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Brief: #881, a String local that starts as “” and is assigned later is lowered into __keep_frame

Score: bugs lane (docs/priorities.md, bugs before moving): a checked program rustc refuses; found by rs2nika (#877), same family as #880 (tools/buffers.nika origins).

Issue text (#881, copied)

Probe (run on c7b90a1, before fef8e78):

fn tail(head: ref String) -> String {
    let mut st: String = (head.trim().strip_prefix("impl") ?? "").clone()
    if st.starts_with("<") {
        let mut after: String = ""
        for ch in st.chars() {
            after.push(ch)
        }
        st = after
    }
    let f = st.rfind(" for ") ?? 0
    if f > 0 {
        st = st[f + " for ".len()..].clone()
    }
    let mut name: String = ""
    for ch in st.trim().chars() {
        if ch == ' ' {
            return name
        }
        name.push(ch)
    }
    return name
}

fn main() {
    print(tail("impl<T> Foo<T> {"))
}

Seen: the local after is lowered as __keep_frame.put(..), and st = after fails in rustc with “Mismatched types”. Expected: after is an owned String, st = after is a move; the program prints what Nikaia means (the issue says Foo<T>; check by reading the code, as b880 found its expected output was a slip).

Locations

To build

  1. Run the probe on today’s develop. If it builds and prints the right line, the result is still done: do step 3 (the test pins it), and say under Tests: that it did not reproduce; commit Fixes #881.
  2. Otherwise find why after gets origins (a "" start, push of a char from st.chars(), the move st = after) and fix it in buffers.nika (then lower-std, commit the lowered file; docs/builder-facts.md). Probe also: without the loop (after = "x" then st = after), and with after.push_str(st.trim()).
  3. Test tests/compiler/tests/string_empty_then_moved.nika with its .stdout (the probe). Commit ... (Fixes #881).

Done when

All steps; nikaia test tests/compiler --both-settings; it filters keep, view, move, and the guard set of .claude/agents/builder.md. Changelog: one line if the compiler changed, else none.

Stop and report if

The fix needs a language decision (what a lent binding’s move means), or an error in another agent’s file. #882 (a String parameter read through ?? counted as kept) is another lane’s: do not touch it.

Read docs/builder-facts.md.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/b881-empty-string-local.md.report.md)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word alone on that line: Result: done, never a summary after it, which cuts the batch, b869a 2026-10-27; Owner: is a question for the owner or none, never a role), Files, Commit, Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed. none where nothing, and then the bare word: Deviation: none (a note) cuts the batch (batch.nika compares exactly); a note goes into the commit message. Found: is only for a defect outside your change (it cuts the batch); how you solved your own step goes into the commit message. Deviation: is only a step of the brief built otherwise than written or not built (a choice the brief left to you, such as keep what X needs and say so, is no deviation: it goes into the commit message); how you verified (e.g. a test not run against the unfixed compiler) goes under Tests:. A noop says why under Tests: (what was probed), not under Deviation:. Final answer, one line: done <sha>, report <path> (or BLOCKED: ... / PUSH REJECTED ...).

Attribution: Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01GFeMqxRbtUMiGVyDMxsvEq

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