Checkpoint 4: Build Variable-Width Rows Transactionally

Checkpoint 3 can lift a scalar function when one owned value represents an output row. A string row is different: its UTF-8 bytes, terminal offset, and validity bit must become visible together. This checkpoint makes that publication boundary explicit.

Start from your completed Checkpoint 3 workspace, copy the cumulative tests, and run the focused test once before editing:

cargo x copy-test --chapter 4
cargo test -p type-exercise-starter-supplied-tests chapter_4 --locked

The first run should fail on the missing writer surface. Shape specialization and runtime expression types are not needed for this change.

Consume the only unpublished handle

In core/src/array/string_array.rs, add Writer<'a> and WriterUsed<'a> around a borrowed StringArrayBuilder. The transition has this shape:

impl<'a> Writer<'a> {
    pub fn write(
        self,
        write: impl FnOnce(&mut StringValueWriter<'_>),
    ) -> WriterUsed<'a>;
}

The closure may append several borrowed fragments. A successful call then commits one terminal offset and one true validity bit. A null row appends no bytes, repeats the terminal offset, and commits one false bit. If a fallible builder callback stops before publication, truncate the bytes back to their starting length and leave offsets and validity unchanged.

Consuming Writer prevents a scalar callback from skipping publication or publishing twice. The core evaluator recovers the builder from WriterUsed only after that row is complete, then begins the next row.

Lift one borrowed string operation

Add evaluate_writer_binary to core/src/expression.rs. It validates two String inputs and their lengths before reading a row, converts both inputs to typed borrowed views once, and owns the only batch loop. For a non-null pair it passes &str, &str, and a fresh Writer to the callback. For a null pair it publishes one null directly.

Keep this boundary in core. The supplied test passes its borrowed concatenation callback directly, so the concrete String facade is not needed yet.

The callback can concatenate two borrowed strings without allocating a temporary String:

|left, right, writer| writer.write(|value| {
    value.push_str(left);
    value.push_str(right);
})

Run the focused and cumulative contracts:

cargo test -p type-exercise-starter-supplied-tests chapter_4 --locked
cargo test -p type-exercise-starter-supplied-tests --lib --locked

The two Checkpoint 4 tests distinguish empty from null strings, pin bytes and offsets, and prove failed writes do not leak partial bytes. Together with Checkpoints 1–3, the cumulative suite has 14 tests. With publication now transactional, the next checkpoint can change the loop shape without changing its visible results.