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bundler: let DCE drop unreachable top-level await before rejecting CJS - #47812
bun build --format=cjs rejected any top-level await at parse time, even when the await sat inside a branch that constant folding / dead code elimination would have removed:
// entry.jsif(typeofTEST==="undefined" ? false : TEST){awaitimport("node:fs");// Do something with the filesystem}foo();
$ bun build entry.js --minify --format=cjs --define TEST=false2 | await import("node:fs"); ^error: "await" can only be used inside an "async" function
esbuild handles this fine: the if (false) { ... } block is dropped by DCE and the unreachable await is never reported. That blocks users who use --define to gate test-only code paths behind an import("node:fs") that never runs in production.
Cause
At module scope, when the target output format didn't support top-level await (--format=cjs), the parser used allow_await = .allow_ident. That treats await as a plain identifier and relies on a lexer-level prev_token_was_await_keyword flag to emit the TLA error the moment a follow-up token can't be interpreted as an identifier continuation (e.g. the import keyword). The diagnostic fires at lex/parse time — well before constant folding has a chance to eliminate the dead branch.
Fix
Parse await EXPR as an await expression at module scope even when the output format is CJS, as long as the following token unambiguously starts an expression rather than continuing an identifier (so var await = 42 and console.log(await) still work as identifier uses). Track "live" top-level awaits separately in the visit pass (has_live_top_level_await), and only let them imply .esm after DCE has run. If a live top-level await survives DCE in a CJS target, emit the diagnostic after visit:
error: Top-level await is currently not supported with the "cjs" output format
This matches esbuild's behaviour.
Verification
test/regression/issue/29243.test.ts covers 26 cases. The core four:
CJS + dead await → builds to foo();, no error (the bug).
CJS + live await → rejected with the new diagnostic, not the old lex-level one.
var await = 42; console.log(await) in a CJS file → still parses as identifier use.
await inside a non-async function → still reports the existing "await" can only be used inside an "async" function error with the "Consider adding the async keyword here" note.
Review rounds added coverage for: dead/live for await loops and their diagnostic ranges, tagged templates, await {}, default parameters of non-async functions and arrows, top-level return plus a dead await, TS await! / await as T / await satisfies T, await using declarations, TS namespace and declare enum bodies, and dead switch/with statements. The last case also fixes the dead-control-flow filter (should_keep_stmt_in_dead_control_flow): switch/with husks were kept wholesale in dead branches, which would have leaked their await-bearing test expressions into non-async CJS output; they now recurse into their bodies like the other statement kinds. The AOT compiler keeps a model of that filter (src/sema/fold/removed_by_bun_run.rs) to predict what bun run removes, and it is updated to the same rules so a file whose only CommonJS name sits beside a dead switch or async function is still compiled as a module. A var in the brace-less body of a dead if/while/do at module scope now relocates out like the braced form does (maybe_relocate_vars_to_top_level no longer bails at module scope in dead control flow), so the husk and its head expression drop; the one visible output change is if (false) var x = 2; printing as if (false) ; plus a hoisted var x;. The todo esbuild parity cases for top-level await in cjs and iife output, including the dead-branch variants, are enabled.
The dead-await cases regress on the installed bun but pass on this build.
Supersedes #29246, which the stale bot closed after 90 days without a maintainer review. Same change, rebased onto current main (one new S::ForOf construction site in src/lint/react_compiler/convert.rs needed the await_range field).
ASAN without fix: 19 failed, 107 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test"--reporter=junit""--reporter-outfile=/tmp/pr_gate.xml" test/bundler/esbuild/default.test.ts test/bundler/transpiler/transpiler.test.js "test/regression/issue/29243.test.ts"bun test v1.4.4 (367d939d9)test/regression/issue/29243.test.ts:26 | stderr: "pipe",27 | stdout: "pipe",28 | });29 | 30 | const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);31 | expect(stderr).toBe(""); ^error: expect(received).toBe(expected)- ""+ "2 | await import("node:fs");+ ^+ error: "await" can only be used inside an "async" function+ at /tmp/issue-29243-dead-tla_tUoz2G/entry.js:2:9+ "- Expected - 1+ Received + 5 at <anonymous> (/workspace/bun/test/regression/issue/29243.test.ts:31:18)(fail) bun build --format=cjs drops an unreachable top-level await before reporting TLA [497.51ms]49 | });50 | 51 | const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);52 | expect(stdout).toBe("");53 | 54 | expect(stderr).toContai... (truncated)release without fix: 3 failed, 107 skippedbun test v1.4.4-canary.1 (e43def048)test/regression/issue/29243.test.ts:(pass) bun build --format=cjs still rejects a live top-level await [20.39ms](pass) await inside a non-async function nested in a CJS file still reports a useful error [20.13ms](pass) bun build --format=cjs still rejects a live top-level for await loop [18.51ms](pass) bun build --format=cjs rejects await in a default parameter of a non-async function [18.72ms](pass) bun build --format=cjs rejects await in a default parameter of a non-async arrow [19.77ms](pass) bun build --format=cjs drops an unreachable top-level await before reporting TLA [27.69ms](pass) bun build --format=cjs drops a literal if (false) top-level await [24.41ms](pass) bun build --format=cjs drops an unreachable await of an object literal [23.56ms](pass) await as a tagged-template call identifier keeps working in CJS [24.25ms](pass) bun build --format=cjs underlines the await token of a live for-await loop [20.68ms](pass) await can still be used as an identifier at module scope in CJS output [25.78ms](pass) bun build --format=cjs drops an unreachable top-level for await loop [23.67ms](pass) bun build --format=cjs ... (truncated)
passes on PR (with fix)
ASAN with fix: 107 skipped
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test"--reporter=junit""--reporter-outfile=/tmp/pr_gate.xml" test/bundler/esbuild/default.test.ts test/bundler/transpiler/transpiler.test.js "test/regression/issue/29243.test.ts"bun test v1.4.4 (367d939d9)test/regression/issue/29243.test.ts:(pass) await inside a non-async function nested in a CJS file still reports a useful error [348.25ms](pass) bun build --format=cjs still rejects a live top-level await [383.67ms](pass) bun build --format=cjs drops a literal if (false) top-level await [365.95ms](pass) bun build --format=cjs drops an unreachable top-level await before reporting TLA [444.88ms](pass) await can still be used as an identifier at module scope in CJS output [407.68ms](pass) await as a tagged-template call identifier keeps working in CJS [369.33ms](pass) bun build --format=cjs still rejects a live top-level for await loop [343.95ms](pass) bun build --format=cjs rejects await in a default parameter of a non-async function [346.50ms](pass) bun build --format=cjs drops an unreachable top-level for await loop [384.44ms](pass) bun build --format=cjs drops an unreachable await o... (truncated)release with fix: 107 skipped
$ bun scripts/build.ts --profile=release[configured] bun-profile → bun (stripped) in 794ms (unchanged)ninja: Entering directory `/workspace/bun/build/release'[1/64] rustc bun_resolve_builtins [2/64] rustc bun_install_types [3/64] rustc bun_options_types [4/64] rustc bun_ast [5/64] rustc bun_ini [6/64] rustc bun_sourcemap [7/64] rustc bun_trim [8/64] rustc bun_http [9/64] rustc bun_js_printer [10/64] rustc bun_parsers [11/64] rustc bun_router [12/64] rustc bun_resolver [13/64] rustc bun_react_compiler [14/64] rustc bun_css [15/64] rustc bun_js_parser [16/64] rustc bun_aot_interface [17/64] rustc bun_sema_baselines [18/64] rustc bun_aot_builtins [19/64] rustc bun_transpiler [20/64] rustc bun_lint_json [21/64] rustc bun_sema_parser [22/64] rustc bun_bunfig [23/64] rustc bun_lint_oxlint [24/64] rustc bun_lint_graph [25/64] rustc bun_standalone_graph [26/64] rustc bun_lint_react_compiler [27/64] rustc bun_sema_driver [28/64] rustc bun_lint_jest [29/64] rustc bun_lint_unicorn [30/64] rustc bun_format_conformance [31/64] rustc bun_lint_conformance [32/64] rustc bun_lint_markdown [33/64] rustc bun_http_jsc [34/64] rustc bun_js_parser_js... (truncated)
The parser rejected any module-scope await while targeting CJS or IIFE output, even when the statement containing it had already been proven unreachable by --define constant folding, so the dead branch triggered a hard error before dead-code elimination could remove it. The fix tracks only live module-scope await (and for await) during the visit pass, clears the recorded state when the await sits in dead control flow, and extends dead-statement handling so brace-less and nested dead bodies relocate their var declarations and drop the await-bearing remainder. The diagnostic is stil…
`bun build --format=cjs` rejected any top-level `await` at parse time,
even when it sat inside a branch that dead-code elimination would
remove (e.g. behind a `--define`-controlled `if (false) { await
import(...) }`). esbuild parses the `await`, lets DCE drop the
unreachable branch, and only reports the CJS / TLA incompatibility if
a live `await` survives.
Match that behavior:
- `P::is_at_module_scope()` walks the scope stack (stops at the real
module `Entry`, returns false on function/class/TS-namespace
scopes) to decide when an `await` identifier at module scope in a
non-ESM target should be upgraded to an `await` expression.
- `token_starts_await_expr()` gates the upgrade on the following
token: keywords / literals / `{` / `~` / a non-contextual
identifier force the keyword reading, while `of`/`in`/`as`/
`satisfies` and template/operator continuations keep `await` as an
identifier. `!` is left out so TS `await!.foo` non-null assertions
parse.
- `has_live_top_level_await` is set by the `e_await` / `s_for_of`
visitors only in reachable control flow at module scope, and
records the surviving `await`'s range for the diagnostic.
- post-visit in `parse_entry`: in non-ESM targets, clear the pre-DCE
range if nothing survived, or emit "Top-level await is currently
not supported with the \"X\" output format" if it did. Gated on
`!features.top_level_await` so ESM / bake-dev builds are untouched.
- `for await` and the `s_return` ESM-classification check get the
same module-scope / feature gating.
Regression test covers dead/live TLA, `for await`, `await` as an
identifier (incl. tagged template / `as` / `satisfies`), default
params, TS namespaces, and the diagnostic range.
Fixes#29243
…located
A switch or with kept wholesale in dead control flow leaked its
await-bearing test expression into non-async CJS output. Recurse into
their bodies like the other statement kinds; hoisted declarations are
already relocated out by the visit pass. Also remove the unreachable
in exclusion (reserved word, never tokenizes as an identifier).
…bodies
An async or generator function declaration surviving in a dead switch
case, with body, or loop body kept the whole statement, leaking its
await-bearing test expression into non-async CJS output. Statements
reached through the filter's body recursion can drop such declarations:
they are block-scoped there, and plain sloppy-mode functions were
already relocated. A dead module-level function still hoists and is
kept.
maybe_relocate_vars_to_top_level returns ok with no statement when every
decl was value-less and absorbed into the hoisted vars; s_for kept the
stale SLocal in the init slot, duplicating the declaration and keeping a
dead loop (and its await-bearing test) alive in non-ESM output. Assign
the relocation result whenever ok, matching s_for_in/s_for_of.
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Walkthrough
The parser distinguishes live module-scope await from unreachable or identifier-form await. It records diagnostic ranges, applies feature-gated top-level-await checks, updates dead-control-flow analysis, and adds regression coverage.
The parser identifies module scope, distinguishes identifier-form await, applies feature-gated top-level-await semantics, and stores the await range on ForOf. Related constructors initialize the range to Range::NONE.
Live await tracking and diagnostics src/js_parser/visit/visit_expr.rs, src/js_parser/visit/visit_stmt.rs, src/js_parser/parse/parse_entry.rs
The visitor records the first live module-scope await or for await. Parse completion clears unreachable await state or reports disabled top-level await at the recorded range.
Dead-code analysis applies nested-statement rules to function declarations and checks loop, label, with, and switch bodies. Variable relocation handles declarations in dead control flow. Related expectations cover module format and transpiler output.
Tests cover unreachable and live await forms, identifier parsing, diagnostic locations, TypeScript scopes, dead statement bodies, and preserved declarations. IIFE and CJS top-level-await tests are active.
Possibly related issues
oven-sh/bun#40968: Both concern top-level await diagnostics in CJS output. This change does not address the issue’s --bytecode behavior.
Possibly related PRs
oven-sh/bun#46615: Both handle live top-level await in CommonJS contexts.
oven-sh/bun#46748: Both address dead-code elimination and top-level-await behavior in CommonJS output.
Suggested reviewers: alii
Severity of issue fixed: Medium
Merge Risk: Minimal · up to e362b
No actionable merge-blocking risk remains in the examined changes. The regression assertions cover the previously identified gaps, and live-await diagnostic tracking preserves the outer await location.
Pre-merge checks: 4
Check name
Status
Explanation
Title check
The title clearly and concisely describes the main change: allowing dead-code elimination to remove unreachable top-level await before CJS rejection.
Linked Issues check
Issue #29243 requires bun build --format=cjs to remove an unreachable top-level await import("node:fs") after --define TEST=false folds the enclosing branch. The parser now keeps eligible module…
Out of Scope Changes check
Each changed area connects to the #29243 fix. Parser, AST (ForOf.await_range in src/ast/s.rs), visitor, and diagnostic changes implement deferred top-level-await validation. Changes to src/js_parser…
Description check
The description clearly explains the problem, cause, fix, scope, expected behavior, regression coverage, and verification results. It does not use the exact template headings, but it provides the requ…
Full details: Linked Issues check
Explanation
Issue #29243 requires bun build --format=cjs to remove an unreachable top-level await import("node:fs") after --define TEST=false folds the enclosing branch. The parser now keeps eligible module-scope await expressions as await expressions (src/js_parser/parse/parse_prefix.rs). The visit pass records only live top-level awaits (has_live_top_level_await in src/js_parser/p.rs and src/js_parser/visit/visit_expr.rs). The post-visit check in src/js_parser/parse/parse_entry.rs reports the CJS diagnostic only when a live await survives DCE. Dead-control-flow handling was corrected for switch, with, and nested function declarations (src/js_parser/scan/scan_side_effects.rs), so await-bearing husks do not leak into CJS output. test/regression/issue/29243.test.ts covers the issue's TEST=false case (dead await builds to foo(); with no error), live await and live for await rejection with the new diagnostic, identifier uses of await in CJS, and errors for await in non-async functions. The regression cases are reported to fail on the installed Bun and pass on the PR build, which is consistent with the fix. Remaining CI failures are reported by the author as pre-existing on main and are not part of this requirement. No non-coding task is required by the issue.
Full details: Out of Scope Changes check
Explanation
Each changed area connects to the #29243 fix. Parser, AST (ForOf.await_range in src/ast/s.rs), visitor, and diagnostic changes implement deferred top-level-await validation. Changes to src/js_parser/fold.rs, src/js_parser/visit/visit_stmt.rs, and src/js_parser/scan/scan_side_effects.rs prevent dead-branch declarations and await-bearing statements from being emitted into CJS output. The AOT model in src/sema/fold/removed_by_bun_run.rs is updated to match these parser rules, which the author states is needed so that AOT and bun run predict the same removals. The ForOf.await_range field is initialized in src/lint/react_compiler/convert.rs and src/react_compiler/codegen.rs. This is required by the new field and does not change behavior. Test expectation updates in test/bundler/transpiler/transpiler.test.js, test/bundler/esbuild/default.test.ts, and src/sema/file_analysis/source_tests/module_format.rs follow from the same dead-control-flow rules. One point needs attention: module_format.rs expectations for an async function declaration, a generator function declaration, and a switch inside if (typeof require === "undefined") changed from CommonJs to Esm. This is a format-detection effect of the shared dead-code rules. It is not a separate feature. The author describes it as intended, and the change is not shown to be unrelated to #29243. No unrelated change is demonstrated.
Full details: Description check
Explanation
The description clearly explains the problem, cause, fix, scope, expected behavior, regression coverage, and verification results. It does not use the exact template headings, but it provides the required information in equivalent sections.
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…model
The AOT compiler predicts what bun run removes from code that never runs.
The parser now drops block-scoped function declarations reached through a
dead statement's body and recurses into switch cases and with bodies, so
the model does the same. Without this, a file whose only CommonJS name sits
beside a dead switch or async function was compiled as sloppy while bun run
treats it as a module.
Status: not ready to merge. This PR is a draft again. The current head (e362be7) has regressions, and the fix needs a decision from a maintainer first.
What is wrong with the current head
The head gets an unreachable await out of cjs output by removing the statement around it. The engine never sees a removed statement. Reproduced on a debug build of this head merged with main b1007f1:
Bun runs programs that main and Node refuse: var c = 0; for (var { a }; c; ) {}, function f() { return; switch (0) { case 1: a: a: ; } } f();, if (false) { with (/(/) {} }.
A valid program is a SyntaxError: function f(x) { return; switch (0) { case 0: for (var [a, b] of x) ; } } f([]);.
A lazily imported module with for (var x; !x; ) x = "body"; var x = 0; returns "body". Main returns 0.
if (false) { while (await x()) { /*! legal */ } } still emits the await with --format=cjs.
What exists, not pushed here
Branch robobun/469263e7/tla-strip-rework (a7d9f8a) replaces that with the rule esbuild has: in code that never runs, await x becomes x and for await becomes for, and the statement stays. scan_side_effects.rs, fold.rs and src/sema are identical to main again.
Direct runs and default bundles against main: 318 of 318 cells of a 53-source matrix and 4,275 of 4,275 generated programs give the same verdict and message.
Bun.Transpiler over 78,597 real files: 0 outputs differ from main.
It is not pushed, because the parse half of this PR is still wrong for cjs and iife output. Checked with release builds of main and of that branch:
Input, built with --format=cjs
main
that branch
if (false) { await x; } then const v = await (Promise.resolve(1));
build fails
exit 0, Node throws ReferenceError: await is not defined
if (false) { switch (await f((x = await y) => x)) { case 1: var v; } }
build fails
exit 0, the output fails node --check
if (false) { const x = (await import("node:fs")).default; }
build fails
build fails, same message
if (false) { await using x = foo(); }
build fails
build fails, same message
Decision needed
This PR keeps a module-scope await an identifier in cjs and iife bundles, and a next-token lookahead decides when it is an await expression. A lookahead cannot decide await (x), (await x).y or await using. #36190 takes the other rule: every bundle format parses a module-scope await as a keyword, as esbuild does, and the linker rejects a top-level await for cjs and iife output. The two PRs cannot both land as they are. With #36190's parse change, the strip in this PR never runs.
The cost of that rule: var await = 1 at module scope stops building for cjs and iife output, as it already does for esm output and in esbuild.
@Jarred-Sumner, you reviewed #36190. Is that the rule you want for cjs and iife bundles? If yes, this PR becomes the change above and #36190 folds into it.
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Follow-up Comments
Keep the of exclusion; narrow it to the for-of header only.
47-50: Minor · Functional Correctness · Poor tradeoff
Keep the of exclusion; narrow it to the for-of header only.
In a CJS build, a dead top-level await of; in statement position does not reach DCE. The parser treats await as an identifier, and the trailing of is a syntax error. Dead await <identifier> statements are removed. Removing the of exclusion globally would break for (await of ...), which the regression test at test/regression/issue/29243.test.ts checks. A fix must limit the exclusion to the for-of header context.
Scope note
This is a pre-existing gap. The same input was a syntax error before this PR, because no upgrade to an await expression happened at module scope.
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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Review comment at @src/js_parser/parse/parse_prefix.rs around lines 47 - 50:
Update the TIdentifier handling in the prefix parser so the `of` exclusion
applies only while parsing a for-of header, while preserving its existing
behavior there. Outside that context, allow `of` to be parsed so top-level
`await of;` can follow the await-expression path; keep the `as`, `satisfies`,
and `using` exclusions unchanged.
Keep the first live await as the CJS diagnostic range.
127-139: Minor · Functional Correctness · Low value
Keep the first live await as the CJS diagnostic range.
For await await value in a CJS build, the error caret points at the second await. The new visitor block in e_await does not change this. The outer await claims the range only after its operand is visited. The inner await claims it first, so the outer guard fails. The parser also overwrites top_level_await_keyword for each await token while parsing the operand. Both paths leave the second await as the reported range.
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Review comment at @src/js_parser/visit/visit_expr.rs around lines 127 - 139:
Update `e_await` and the parsing path that assigns `top_level_await_keyword` so
nested awaits retain the range of the first live, outermost top-level `await`.
Prevent visiting or parsing an operand from replacing that range with an inner
`await`, while preserving the existing dead-code and function-boundary checks.
Assert the tagged-template call in the CJS output.
Assert the tagged-template call in the CJS output.
The test name says the tagged-template call keeps working in CJS. Its assertions check only the var await = String.raw declaration, empty stderr, and exit code 0. Output that drops the globalThis.output = await assignment still passes. Add an assertion for the assigned call, as the sibling identifier test does.
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Review comment at @test/regression/issue/29243.test.ts around lines 138 - 148:
The CJS regression test does not verify that the tagged-template result is
assigned to the global output. In the test containing the shown `stdout`
assertions, add an assertion that the output includes the `globalThis.output =
await` assignment, while preserving the existing declaration, stderr, and
exit-code checks.
Assert that the TypeScript ! is removed from the await! output.
Assert that the TypeScript ! is removed from the await! output.
The test checks only var await = and .then(console.log). If the ! were kept, the output would contain await!.then(console.log). That output still matches .then(console.log), so this test would not detect the regression. Add a negative assertion for await!.
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Review comment at @test/regression/issue/29243.test.ts around lines 346 - 354:
Update the regression test around the stdout assertions to verify that the
emitted JavaScript does not contain “await!”, while preserving the existing
checks for “var await =”, “.then(console.log)”, and a successful exit code.
Reviewing files that changed from the base of the PR and between 6cdb383 and e43def0.
Files selected for processing: 12
src/ast/s.rs
src/js_parser/p.rs
src/js_parser/parse/parse_entry.rs
src/js_parser/parse/parse_prefix.rs
src/js_parser/parse/parse_stmt.rs
src/js_parser/scan/scan_side_effects.rs
src/js_parser/visit/visit_expr.rs
src/js_parser/visit/visit_stmt.rs
src/lint/react_compiler/convert.rs
src/react_compiler/codegen.rs
src/sema/fold/removed_by_bun_run.rs
test/regression/issue/29243.test.ts
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Nested await await x: the visitor now claims the diagnostic range before visiting the operand, so the caret points at the outermost live await. Added a regression test that checks the column.
Tagged-template test asserts the globalThis.output = await call; the await! test asserts the ! is gone.
Not changed: narrowing the of exclusion to the for-of header. That gap (await of where of is a variable, in a dead branch) is pre-existing and needs for-of header context threaded into the prefix parser for a case nobody writes. Left as is.
On the two related issues the summary lists: #40968 is a --bytecode parse failure on live top-level await (Expected "=>"), and #40387 is --outfile out.cjs choosing a format that silently emits live top-level await. Both concern live awaits and output-format selection, not dead-branch elimination, so this PR does not close them.
If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.
A var in the brace-less body of a dead if/while/do at module scope was not
relocated, because the relocation bails when the current scope is already
the module scope. The var then pinned the dead statement, which kept its
head expression, including an await, in non-async output. Relocate in dead
control flow at module scope too, as the braced form already does.
Assert the dead branch calls are absent, not only the await keywords.
The test name says the dead if, while, and do bodies are dropped. The assertions check only that await is absent and that the three var names are present. Output that keeps a(), b(), or c() with the await removed would still pass. The if (false) husk is expected to remain, so assert the calls rather than the if keyword.
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Review comment at @test/regression/issue/29243.test.ts around lines 690 - 693:
Update the regression test assertions around the dead `if`, `while`, and `do`
bodies to verify that calls to `a()`, `b()`, and `c()` are absent from `stdout`,
while preserving the existing assertions and allowing the `if (false)` husk to
remain.
Reviewing files that changed from the base of the PR and between bdcd699 and 80677ed.
Files selected for processing: 3
src/js_parser/fold.rs
test/bundler/transpiler/transpiler.test.js
test/regression/issue/29243.test.ts
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…mmonJS name
The AOT model now drops them like the parser does, so a file whose only
CommonJS name guards such a husk is a module. Also assert the dead calls are
gone in the brace-less body test.
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Fixes #29243
Problem
bun build --format=cjsrejected any top-levelawaitat parse time, even when theawaitsat inside a branch that constant folding / dead code elimination would have removed:esbuild handles this fine: the
if (false) { ... }block is dropped by DCE and the unreachableawaitis never reported. That blocks users who use--defineto gate test-only code paths behind animport("node:fs")that never runs in production.Cause
At module scope, when the target output format didn't support top-level await (
--format=cjs), the parser usedallow_await = .allow_ident. That treatsawaitas a plain identifier and relies on a lexer-levelprev_token_was_await_keywordflag to emit the TLA error the moment a follow-up token can't be interpreted as an identifier continuation (e.g. theimportkeyword). The diagnostic fires at lex/parse time — well before constant folding has a chance to eliminate the dead branch.Fix
Parse
await EXPRas anawaitexpression at module scope even when the output format is CJS, as long as the following token unambiguously starts an expression rather than continuing an identifier (sovar await = 42andconsole.log(await)still work as identifier uses). Track "live" top-level awaits separately in the visit pass (has_live_top_level_await), and only let them imply.esmafter DCE has run. If a live top-level await survives DCE in a CJS target, emit the diagnostic after visit:This matches esbuild's behaviour.
Verification
test/regression/issue/29243.test.tscovers 26 cases. The core four:await→ builds tofoo();, no error (the bug).await→ rejected with the new diagnostic, not the old lex-level one.var await = 42; console.log(await)in a CJS file → still parses as identifier use.awaitinside a non-async function → still reports the existing"await" can only be used inside an "async" functionerror with the "Consider adding the async keyword here" note.Review rounds added coverage for: dead/live
for awaitloops and their diagnostic ranges, tagged templates,await {}, default parameters of non-async functions and arrows, top-levelreturnplus a deadawait, TSawait!/await as T/await satisfies T,await usingdeclarations, TS namespace anddeclare enumbodies, and deadswitch/withstatements. The last case also fixes the dead-control-flow filter (should_keep_stmt_in_dead_control_flow):switch/withhusks were kept wholesale in dead branches, which would have leaked their await-bearing test expressions into non-async CJS output; they now recurse into their bodies like the other statement kinds. The AOT compiler keeps a model of that filter (src/sema/fold/removed_by_bun_run.rs) to predict whatbun runremoves, and it is updated to the same rules so a file whose only CommonJS name sits beside a deadswitchor async function is still compiled as a module. Avarin the brace-less body of a deadif/while/doat module scope now relocates out like the braced form does (maybe_relocate_vars_to_top_levelno longer bails at module scope in dead control flow), so the husk and its head expression drop; the one visible output change isif (false) var x = 2;printing asif (false) ;plus a hoistedvar x;. Thetodoesbuild parity cases for top-level await in cjs and iife output, including the dead-branch variants, are enabled.The dead-await cases regress on the installed
bunbut pass on this build.Fixes #29243
Supersedes #29246, which the stale bot closed after 90 days without a maintainer review. Same change, rebased onto current main (one new
S::ForOfconstruction site insrc/lint/react_compiler/convert.rsneeded theawait_rangefield).[human-review] gate passed · iteration 20 · 16 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 3 passed · 0 rejected · iteration 20
evidence per changed file
root cause · written by the author bot
The parser rejected any module-scope
awaitwhile targeting CJS or IIFE output, even when the statement containing it had already been proven unreachable by--defineconstant folding, so the dead branch triggered a hard error before dead-code elimination could remove it. The fix tracks only live module-scopeawait(andfor await) during the visit pass, clears the recorded state when the await sits in dead control flow, and extends dead-statement handling so brace-less and nested dead bodies relocate theirvardeclarations and drop the await-bearing remainder. The diagnostic is stil…