Key the runtime transpiler cache on the define table and --drop

[robobun]

Problem

• bun run ./a.ts with a bunfig [define] serves stale output after the value changes. A second project with the same source bytes (4 KiB or more) gets the first project's values on its first run. --drop=console has the same hole. Reproduces on 1.4.1 and main.
• Cause: the cache key is the source bytes plus a features hash (Features::hash_for_runtime_transpiler, src/js_parser/parser.rs) that never covered the define table. CLI --define was safe only because Arguments.rs:1573 disabled the cache, and bun run <file> loads bunfig.toml after that check (RunCommand::boot, run_command.rs:931).

Fix

• defines_from_transform_options (src/bundler/options.rs) hashes what it adds to the Define table beyond the constant tables: the resolved define map, the inlined env string entries and the --drop list, each sorted, length-prefixed and in its own section (Define::hash_user_inputs). None when there are none.
• Define::user_hash is copied into Features::define_hash per parse (src/bundler/transpiler.rs) and folded into the features hash like --feature flags.
• Cache version 27 to 28, because old entries can hold define-substituted output under a define-free hash. The --define cache disable is removed: the key covers it now.
• Self-reviewed: 3 concerns raised, 1 addressed (user and env pairs hash as separate sections). Rejected: fold this into Key the runtime transpiler cache on the define table, the module type and --jsx-side-effects #38590 so the cache version bump is spent once (the maintainer asked for this smaller PR), and add remove_cjs_module_wrapper and is_macro_runtime to the hashed bools here (out of scope, tracked separately, the second is Key the runtime transpiler cache on macro mode #38683).
• Verified: four new tests in test/cli/run/transpiler-cache.test.ts (bunfig define across two projects through bun run, --define with flag order and a repeated key, --define on bun test, --drop). Each fails on the released binary. The rest of that file, bundler_drop.test.ts and regression tests 28159, 30887, 32686 pass.

Background

• The runtime transpiler cache stores the transpiled output of every source file of 4 KiB or more in a per-user directory (.pile files) and reuses it across processes and projects. The file name is a hash of the source bytes. The entry header carries a features hash of the options that change the output.
• The define table (Define, src/js_parser/lib.rs) makes the parser replace an expression such as BVAL with a constant. --define, bunfig [define] and --drop all feed it through defines_from_transform_options, once per VM.
• At runtime every boot path disables env inlining, so the inlined env entries hold only the window placeholder, which has no string value.

Notes

Repro on the released binary, with the cache in an empty directory:

mkdir tc pA pB
# big.ts: 'declare const BVAL: string; console.log("bval:", BVAL);' plus 5 KiB of comments
cp big.ts pA/; cp big.ts pB/
printf '[define]\nBVAL = "\\"one\\""\n' > pA/bunfig.toml
(cd pA; BUN_RUNTIME_TRANSPILER_CACHE_PATH=$PWD/../tc bun run ./big.ts)   # bval: one
printf '[define]\nBVAL = "\\"two\\""\n' > pA/bunfig.toml
(cd pA; BUN_RUNTIME_TRANSPILER_CACHE_PATH=$PWD/../tc bun run ./big.ts)   # bval: one   (expected two)
printf '[define]\nBVAL = "\\"mine\\""\n' > pB/bunfig.toml
(cd pB; BUN_RUNTIME_TRANSPILER_CACHE_PATH=$PWD/../tc bun run ./big.ts)   # bval: one   (expected mine)

bun ./big.ts (no run) was already safe: bun file.ts loads bunfig.toml inside Arguments::parse, before the IS_DISABLED check, so a bunfig define turned the cache off on that path.

Why the resolved map and not the raw flag list: the table keeps one value per key, so --define X=1 --define X=2 must key like --define X=2, and sorting a raw list with repeated keys would make X=1 X=2 and X=2 X=1 collide. A map has unique keys, so sorting by key is unambiguous.

Why the user and env pairs are separate sections: Define::init inserts the env pairs after the user pairs and DefineData::merge keeps the later value, so the same pair gives a different table depending on its source. One pooled list would key --define process.env.FLAG='"a"' with FLAG=b like --define process.env.FLAG='"b"' with FLAG=a.

Why the env entries are hashed although the runtime never inlines them: the hash is taken from the same inputs the table is built from, so the key and the table cannot disagree if a boot path ever runs with env inlining on.

On a features hash mismatch the cache entry is deleted and rewritten, so the tests count entries (net zero on a rewrite) and compare entry bytes to tell a hit from a rewrite.

Found while testing, not part of this change: under bun run <file> a bunfig [define] overrides a CLI --define, because the lazy bunfig load replaces ctx.args.define. bun <file> lets the CLI win. This is the precedence problem of #12216.

#38590 is a wider change for the same cache key (it also adds the module type and --jsx-side-effects). This PR is the smaller one.

[robobun]

Status: ready for review. The fix is green: build 108571 (76ecb70) passed on every lane. Build 108596 runs the test-only commit 11a945f; its one red lane so far is test/cli/install/bun-patch.test.ts on Windows (a long-label install path, unrelated to this diff, reported to triage).

Reproduced on the released 1.4.1 and on main (41906a4) with BUN_RUNTIME_TRANSPILER_CACHE_PATH pointed at an empty directory: bun run ./big.ts with a bunfig [define] printed the first value after the bunfig changed, and a second project with the same file printed the first project's value on its first run. --drop=console was served the plain run's entry.

The four new tests in test/cli/run/transpiler-cache.test.ts (bunfig define across two projects, --define on bun run, --define on bun test, --drop) fail on the released binary and pass with the change. The rest of that file, test/bundler/bundler_drop.test.ts and the cache regression tests 28159, 30887 and 32686 pass with the debug build.

Review: the user define pairs and the inlined env define pairs are hashed as separate sections since 55b6ec0 (a later env pair overrides a user pair in Define::init, so the source of a pair is part of the key).

[coderabbitai]

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Walkthrough

Changes

The transpiler cache now includes user defines and --drop entries. Define inputs receive a deterministic hash, which passes through parser features and affects cache invalidation. Cache format version 28 replaces version 27.

Define-aware transpiler cache

Layer / File(s)	Summary
Define hashing and propagation src/js_parser/lib.rs, src/bundler/defines.rs, src/bundler/options.rs, src/bundler/transpiler.rs, src/js_parser/parser.rs, src/js_parser/parse/parse_entry.rs	Define pairs and non-empty drop entries receive an order-independent hash. The hash propagates through worker options and parser features, including lazy exports.
Cache identity and validation src/js_parser/parser.rs, src/jsc/RuntimeTranspilerCache.rs, src/runtime/cli/Arguments.rs, test/cli/run/transpiler-cache.test.ts	The cache hash includes the define hash. The cache format version increases to 28, CLI defines no longer disable caching, and tests cover define and drop invalidation.

Suggested reviewers: jarred-sumner

Merge Risk: ⚪ Minimal · up to 11a94

The runtime transpiler cache now keys entries on define and --drop inputs, preventing stale values from being reused across changes or projects, with a cache version bump to invalidate incompatible entries. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely summarizes the primary change: adding the define table and --drop entries to the runtime transpiler cache key.
Description check	✅ Passed	The description explains the problem, fix, scope, background, and verification results. It provides the information required by the template, although it uses Problem, Fix, and Verified sections inste…

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The description explains the problem, fix, scope, background, and verification results. It provides the information required by the template, although it uses Problem, Fix, and Verified sections instead of the template headings.

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[coderabbitai]

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Inline comments:
In `@src/bundler/options.rs`:
- Line 958: Update the cache-hash construction around the chain passed to
Define::hash_user_inputs so define-source precedence is preserved. Hash user
defines and inlined environment defines separately, or hash the effective map
after environment overrides are applied, ensuring duplicate keys with different
source values produce distinct hashes while retaining the existing precedence
behavior.

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[robobun]

^{Updated 12:15 PM PT - Aug 30th, 2026}

❌ @robobun, your commit 11a945f has 1 failures in Build #108596 (All Failures):

• test/cli/install/bun-patch.test.ts - code 1 on 🪟 2019 x64

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[claude]

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[claude]

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