Adds `write_message::encode_to_bytes_mut` (and the timestamped variant)
returning a freshly-allocated `BytesMut`. Allocation count is identical
to `encode` (2 allocs/op for fixed-width scalars); the benefit is
downstream — consumers can `BytesMut::split_to` / `freeze` and forward
the body bytes to a wire-level sink without an intermediate copy.
The body builders (`encode_boolean` / `encode_fixed` / `encode_variable`
/ `encode_array`) were refactored to fill a pre-sized `&mut [u8]`
rather than each allocating their own `Vec<u8>`. The dispatcher
computes the body size up front via small `*_body_size` helpers and
resizes the destination buffer (Vec or BytesMut) once. This is also
the prerequisite refactor for F52.3.
Bench delta in `design/M6-bench-baseline.md` § F52.1; existing
`encode` row unchanged at 2 allocs/op. All 265 round-trip tests
unchanged.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hand-rolled GlobalAlloc wrapper around System that tracks allocs +
bytes + deallocs via two atomics. Each scenario runs 10k iterations
after a 1k warm-up; output is a markdown table with allocs/op,
bytes/op, deallocs/op.
Why hand-rolled (not dhat/criterion): R12 gates on a single number
("< 5 allocs/write"). dhat is heap-profiling-oriented (call-stack
attribution, JSON snapshots); criterion measures wall-clock latency
which is reported-but-not-gated per 60-roadmap.md:104. A 50-line
GlobalAlloc + atomic counters is the simplest thing that answers
the gate.
Run: `cargo bench -p mxaccess-codec`
Baseline numbers (release, Windows x64):
- Bool write: 1.00 allocs/op
- Int32 write: 2.00 allocs/op
- Float32 write: 2.00 allocs/op
- Float64 write: 2.00 allocs/op
- String write: 4.00 allocs/op (5-char string)
- Handle from_names: 2.00 allocs/op
- DataUpdate decode: 1.00 alloc/op
R12's < 5 allocs/write target is **already met** across the proven
matrix without any zero-copy work. The bench gates on this — any
write_message::encode scenario at >= 5 allocs/op exits the harness
with code 1.
Companion: `design/M6-bench-baseline.md` documents the numbers,
explains the per-scenario breakdown, and tightens F39's scope from
"hit the target" to "nice-to-have optimisations" (BytesMut output
buffer, name-signature cache, session-level scratch pool).
Workspace: 759 tests still pass; clippy --benches clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty