`OxcDiagnostic` is an unboxed ~300-byte struct constructed at thousands of callsites. Boxing its inner data behind an 8-byte handle means every callsite, every `Result<T, OxcDiagnostic>`, and every move copies 8 bytes instead of ~300, so the compiler emits far less inlined construction/copy code.

## Binary size (stripped, real release builds, aarch64-apple-darwin)

| Binary | Before | After | Delta |
|---|---:|---:|---:|
| `oxlint` | 14,177,616 B | 13,665,680 B | **−499.9 KiB (−3.61%)** |
| napi transform (`.node`) | 3,482,624 B | 3,433,040 B | **−48.4 KiB (−1.42%)** |

Most of the win lands in `oxc_linter`, whose ~1,500 diagnostic-construction sites inline into rule bodies; `oxc_react_compiler` also contributes.

The `error`/`warn` constructors delegate to an outlined `#[inline(never)] fn new`, so the `Box` allocation + field initialization exist once in the binary rather than inlined at every construction site.

This reverses #22406, which unboxed the inner to save a heap allocation. That is now safe: the parser no longer constructs diagnostics on its hot path (#24663).

Supersedes the draft #24656, which additionally carried a now-redundant copy of the parser deferral that has since landed via #24663.

### 💥 BREAKING CHANGES

- 54cc121 ast: [**BREAKING**] Split `MetaProperty` into `ImportMeta` and
`NewTarget` (#24557) (camc314)

### 🚀 Features

- 4c71560 parser: More friendly error for spread element in dynamic
imports (#24705) (sapphi-red)
- 7b045cd minfier: Drop last break from last switch case (#24673)
(Armano)
- 7d3c178 minifier: Remove unreachable recursive functions (#24125)
(Dunqing)
- 94f99b3 ast: Allow `NONE` to be passed to AST builder methods where
`Option<ArenaVec>` is expected (#24629) (overlookmotel)
- 77230c5 ast: Accept arrays for `ArenaVec` params of AST builder
methods (#24621) (overlookmotel)
- f08b152 allocator: Implement `FromIn` for array to `Vec` conversion
(#24620) (overlookmotel)
- 2338c13 track-memory-allocations: Track heap deallocs, alloc bytes,
and peak growth (#24619) (Boshen)
- 7aa4739 syntax,transformer: Move JSX entity decoder to `oxc_syntax`
(#24617) (camc314)
- 2b097c4 str: Export `Str` as `ArenaStr` (#24604) (overlookmotel)
- 3acf4c1 minifier: Expand switch optimiation to remove empty cases
(#24520) (Armano)
- 129b759 parser: Improve diagnostics for unparenthesized LHS on
exponential expr (#24569) (camc314)
- 4d0c601 minifier: Fold arithmetic over undefined and null operands
(#24485) (Dunqing)
- 91541dd minifier: Drop empty switch statements  (#24527) (Armano)
- d05224d ast_visit: Generate VisitJs visitor that skips TypeScript
type-space (#24499) (Boshen)
- 3d22307 parser: Add `ParseOptions::enable_ident_hashes` (#24491)
(Boshen)

### 🐛 Bug Fixes

- 64c2241 minifier: Align class heritage removal with assumptions
(#24533) (Dunqing)
- 48b59f4 parser: Span ambient generator diagnostics (#24711) (camc314)
- e750a82 ecmascript: Fix false negative for may_have_side_effects on
dynamic property access (#24709) (sapphi-red)
- f145d73 minifier: Guard reordered identifier reads (#24698) (Dunqing)
- a2ef382 isolated-declarations: Reject `window.Symbol` as global symbol
reference (#24689) (camc314)
- b1bcf72 minifier: Invalidate facts for redeclared bindings (#24658)
(Dunqing)
- 921b834 minifier: Don't treat a conditionally-assigned var as
write-once (#24650) (Dunqing)
- 061af1f minifier: Avoid stale pure function summaries (#24636)
(Dunqing)
- 40c2f43 allocator: `Vec::from_array_in` do not allocate zero-length
array (#24628) (overlookmotel)
- 70994ae codegen: Preserve comments before expression operands (#24510)
(Dunqing)
- 7b4baff parser: Reject new import member access (#23459) (camc314)
- 128b385 minifier: Clippy warning with no-debug-assertions (#24547)
(camc314)
- 8421feb parser: Use first `as` span for imported name (#24537)
(leaysgur)
- c517aa0 parser: Reject invalid accessor assertions (#24504) (camc314)

### ⚡ Performance

- 884d9eb parser: Pre-size cover-grammar assignment target buffers
(#24683) (Boshen)
- d3f07a0 diagnostics: Box OxcDiagnosticInner to reduce binary size
(#24665) (Boshen)
- bcc9de0 parser: Defer diagnostic creation until parse exit (#24663)
(Boshen)
- c35d8ab allocator: Mark `ReplaceWith` panic path cold (#24515)
(camc314)
- ba65790 semantic, allocator: Branchless `clone_in` for semantic IDs
(#24564) (overlookmotel)
- 747feec parser: Build AST nodes with the AST builder instead of
cloning (#24540) (Boshen)
- ba35a0d react_compiler: Use IndexVec for dense id-keyed maps (#24549)
(Boshen)
- b685062 react_compiler: Keep small hot-path collections inline
(#24514) (Marius Schulz)
- a149e95 transformer: Outline rare expression exits (#24512) (camc314)
- 7808a6e react_compiler: Make aliasing effects cheap to intern and
clone (#24506) (Marius Schulz)
- 3a36f2a react_compiler: Store AbstractValue reasons as a u16 bitmask
(#24480) (Boshen)
- 1c96753 react_compiler: Use FxHashMap for the lookup-only aliasing
node map (#24490) (Boshen)

Co-authored-by: Cameron <[email protected]>