fix(minifier): track pure functions in DCE mode#21722
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sapphi-red
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## Summary
In DCE mode, `enter_statement` was skipping `keep_track_of_pure_functions` via an `if ctx.state.dce { return; }` guard, so user-defined functions with empty or side-effect-free bodies were never recorded as pure. The two consumers of that map — `remove_unused_call_expr` (via `try_fold_expression_stmt`) and `remove_dead_code_call_expression` — **already run in DCE mode**, so the data was needed; only the producer was gated off.
The guard was introduced during the consolidation refactor in #18128, which preserved pre-consolidation behavior rather than being an intentional choice. Eliminating calls to side-effect-free functions is classic DCE, not a full-minify extra, so the tracker now runs in both modes.
```js
// before (minify: "dce-only" / rolldown treeshake: true)
function noop() {}
noop(); // kept
// after
// (empty)
```
## Test plan
- New test case `remove_pure_function_calls` covering the issue repro
- Two existing expectations in `dce_if_statement` / `dce_var_hoisting` updated where DCE now produces strictly smaller output (pure function + its only call both collapse)
- All 484 minifier unit tests pass
- `just minsize` shows no size-snapshot changes (DCE mode isn't exercised by minsize benchmarks)
Closes rolldown/rolldown#9211
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### 💥 BREAKING CHANGES - 502e804 ast: [**BREAKING**] Reduce size of `TSTypePredicateName` (#21711) (overlookmotel) - 5651539 ast: [**BREAKING**] Reduce size of `JSXExpression` (#21710) (overlookmotel) - c44e280 ast: [**BREAKING**] Reduce size of `ArrayExpressionElement` (#21709) (overlookmotel) - c5b3deb syntax: [**BREAKING**] Remove `CommentNodeId` (#21679) (overlookmotel) ### 🚀 Features - b738a39 allocator: Add `Allocator::cursor_ptr` method (#21773) (overlookmotel) - 678767e ast: Generate node_id accessors for AST enum wrappers (#21653) (camc314) - f091d77 minifier: Inline constant spread elements into arrays (#21095) (Armano) ### 🐛 Bug Fixes - 0d608c2 minifier: Preserve raw CR in template literals (#21645) (Dunqing) - a889ea9 minifier: Track pure functions in DCE mode (#21722) (Dunqing) - 674dfac allocator: `Arena` retry allocation when chunk size approaches maximum (#21777) (overlookmotel) - f130cc0 allocator: Fix arithmetic overflow in `Arena::new_chunk_memory_details` (#21745) (overlookmotel) - b9bf239 allocator: Fix UB in `Arena::grow_zeroed` (#21739) (overlookmotel) - d2b9389 allocator: Clippy warning when building without `testing` feature (#21681) (camc314) - 503dc86 codegen: Map sourcemaps from visible output starts (#21662) (Dunqing) - c92bd3b transformer: Use SPAN for synthesized helper calls to prevent comment misattribution (#21578) (Dunqing) - 0d80441 codegen: Add mapping before printing `#` for private ident (#21619) (camc314) ### ⚡ Performance - 9fa362e napi/parser: Do not generate tokens except in tests (#21811) (overlookmotel) - 0044392 allocator: Reduce branches when allocating new chunk (#21776) (overlookmotel) - 7896bd0 allocator: `Allocator::used_bytes` do not use chunk iterator (#21771) (overlookmotel) - a5c562f allocator: Remove check in `Arena::new_chunk_memory_details` (#21750) (overlookmotel) - 35bbe1f allocator: `Arena` use unchecked size round up where guaranteed no overflow (#21743) (overlookmotel) - ffe229b allocator: Remove unnecessary check from `Arena::try_alloc_layout_slow_impl` (#21732) (overlookmotel) - 72fece5 allocator: Use `NonNull::offset_from_unsigned` in `Arena::chunk_capacity` (#21731) (overlookmotel) - cab32ae ast: Add `#[inline(always)]` to `node_id` methods on enums with all variants unboxed (#21707) (overlookmotel) - b179688 parser: Allocate `TriviaBuilder` comments in the arena (#21512) (Boshen) - 2290f31 lexer: Fix perf of `Token::set_*` methods on Rust 1.95.0 (#21659) (overlookmotel) - 1b58029 allocator: Move code into cold path in `Arena::alloc_layout` (#21622) (overlookmotel) - 3cf7cef allocator: Reduce instructions on allocation hot path (#21510) (overlookmotel) ### 📚 Documentation - ce65070 data_structures: Document why `as_ref` and `as_mut` on `NonNullConst` and `NonNullMut` take `self` (#21800) (overlookmotel) - 93b7dbd allocator: Improve doc comments for `ChunkFooter` (#21733) (overlookmotel) - 295db8d transformer: Fix comment (#21717) (overlookmotel) - 5c93af8 ast: Add comments explaining `#[inline(always)]` to `node_id` methods on enums (#21706) (overlookmotel) - e4cea25 transform: Use the `node:` namespace in the example (#19998) (루밀LuMir) ### 🛡️ Security - d8076c9 deps: Update rolldown (#21639) (renovate) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com>
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Summary
In DCE mode,
enter_statementwas skippingkeep_track_of_pure_functionsvia anif ctx.state.dce { return; }guard, so user-defined functions with empty or side-effect-free bodies were never recorded as pure. The two consumers of that map —remove_unused_call_expr(viatry_fold_expression_stmt) andremove_dead_code_call_expression— already run in DCE mode, so the data was needed; only the producer was gated off.The guard was introduced during the consolidation refactor in #18128, which preserved pre-consolidation behavior rather than being an intentional choice. Eliminating calls to side-effect-free functions is classic DCE, not a full-minify extra, so the tracker now runs in both modes.
Test plan
remove_pure_function_callscovering the issue reprodce_if_statement/dce_var_hoistingupdated where DCE now produces strictly smaller output (pure function + its only call both collapse)just minsizeshows no size-snapshot changes (DCE mode isn't exercised by minsize benchmarks)Closes rolldown/rolldown#9211