fix(ecmascript): resolve identifier value type via tracked constants#22234
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Dunqing merged 3 commits intoMay 18, 2026
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`value_type` returned `Undetermined` for any non-global identifier, even when the binding had zero writes and a known constant initializer. As a result `slot?.()` on a `let slot;` (with all writers tree-shaken away) was not folded to `void 0`, leaving dead optional calls in the output. Consult `get_constant_value_for_reference_id` for non-global identifier references and map the resulting `ConstantValue` to a `ValueType`. This lets `fold_chain_expr` (and other consumers of `value_type`) recognise read-only `let`/`const` bindings whose value is statically known. Closes rolldown/rolldown#9281 # Conflicts: # crates/oxc_minifier/tests/peephole/dead_code_elimination.rs
The Identifier branch in `Expression::value_type` cloned the tracked ConstantValue (potential BigInt / owned-String heap clone) only to read its discriminant. Add `GlobalContext::value_type_for_reference_id` and a `ConstantValue::value_type` projection, so the minifier impl can borrow the tracked value and skip the clone. The existing `get_constant_value_for_reference_id` path is unchanged for callers that need the full value. Extend the dead-code-elimination tests with Null (via `?.`) and non-nullish (String/BigInt via `??`) cases.
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LGTM! I sent a few commits that polished and fixed CI.
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### 🐛 Bug Fixes - 0f26de6 ecmascript: Resolve identifier value type via tracked constants (#22234) (Alexander Lichter) - c27a8cf minifier: Normalize `{ x: x }` shorthand so adjacent-if merge is idempotent (#22401) (Dunqing) - e431a0e parser: Break extends clause loop on fatal error (#22517) (Boshen) - e9ec7c6 minifier: Fold optional chains by base nullishness (#22236) (Alexander Lichter) - e6090e7 transformer: Keep enum IIFE when a non-inlinable value reference remains (#22501) (Dunqing) - 931b7d6 transformer: Inline const enum members through type-cast wrappers (#22500) (Dunqing) - b9615b2 codegen: Preserve string quotes in require() calls during minification (#22475) (zennnnnnn11) - c73c159 transformer/async-to-generator: Reparent parameter initializer scopes (#22507) (camc314) - ecfd3ca transformer/async-to-generator: Move only parameter bindings (#22503) (camc314) - 3ce3431 transformer/explicit-resource-managment: Preserve shadowed for-head block (#22451) (camc314) ### ⚡ Performance - ce92c6c semantic: `#[inline]` `Scoping::get_binding` (#22414) (Dunqing) - 98be95c regular_expression: Track regex flags via bitflags (#22427) (Boshen) - dbbc059 jsdoc: Skip should_attach_jsdoc when no remaining comments (#22409) (Boshen) - 217d7d8 minifier: Index `SymbolValues` by `SymbolId` (#22441) (Dunqing) - d782b78 minifier: Use BitSet for LiveUsageCollector live references (#22425) (Boshen)
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value_typeforExpression::IdentifierreturnedUndeterminedfor any non-global identifier, even when the binding had zero writes and a known constant initializer. This causes code likeslot?.()on alet slot;with all writers tree-shaken away not being folded tovoid 0, leaving dead code in the output.This PR changes that by checking
GlobalContext::get_constant_value_for_reference_idfor non-global identifier references and mapping the resultingConstantValueto aValueTypevia a new private helper.Once the identifier resolves to a tracked nullish value, the existing nullish branch in
fold_chain_exprcollapses the chain tovoid 0. The same lookup also works for the opposite case (#21923).The lookup is gated by making sure there are no write references, so anything that is being written anywhere keeps its
Undeterminedtype.Closes rolldown/rolldown#9281
Related (but does not resolve) #21923