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b41: fn(mut S) in a function type (#1134)

The owner chose option A on #1134; Part I says it since 67614ebd (a function type keeps a parameter’s mut: the argument is lent as to a named function’s mut parameter). Build it. Read #1134 (probe and To build) first.

Probe (today: a parse error at fn(mut)

struct S { pub n: i64 }
impl S {
    fn apply(ref mut self, f: fn(mut S)) {
        f(self)
        self.n += 1
    }
}
fn bump(mut s: S) { s.n += 10 }
fn main() {
    let mut s = S { n: 1 }
    s.apply(bump)
    s.apply(fn(mut t) { t.n += 100 })
    println(f"{s.n}")
}

Expected output: 112.

Steps

  1. Parser and types: mut before a parameter type in a function type. A named fn f(mut s: S) and a lambda fn(mut t) { … } fit fn(mut S); fn(S) and fn(mut S) are different types (a function without the mut does not fit and the refusal says which parameter).
  2. Checker and lowering: the argument at the call is lent as mut, the same rule as a mut parameter of a named function; lowered as impl FnMut(&mut S) for an immediate parameter, Box<dyn FnMut(&mut S)> for a kept one. A test tests/compiler/fn_type_mut_param.nika with .stdout (the probe, 112) and one refusal with .stderr. Commit with Fixes #1134.

Where the change sits in a tools/*.nika file, change the .nika and re-lower; new Rust only where the existing Rust code cannot be converted (CLAUDE.md, Written in Nikaia only), else stop and report blocked.

Rules (CLAUDE.md; shared checkout)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/bugs/b41-1134-fn-mut-param-type.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, Body (optional, one paragraph for the commit message), Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed, and for a move Rs2nika: (the kept N of M lines of rs2nika --score, then the three costliest hand edits; none for a task that moves no Rust). 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); a converter draft that came out wrong is no Found:, it is one of the costliest edits under Rs2nika:, which the converter lane reads (e12-4b 2026-10-27); 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; nor is building two steps in one commit when every step is built, e12-3a 2026-10-27, nor a step another commit on develop already built, shown by your test failing with that commit’s change reverted, b889 2026-10-27: name the commit in Body:); 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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