Accept and lower the accessor keyword in TypeScript files using experimentalDecorators - #38125

Fixes #29197

Replaces #29201 (closed) and the parked branch farm/57075cac/ts-legacy-decorators-accessor. #29201 routed these classes through the standard-decorator lowering and reported a compile error for a class that mixes legacy decorators with accessor members; tsc accepts that combination, so the in-place lowering here was kept. Its test file was run against this branch: every case passes except that deliberate error and a standard-mode (experimentalDecorators off) computed-key case, which belongs to #31926. The one case it covered that this suite lacked (TypeScript modifiers on accessors) was added here, together with the mixed class.

Problem

• In a project whose tsconfig sets experimentalDecorators (or emitDecoratorMetadata), any .ts class using the accessor keyword fails to parse: class A { accessor name = "A" } gives error: Expected ";" but found "name". The same file works when the tsconfig does not enable legacy decorators. tsc accepts the keyword in both decorator modes (it is class syntax from the decorators proposal, independent of which decorator flavor is configured).
• Cause: src/js_parser/parse/parse_property.rs only recognized the keyword when features.standard_decorators was set, and ParseTask.rs / transpiler.rs clear that flag for TypeScript files with experimentalDecorators or emitDecoratorMetadata. accessor then parsed as a field named accessor.
• Parsing alone is not enough: JavaScriptCore does not implement accessor, and the only lowering for it lives in the standard-decorator path (lower_decorators.rs), which these classes never take because their decorators are TypeScript legacy decorators lowered by P::lower_class.
• Adjacent bug, both modes: Class::can_be_moved (src/ast/g.rs) only looked at plain static fields, so a class whose only side effect is a static accessor initializer was hoisted to the top of the module by the runtime transpiler and read bindings before their declaration (ReferenceError: Cannot access 'names' before initialization). Reproduces on main with a .js file too.

Fix

• parse_property.rs: recognize accessor in class bodies in every decorator mode. The newline (ASI) and raw-spelling checks are unchanged, so accessor as a member name still parses the way tsc parses it (covered by a snapshot test). In experimentalDecorators mode the abstract branch there turns a decorated abstract accessor x into the same Abstract property a decorated abstract x becomes, so its decorators are emitted as __legacyDecorateClassTS(decorators, proto, "x", undefined) plus design:type, which is what tsc emits; before, the member (and its decorators) was dropped like an undecorated abstract member. Standard-decorator mode is left as it was (the member is still dropped; tsc rejects standard decorators on abstract members), covered in es-decorators.test.ts.
• New src/js_parser/lower/lower_auto_accessors.rs, called from visit_class for classes that should_lower_standard_decorators does not claim. Each accessor is replaced at its own position by the desugaring the proposal defines and tsc emits:

  accessor x = 1;   // -> #x = 1; get x() { return this.#x; } set x(v) { this.#x = v; }

  • Why in place: nothing leaves the class body, so key evaluation order, initializer order and private-name scoping are exactly those of the source, and it composes with the legacy decorator lowering that runs afterwards on the same class. Static accessors use this, like the proposal and the existing standard-mode output.
  • Why in visit_class, before the useDefineForClassFields: false block: the generated backing field is then relocated into the constructor in source order together with the other instance fields, which is what tsc emits for that option.
  • Backing name: #x (#_p for accessor #p, #_accessor_storage for other keys), with a numeric suffix when that spelling is declared by this class or by an enclosing class, found by walking the scope chain while the class body scope is still current. Private names are printed verbatim by the non-minifying renamer, and a name declared by an enclosing class may be referenced from inside this class body, so both sets have to be avoided.
  • Computed keys are evaluated once, at the accessor's position: get [_computedKey = expr]() {} set [_computedKey]() {}. The temporary comes from generate_temp_ref (collision-safe name in the runtime transpiler), is registered in the scope a var there hoists to, and is recorded as a top-level declared symbol of the part when that scope is the module scope, so the bundler's renamer gives it a name distinct from other files' top-level bindings. Without the last step, the temporary of an earlier file and a top-level binding of a later file could be assigned the same name (verified while writing the bundling test).
  • Legacy decorators on an accessor move to the generated setter. lower_class then emits __legacyDecorateClassTS(decorators, target, key, null), which is what tsc's __decorate receives for a legacy-decorated accessor (the decorator gets the get/set descriptor and can replace it). The setter rather than the getter because lower_class reuses the decorated member's key expression in that call: the setter's key is the bare _computedKey temporary, so a computed key is still evaluated once (tsc emits __decorate(..., _a, null) the same way). The pair is flagged IsLoweredAutoAccessor (new flags::Property variant) and the setter carries the member's declared type, so emitDecoratorMetadata emits design:type only, as tsc does for accessors, instead of a getter's or setter's design:type + design:paramtypes.
  • Standard-decorator mode is unchanged: those classes always have should_lower_standard_decorators set, so the new pass never runs for them (checked that bun build --no-bundle output for JS/TS files without experimentalDecorators is byte-identical before and after). Lower accessor-only classes in place instead of relocating static elements #31926, which makes the standard-mode lowering of accessor-only classes in place as well, is independent of this; the new pass is written so that path could share it later.
• src/ast/g.rs: can_be_moved also inspects static accessor initializers. Because this changes the output of files that already transpiled before (the other changes only affect files that used to fail to parse), EXPECTED_VERSION in src/jsc/RuntimeTranspilerCache.rs is bumped to 26; the cache key is source hash plus features, not the bun version, so without the bump a warm cache would keep serving the hoisted output for such files until they are edited.
• Small enablers: four helpers in lower_decorators.rs became pub(crate) for reuse; the obsolete accessor TODO in lower_class and the stale comment in emit_decorator_metadata_for_prop were removed.
• Verification:
  • test/bundler/transpiler/decorators.test.ts, new describe("accessor keyword with experimentalDecorators"): output shapes via Bun.Transpiler (basic, static/private/literal/computed keys, name collisions incl. an enclosing class, decorated instance/static/computed-key accessors, private / protected readonly / public static / static override / abstract / decorated abstract modifiers in a class whose other members carry legacy decorators, metadata vs. a real getter, useDefineForClassFields: false, accessor as a plain member name) and runtime fixtures with a tsconfig on disk (getter/setter semantics incl. subclass override and super, computed key evaluated once, anonymous export default keeps .name === "default", legacy decorators receiving and replacing the descriptor plus design:type, including on a computed-key accessor whose key function must run exactly once, useDefineForClassFields: false ordering, and a two-file Bun.build where the temporaries, a user function and an export all want the same name). 11 of the 12 fail on the released build (parse error), all pass with this branch.
  • test/bundler/transpiler/es-decorators.test.ts: .js class statement and export default class with static accessor initializers are no longer hoisted above the binding they read; fails on the released build with the ReferenceError above.
  • decorators, decorator-metadata, es-decorators, es-decorators-esbuild, ts-use-define-for-class-fields, bundler_decorator_metadata, esbuild/ts, esbuild/lower, bundler_edgecase and the internal source lints pass with a debug build; cargo clippy -p bun_js_parser is clean.

Background

• Auto-accessor: accessor x = v declares a getter/setter pair backed by a hidden per-instance slot; the proposal specifies it as exactly the #x + get/set desugaring above. TypeScript has accepted it since 4.9 under both experimentalDecorators and standard decorators, and its legacy decorator transform decorates such a member with the descriptor form (__decorate(..., key, null)), emitting only design:type as metadata.
• Decorator modes in bun: .js files and .ts files without experimentalDecorators use the standard (TC39) lowering in lower_decorators.rs, which also lowers accessors; .ts files with experimentalDecorators / emitDecoratorMetadata have features.standard_decorators cleared and get their decorators lowered by P::lower_class into __legacyDecorateClassTS calls after the class. G::Class::should_lower_standard_decorators is what routes a class to the former.
• useDefineForClassFields: false: tsc option under which instance field initializers are turned into this.x = init assignments in the constructor; bun implements it at the end of visit_class.
• Renamers: the runtime transpiler prints symbol names verbatim (generated names must be unique by construction, which generate_temp_ref provides), the bundler renames top-level symbols across all files of a chunk and then nested scopes per part (so a symbol declared by a top-level var has to be registered as a top-level declaration to take part in the cross-file pass), and private names are never renamed except by the minifier.
• Class::can_be_moved: the runtime transpiler hoists top-level classes without observable side effects to the top of the module to make some import cycles work; a static initializer is such a side effect.

[review] gate passed · iteration 0 · 11 files touched

ASAN without fix: 12 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/bundler/transpiler/decorators.test.ts test/bundler/transpiler/es-decorators.test.ts
bun test v1.4.1 (65362b53b)

test/bundler/transpiler/decorators.test.ts:
(pass) decorator order of evaluation [49.87ms]
(pass) decorator factories order of evaluation [42.99ms]
(pass) parameter decorators [32.11ms]
(pass) decorators random [71.12ms]
(pass) class field order [9.58ms]
(pass) changing static method [10.67ms]
(pass) class extending from another class [16.62ms]
(pass) decorated fields moving to constructor [10.21ms]
(pass) only class decorator [5.48ms]
(pass) decorators with different property key types [13.39ms]
(pass) only property decorators [13.46ms]
(pass) only argument decorators [5.95ms]
(pass) no decorators [5.01ms]
(pass) constructor statements > with parameter properties [14.59ms]
(pass) constructor statements > class expressions (no decorators) [13.47ms]
(pass) constructor statements > with parameter properties and statements [7.93ms]
(pass) constructor statements > with parameter properties, statements, and decorators [10.97
... (truncated)

release without fix: 12 FAILED
bun test v1.4.1-canary.1 (65362b53b)

test/bundler/transpiler/decorators.test.ts:
(pass) decorator order of evaluation [0.46ms]
(pass) decorator factories order of evaluation [0.41ms]
(pass) parameter decorators [0.40ms]
(pass) decorators random [0.96ms]
(pass) class field order [0.12ms]
(pass) changing static method [0.11ms]
(pass) class extending from another class [0.18ms]
(pass) decorated fields moving to constructor [0.11ms]
(pass) only class decorator [0.05ms]
(pass) decorators with different property key types [0.20ms]
(pass) only property decorators [0.17ms]
(pass) only argument decorators [0.06ms]
(pass) no decorators [0.04ms]
(pass) constructor statements > with parameter properties [0.13ms]
(pass) constructor statements > class expressions (no decorators) [0.12ms]
(pass) constructor statements > with parameter properties and statements [0.07ms]
(pass) constructor statements > with parameter properties, statements, and decorators [0.10ms]
(pass) constructor statements > with more parameter properties, statements, and decorators [0.17ms]
(pass) constructor statements > expression with parameter properties and statements [0.10ms]
(pass) export default class 
... (truncated)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/mechgate.xml" test/bundler/transpiler/decorators.test.ts test/bundler/transpiler/es-decorators.test.ts
bun test v1.4.1 (65362b53b)

test/bundler/transpiler/decorators.test.ts:
(pass) decorator order of evaluation [51.04ms]
(pass) decorator factories order of evaluation [39.84ms]
(pass) parameter decorators [30.21ms]
(pass) decorators random [66.84ms]
(pass) class field order [9.08ms]
(pass) changing static method [10.28ms]
(pass) class extending from another class [16.05ms]
(pass) decorated fields moving to constructor [9.51ms]
(pass) only class decorator [4.93ms]
(pass) decorators with different property key types [12.00ms]
(pass) only property decorators [11.36ms]
(pass) only argument decorators [5.53ms]
(pass) no decorators [4.32ms]
(pass) constructor statements > with parameter properties [12.95ms]
(pass) constructor statements > class expressions (no decorators) [12.28ms]
(pass) constructor statements > with parameter properties and statements [7.28ms]
(pass) constructor statements > with parameter properties, statements, and decorators [9.83ms
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     e9b7b98728
  features     baseline

23 deps, 131 codegen, 1172 objects in 1260ms

ninja: Entering directory `/workspace/bun/build/release'
[1/1244] gen bindgenv2
[2/1244] install /workspace/bun
bun install v1.4.1-canary.1 (65362b53b)

Checked 26 installs across 63 packages (no changes) [24.00ms]
[3/1244] fetch zlib
[zlib] up to date
[4/1244] install /workspace/bun/packages/bun-error
bun install v1.4.1-canary.1 (65362b53b)

Checked 1 install across 2 packages (no changes) [1.00ms]
[5/1244] gen ProcessBindingConstants.lut.h
Generating /workspace/bun/build/release/codegen/ProcessBindingConstants.lut.h from /workspace/bun/src/jsc/bindings/ProcessBindingConstants.cpp
[6/1244] fetch libjpeg-turbo
[libjpeg-turbo] up to date
[7/1217] fetch tinycc
[tinycc] up to date
[8/1216] gen JSBuffer.lut.h
Generating /workspace/bun/build/release/codegen/JSBuffer.lut.h from /workspace/bun/src/jsc/bindings/JSBuffer.cpp
[9/1216] gen bake.{client,server,error}.js
-> bake.client.js, bake.server.js, bake.error.js
... (truncated)

src/ast/g.rs                                  |   6 +-
 src/ast/lib.rs                                |   3 +
 src/js_parser/lower/lower_auto_accessors.rs   | 266 ++++++++++++
 src/js_parser/lower/lower_decorators.rs       |  10 +-
 src/js_parser/lower/mod.rs                    |   1 +
 src/js_parser/p.rs                            |  14 +-
 src/js_parser/parse/parse_property.rs         |  10 +-
 src/js_parser/visit/mod.rs                    |   6 +
 src/jsc/RuntimeTranspilerCache.rs             |   5 +-
 test/bundler/transpiler/decorators.test.ts    | 598 +++++++++++++++++++++++++-
 test/bundler/transpiler/es-decorators.test.ts |  37 ++
 11 files changed, 942 insertions(+), 14 deletions(-)

gate history · 4 passed · 0 rejected · iteration 0

file                                           reads  edits  tests
src/ast/g.rs                                       0      0      0
src/ast/lib.rs                                     0      0      0
src/js_parser/lower/lower_auto_accessors.rs        0      0      0
src/js_parser/lower/lower_decorators.rs            0      0      0
src/js_parser/lower/mod.rs                         0      0      0
src/js_parser/p.rs                                 0      0      0
src/js_parser/parse/parse_property.rs              3      2      0
src/js_parser/visit/mod.rs                         0      0      0
src/jsc/RuntimeTranspilerCache.rs                  2      2      0
test/bundler/transpiler/decorators.test.ts         0      0      0
test/bundler/transpiler/es-decorators.test.ts      1      2      0

root cause · written by the author bot

The parser did not support the accessor class-member keyword under TypeScript's experimentalDecorators mode, so auto-accessors were rejected or mishandled instead of being desugared like tsc does. The fix teaches the parser to accept accessor and adds a dedicated lowering pass that rewrites each accessor x = init into a private backing field with a generated getter and setter pair, attaching decorators and metadata to the setter so computed keys evaluate exactly once and only design:type is emitted. It also marks static accessor initializers as side effects in Class::can_be_moved …