feat(minifier): fold write-once falsy var to false in boolean context#23540
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…#23540) ## What A `var` initialized to a falsy constant and never reassigned is always falsy. oxc already folds such a `let`/`const` everywhere, but a `var` is held back by a hoisting check (a read before the declarator runs sees the hoisted `undefined`), so a multi-read `var` flag past a non-trivial declarative prelude was never folded. In **boolean context** that check is unnecessary: a pre-init `var` read yields `undefined`, which is indistinguishable from the falsy init inside `if (x)` / `x ? … : …` / `!x`. So fold such reads to `false` there; the existing `if (false)` dead-code pass removes the branch. This is the bundled-flag shape behind this issue. Svelte's `var hydrating = false` is read by `if (hydrating)` throughout the runtime; once a client-only bundle tree-shakes the `set_hydrating` setter, `hydrating` is a write-once falsy `var` and the whole hydration path becomes dead and is eliminated. ## Why it is sound - `SymbolValue::boolean_falsy` is set only for a write-once binding with a falsy constant initializer. - It folds to `false` **only** in boolean context; value-context reads keep their hoisting-correct behavior (never folded here). - Gated off for a script's top-level `var` (a global another script can reassign, so an in-module write count of 0 doesn't prove write-once) and for `eval` scopes. ## Verification - `oxc_minifier` tests pass; idempotent. - On a real client-only Svelte (Vite/Rollup) bundle, every `if (hydrating)` check is eliminated. - `minsize`: react 23.14 → 23.12 kB, lodash 70.96 → 70.92 kB (gzip down too). `allocs_minifier.snap`: +20 sys allocs on antd (from evaluating `var` initializers; arena allocs unchanged). - Cross-validated for soundness: on the Svelte bundle, terser `--module` and esbuild `--format=esm` independently fold the same write-once flag (16 → 0), agreeing with oxc. On a Vue 3 client bundle the runtime's flags are multi-write (e.g. `isInSSRComponentSetup` has two assignments, so it can be truthy) and oxc correctly leaves them untouched — the write-once gate is the right line. ## Scope (partial fix) This handles the real-world **bundled** shape — a write-once falsy `var` flag read in boolean context (the Svelte/Vue hydration case, where the bundler has already tree-shaken the setter). It does **not** fold the issue's literal source snippet: ```js let foo = false; function bar(val) { foo = val; } if (foo) { bar(true) } console.log(foo); ``` There `foo` is still reassigned via `bar` (so it isn't write-once), and proving the write dead requires self-referential / flow-sensitive analysis — out of scope here (see #22753 for that direction). So this is a partial step. Refs #14001. Prepared with AI assistance.
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### 💥 BREAKING CHANGES - 94fbacb ast: [**BREAKING**] Only export `AstBuilder` and `NONE` in `builder` module (#23876) (overlookmotel) - 8de5122 ecmascript: [**BREAKING**] Switch to new `AstBuilder` (#23834) (overlookmotel) - dc0ef38 transformer: [**BREAKING**] Switch to new `AstBuilder` (#23831) (overlookmotel) - 88f4455 str: [**BREAKING**] `Str` and `Ident` methods take `&GetAllocator` (#23781) (overlookmotel) - 36009dd allocator: [**BREAKING**] `GetAllocator::allocator` take `&self` (#23676) (overlookmotel) - bd74f9d allocator: [**BREAKING**] Rename `AllocatorAccessor` trait to `GetAllocator` (#23675) (overlookmotel) ### 🚀 Features - 326fe25 transformer_plugins: Support `typeof` `define` keys (#23605) (Alexander Lichter) - f2091b3 ast: Unify old and new `AstBuilder`s (#23875) (overlookmotel) - cd1fd12 codegen: Expose `Codegen::print_string` API (#23785) (camc314) - 785461b ast: Add custom builder methods to AST types (#23651) (overlookmotel) - 05d1357 ast: Add AST creation methods to AST types (#23650) (overlookmotel) - 2580eda str: Add `Str::from_str_in` and `Ident::from_str_in` methods (#23767) (overlookmotel) - 6883fcf minifier: Fold write-once falsy var to false in boolean context (#23540) (Dunqing) - fcbf993 allocator: Add `Vec::from_value_in` method (#23718) (overlookmotel) - 989ddb7 allocator: Add `Vec::from_box_in` method (#23717) (overlookmotel) - 9d1aa7f allocator: Improve `PartialEq` for `Vec` (#23716) (overlookmotel) ### 🐛 Bug Fixes - da0e5bf minifier: Don't reorder a closed-over TDZ read when inlining a var (#23771) (Dunqing) - 0b3021f allocator: Remove `Vec::from_box_in` (#23873) (overlookmotel) - 0ab64ec ast: Silence deprecation warnings within files defining deprecated `AstBuilder` methods (#23889) (overlookmotel) - 8c07cad all: Enable `disable_old_builder` Cargo feature for `oxc_ast` crate in tests (#23888) (overlookmotel) - 3800f01 ast: Legacy `AstBuilder` methods take `self` not `&self` (#23891) (overlookmotel) - 869ac20 semantic/cfg: Connect for update exit to loop test (#23791) (camc314) - d3e92d5 semantic/cfg: Connect while branches from condition exit (#23790) (camc314) - 025045d ast: `ExportNamedDeclaration` plain builder methods return boxed nodes (#23783) (overlookmotel) - 7537c58 ast: Fix name of `AstBuilder` method for `Expression::V8IntrinsicExpression` (#23766) (overlookmotel) - 3f574f5 traverse: Fix unsoundness in `Traverse` walk functions (#23745) (overlookmotel) - 585760f parser: String in AST reference arena (#23721) (overlookmotel) - 7231d55 allocator: Fix unsound lifetime extension in `Box::new_in` (#23685) (overlookmotel) ### ⚡ Performance - d5c916a semantic: Flatten hoisting_variables to avoid per-scope map allocation (#23927) (Lawrence Lin) - e71609d minifier: Bail member-expr folding before the side-effect walk (#23924) (Lawrence Lin) - e1f89ab minifier: Reduce string allocations folding addition (#23846) (overlookmotel) - 9f6ee3b isolated-declarations: Pool scope maps to avoid per-scope alloc/rehash (#23761) (Boshen) - 0b07c4c semantic: Avoid heap alloc for catch-clause binding ids (#23911) (Lawrence Lin) - c5eef8b regular_expression: Skip capturing-group pre-parse when pattern has no `(` (#23908) (Lawrence Lin) - b4f5b4b isolated_declarations: Remove redundant clone of formal parameter pattern (#23912) (Lawrence Lin) - 53d083f isolated_declarations: Use `TakeIn` not `CloneIn` (#23847) (overlookmotel) - 3ea9304 react_compiler: Use faster API to arena allocate strings (#23849) (overlookmotel) - a6d8e45 parser: Avoid span lookup for arrow expression body (#23788) (camc314) - e1886a0 transformer, minifier: Use `static_ident!` macro to create static `Ident`s (#23727) (overlookmotel) - 5527bef transformer/object-rest-spread: Reduce iteration (#23720) (overlookmotel) - 680ffbc transformer: Allocate AST nodes in arena directly (#23711) (overlookmotel) - 1c63c66 parser: Allocate AST nodes in arena directly (#23712) (overlookmotel) - 3855f0c minifier: Allocate AST nodes in arena directly (#23710) (overlookmotel) - d025887 isolated_declarations: Allocate AST nodes in arena directly (#23709) (overlookmotel) - 10b96c6 parser: Remove string search from parsing JSX element name (#23713) (overlookmotel) ### 📚 Documentation - 3d61dea all: Correct capitalization in comments (#23887) (overlookmotel) - aa1ad74 ast: Add `#[deprecated]` to legacy `AstBuilder` methods (#23877) (overlookmotel) - a4676db ast: Correct doc comment for `NONE` (#23765) (overlookmotel) - 419ec80 syntax: Fix typo in doc comment (#23674) (overlookmotel) ### 🛡️ Security - 3cdd18f deps: Update npm packages (#23690) (renovate[bot]) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com> Co-authored-by: Cameron <cameron.clark@hey.com>
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Jul 3, 2026
…#23540) ## What A `var` initialized to a falsy constant and never reassigned is always falsy. oxc already folds such a `let`/`const` everywhere, but a `var` is held back by a hoisting check (a read before the declarator runs sees the hoisted `undefined`), so a multi-read `var` flag past a non-trivial declarative prelude was never folded. In **boolean context** that check is unnecessary: a pre-init `var` read yields `undefined`, which is indistinguishable from the falsy init inside `if (x)` / `x ? … : …` / `!x`. So fold such reads to `false` there; the existing `if (false)` dead-code pass removes the branch. This is the bundled-flag shape behind this issue. Svelte's `var hydrating = false` is read by `if (hydrating)` throughout the runtime; once a client-only bundle tree-shakes the `set_hydrating` setter, `hydrating` is a write-once falsy `var` and the whole hydration path becomes dead and is eliminated. ## Why it is sound - `SymbolValue::boolean_falsy` is set only for a write-once binding with a falsy constant initializer. - It folds to `false` **only** in boolean context; value-context reads keep their hoisting-correct behavior (never folded here). - Gated off for a script's top-level `var` (a global another script can reassign, so an in-module write count of 0 doesn't prove write-once) and for `eval` scopes. ## Verification - `oxc_minifier` tests pass; idempotent. - On a real client-only Svelte (Vite/Rollup) bundle, every `if (hydrating)` check is eliminated. - `minsize`: react 23.14 → 23.12 kB, lodash 70.96 → 70.92 kB (gzip down too). `allocs_minifier.snap`: +20 sys allocs on antd (from evaluating `var` initializers; arena allocs unchanged). - Cross-validated for soundness: on the Svelte bundle, terser `--module` and esbuild `--format=esm` independently fold the same write-once flag (16 → 0), agreeing with oxc. On a Vue 3 client bundle the runtime's flags are multi-write (e.g. `isInSSRComponentSetup` has two assignments, so it can be truthy) and oxc correctly leaves them untouched — the write-once gate is the right line. ## Scope (partial fix) This handles the real-world **bundled** shape — a write-once falsy `var` flag read in boolean context (the Svelte/Vue hydration case, where the bundler has already tree-shaken the setter). It does **not** fold the issue's literal source snippet: ```js let foo = false; function bar(val) { foo = val; } if (foo) { bar(true) } console.log(foo); ``` There `foo` is still reassigned via `bar` (so it isn't write-once), and proving the write dead requires self-referential / flow-sensitive analysis — out of scope here (see #22753 for that direction). So this is a partial step. Refs #14001. Prepared with AI assistance.
camc314
added a commit
that referenced
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Jul 3, 2026
### 💥 BREAKING CHANGES - 94fbacb ast: [**BREAKING**] Only export `AstBuilder` and `NONE` in `builder` module (#23876) (overlookmotel) - 8de5122 ecmascript: [**BREAKING**] Switch to new `AstBuilder` (#23834) (overlookmotel) - dc0ef38 transformer: [**BREAKING**] Switch to new `AstBuilder` (#23831) (overlookmotel) - 88f4455 str: [**BREAKING**] `Str` and `Ident` methods take `&GetAllocator` (#23781) (overlookmotel) - 36009dd allocator: [**BREAKING**] `GetAllocator::allocator` take `&self` (#23676) (overlookmotel) - bd74f9d allocator: [**BREAKING**] Rename `AllocatorAccessor` trait to `GetAllocator` (#23675) (overlookmotel) ### 🚀 Features - 326fe25 transformer_plugins: Support `typeof` `define` keys (#23605) (Alexander Lichter) - f2091b3 ast: Unify old and new `AstBuilder`s (#23875) (overlookmotel) - cd1fd12 codegen: Expose `Codegen::print_string` API (#23785) (camc314) - 785461b ast: Add custom builder methods to AST types (#23651) (overlookmotel) - 05d1357 ast: Add AST creation methods to AST types (#23650) (overlookmotel) - 2580eda str: Add `Str::from_str_in` and `Ident::from_str_in` methods (#23767) (overlookmotel) - 6883fcf minifier: Fold write-once falsy var to false in boolean context (#23540) (Dunqing) - fcbf993 allocator: Add `Vec::from_value_in` method (#23718) (overlookmotel) - 989ddb7 allocator: Add `Vec::from_box_in` method (#23717) (overlookmotel) - 9d1aa7f allocator: Improve `PartialEq` for `Vec` (#23716) (overlookmotel) ### 🐛 Bug Fixes - da0e5bf minifier: Don't reorder a closed-over TDZ read when inlining a var (#23771) (Dunqing) - 0b3021f allocator: Remove `Vec::from_box_in` (#23873) (overlookmotel) - 0ab64ec ast: Silence deprecation warnings within files defining deprecated `AstBuilder` methods (#23889) (overlookmotel) - 8c07cad all: Enable `disable_old_builder` Cargo feature for `oxc_ast` crate in tests (#23888) (overlookmotel) - 3800f01 ast: Legacy `AstBuilder` methods take `self` not `&self` (#23891) (overlookmotel) - 869ac20 semantic/cfg: Connect for update exit to loop test (#23791) (camc314) - d3e92d5 semantic/cfg: Connect while branches from condition exit (#23790) (camc314) - 025045d ast: `ExportNamedDeclaration` plain builder methods return boxed nodes (#23783) (overlookmotel) - 7537c58 ast: Fix name of `AstBuilder` method for `Expression::V8IntrinsicExpression` (#23766) (overlookmotel) - 3f574f5 traverse: Fix unsoundness in `Traverse` walk functions (#23745) (overlookmotel) - 585760f parser: String in AST reference arena (#23721) (overlookmotel) - 7231d55 allocator: Fix unsound lifetime extension in `Box::new_in` (#23685) (overlookmotel) ### ⚡ Performance - d5c916a semantic: Flatten hoisting_variables to avoid per-scope map allocation (#23927) (Lawrence Lin) - e71609d minifier: Bail member-expr folding before the side-effect walk (#23924) (Lawrence Lin) - e1f89ab minifier: Reduce string allocations folding addition (#23846) (overlookmotel) - 9f6ee3b isolated-declarations: Pool scope maps to avoid per-scope alloc/rehash (#23761) (Boshen) - 0b07c4c semantic: Avoid heap alloc for catch-clause binding ids (#23911) (Lawrence Lin) - c5eef8b regular_expression: Skip capturing-group pre-parse when pattern has no `(` (#23908) (Lawrence Lin) - b4f5b4b isolated_declarations: Remove redundant clone of formal parameter pattern (#23912) (Lawrence Lin) - 53d083f isolated_declarations: Use `TakeIn` not `CloneIn` (#23847) (overlookmotel) - 3ea9304 react_compiler: Use faster API to arena allocate strings (#23849) (overlookmotel) - a6d8e45 parser: Avoid span lookup for arrow expression body (#23788) (camc314) - e1886a0 transformer, minifier: Use `static_ident!` macro to create static `Ident`s (#23727) (overlookmotel) - 5527bef transformer/object-rest-spread: Reduce iteration (#23720) (overlookmotel) - 680ffbc transformer: Allocate AST nodes in arena directly (#23711) (overlookmotel) - 1c63c66 parser: Allocate AST nodes in arena directly (#23712) (overlookmotel) - 3855f0c minifier: Allocate AST nodes in arena directly (#23710) (overlookmotel) - d025887 isolated_declarations: Allocate AST nodes in arena directly (#23709) (overlookmotel) - 10b96c6 parser: Remove string search from parsing JSX element name (#23713) (overlookmotel) ### 📚 Documentation - 3d61dea all: Correct capitalization in comments (#23887) (overlookmotel) - aa1ad74 ast: Add `#[deprecated]` to legacy `AstBuilder` methods (#23877) (overlookmotel) - a4676db ast: Correct doc comment for `NONE` (#23765) (overlookmotel) - 419ec80 syntax: Fix typo in doc comment (#23674) (overlookmotel) ### 🛡️ Security - 3cdd18f deps: Update npm packages (#23690) (renovate[bot]) Co-authored-by: Boshen <1430279+Boshen@users.noreply.github.com> Co-authored-by: Cameron <cameron.clark@hey.com>
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What
A
varinitialized to a falsy constant and never reassigned is always falsy. oxc already folds such alet/consteverywhere, but avaris held back by a hoisting check (a read before the declarator runs sees the hoistedundefined), so a multi-readvarflag past a non-trivial declarative prelude was never folded.In boolean context that check is unnecessary: a pre-init
varread yieldsundefined, which is indistinguishable from the falsy init insideif (x)/x ? … : …/!x. So fold such reads tofalsethere; the existingif (false)dead-code pass removes the branch.This is the bundled-flag shape behind this issue. Svelte's
var hydrating = falseis read byif (hydrating)throughout the runtime; once a client-only bundle tree-shakes theset_hydratingsetter,hydratingis a write-once falsyvarand the whole hydration path becomes dead and is eliminated.Why it is sound
SymbolValue::boolean_falsyis set only for a write-once binding with a falsy constant initializer.falseonly in boolean context; value-context reads keep their hoisting-correct behavior (never folded here).var(a global another script can reassign, so an in-module write count of 0 doesn't prove write-once) and forevalscopes.Verification
oxc_minifiertests pass; idempotent.if (hydrating)check is eliminated.minsize: react 23.14 → 23.12 kB, lodash 70.96 → 70.92 kB (gzip down too).allocs_minifier.snap: +20 sys allocs on antd (from evaluatingvarinitializers; arena allocs unchanged).--moduleand esbuild--format=esmindependently fold the same write-once flag (16 → 0), agreeing with oxc. On a Vue 3 client bundle the runtime's flags are multi-write (e.g.isInSSRComponentSetuphas two assignments, so it can be truthy) and oxc correctly leaves them untouched — the write-once gate is the right line.Scope (partial fix)
This handles the real-world bundled shape — a write-once falsy
varflag read in boolean context (the Svelte/Vue hydration case, where the bundler has already tree-shaken the setter). It does not fold the issue's literal source snippet:There
foois still reassigned viabar(so it isn't write-once), and proving the write dead requires self-referential / flow-sensitive analysis — out of scope here (see #22753 for that direction). So this is a partial step.Refs #14001.
Prepared with AI assistance.