## Memory reduction: fewer arena reallocations on JSX

`transform_jsx` built the `React.createElement` / jsx props object in a
**zero-capacity** arena vec
(`ctx.ast.vec()`), regrowing it (4 → 8 → 16 …) as it pushed one property
per attribute. The attribute
count is already known, so pre-sizing the vec to it removes those
regrowths — mirroring `arguments`,
pre-sized a few lines above, and the same pattern as #9915.

Measured with `cargo allocs` (`TestFiles::complicated`). **Arena
reallocations drop on every JSX file;
nothing else changes** (system allocs and the arena-alloc count are
essentially flat):

| file | arena reallocs: before → after | **reduction** |
| --- | --- | --- |
| App.tsx | 77 → 25 | **−52 (−68%)** |
| kitchen-sink.tsx | 517 → 304 | **−213 (−41%)** |
| RadixUIAdoptionSection.jsx | 24 → 13 | **−11 (−46%)** |

Each eliminated realloc is a buffer grow + memcpy of the existing
elements, so this is real
work removed on the JSX hot path, not just bytes.

<details><summary>Raw <code>allocs_transformer.snap</code> diff (updated
in this PR)</summary>

```diff
-App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             77
+App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             25
-RadixUIAdoptionSection.jsx | 2.52 kB        ||             21 |              2 ||            259 |             24
+RadixUIAdoptionSection.jsx | 2.52 kB        ||             21 |              2 ||            259 |             13
-kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6689 |            517
+kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6690 |            304
```

(Non-JSX files unchanged. The lone `+1` arena alloc on kitchen-sink is
an eager 1-slot allocation for
a rare element whose attributes are all filtered out — negligible
against −213 reallocs.)

</details>

## Change

```rust
let mut properties =
    ctx.ast.vec_with_capacity(opening_element.as_ref().map_or(0, |e| e.attributes.len()));
```

Capacity-only: the vec's contents, push order, and emitted output are
unchanged — only the initial
allocation differs. CodSpeed (instructions) may pick up the saved
memcpys on JSX-heavy input, but this
is framed as a memory/allocation win.

## Behavior-preserving

`cargo test -p oxc_transformer` passes. The change is a capacity hint
and cannot alter the produced AST.

---

Prepared with AI assistance.

Co-authored-by: Cameron <cameron.clark@hey.com>

## Memory reduction: fewer arena reallocations on JSX

`transform_jsx` built the `React.createElement` / jsx props object in a
**zero-capacity** arena vec
(`ctx.ast.vec()`), regrowing it (4 → 8 → 16 …) as it pushed one property
per attribute. The attribute
count is already known, so pre-sizing the vec to it removes those
regrowths — mirroring `arguments`,
pre-sized a few lines above, and the same pattern as #9915.

Measured with `cargo allocs` (`TestFiles::complicated`). **Arena
reallocations drop on every JSX file;
nothing else changes** (system allocs and the arena-alloc count are
essentially flat):

| file | arena reallocs: before → after | **reduction** |
| --- | --- | --- |
| App.tsx | 77 → 25 | **−52 (−68%)** |
| kitchen-sink.tsx | 517 → 304 | **−213 (−41%)** |
| RadixUIAdoptionSection.jsx | 24 → 13 | **−11 (−46%)** |

Each eliminated realloc is a buffer grow + memcpy of the existing
elements, so this is real
work removed on the JSX hot path, not just bytes.

<details><summary>Raw <code>allocs_transformer.snap</code> diff (updated
in this PR)</summary>

```diff
-App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             77
+App.tsx                    | 415.34 kB      ||             25 |              2 ||            719 |             25
-RadixUIAdoptionSection.jsx | 2.52 kB        ||             21 |              2 ||            259 |             24
+RadixUIAdoptionSection.jsx | 2.52 kB        ||             21 |              2 ||            259 |             13
-kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6689 |            517
+kitchen-sink.tsx           | 732.90 kB      ||            184 |              3 ||           6690 |            304
```

(Non-JSX files unchanged. The lone `+1` arena alloc on kitchen-sink is
an eager 1-slot allocation for
a rare element whose attributes are all filtered out — negligible
against −213 reallocs.)

</details>

## Change

```rust
let mut properties =
    ctx.ast.vec_with_capacity(opening_element.as_ref().map_or(0, |e| e.attributes.len()));
```

Capacity-only: the vec's contents, push order, and emitted output are
unchanged — only the initial
allocation differs. CodSpeed (instructions) may pick up the saved
memcpys on JSX-heavy input, but this
is framed as a memory/allocation win.

## Behavior-preserving

`cargo test -p oxc_transformer` passes. The change is a capacity hint
and cannot alter the produced AST.

---

Prepared with AI assistance.

Co-authored-by: Cameron <cameron.clark@hey.com>