processing: 38

• quest/m3/README.md
• quest/m3/ts-take-value.md
• ts/README.md
• ts/fixtures/codec/expected/harness.ts
• ts/fixtures/lang/args/expected/args.ts
• ts/fixtures/lang/borrows/expected/borrows.ts
• ts/fixtures/lang/boxes/expected/boxes.ts
• ts/fixtures/lang/clones/expected/clones.ts
• ts/fixtures/lang/collections/expected/collections.ts
• ts/fixtures/lang/counts/expected/counts.ts
• ts/fixtures/lang/cows/expected/cows.ts
• ts/fixtures/lang/cycles/expected/cycles.ts
• ts/fixtures/lang/dyns/expected/dyns.ts
• ts/fixtures/lang/edits/expected/core/bool.ts
• ts/fixtures/lang/edits/expected/edits.Down.ts
• ts/fixtures/lang/edits/expected/edits.Logged.ts
• ts/fixtures/lang/edits/expected/edits.ts
• ts/fixtures/lang/edits/expected/lib.ts
• ts/fixtures/lang/edits/src/edits.rs
• ts/fixtures/lang/edits/src/lib.rs
• ts/fixtures/lang/errors/expected/errors.ts
• ts/fixtures/lang/ints/expected/lib.ts
• ts/fixtures/lang/iters/expected/iters.ts
• ts/fixtures/lang/joins/expected/joins.ts
• ts/fixtures/lang/locals/expected/locals.ts
• ts/fixtures/lang/objects/expected/objects.ts
• ts/fixtures/lang/orders/expected/orders.ts
• ts/fixtures/lang/refs/expected/refs.ts
• ts/fixtures/lang/returns/expected/returns.ts
• ts/fixtures/lang/sorts/expected/lib.ts
• ts/fixtures/lang/strs/expected/lib.ts
• ts/fixtures/lang/zeros/expected/lib.ts
• ts/fixtures/stock/expected/buf.ts
• ts/fixtures/stock/expected/harness.ts
• ts/fixtures/task/expected/drops/objects.ts
• ts/src/lower/body.rs
• ts/src/lower/mir/call.rs
• ts/src/lower/mir/mod.rs

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