fix(transpiler): don't inline const values into function bodies

[robobun]

Summary

• Fix const-inlining optimization replacing variable references with literal values inside function bodies, which changed the observable result of Function.prototype.toString()
• Only affects the runtime transpiler (non-bundler mode); bundler path is unchanged since users explicitly opt into minification there
• Add regression tests covering the original issue, same-scope inlining (still works), inner-function inlining (still works), toString() preservation, and let variables

Root Cause

Bun's transpiler has a const-inlining optimization that replaces references to const variables with their literal values. The const_values map was global to the parser and did not respect function scope boundaries — so const hi = "hi"; log(() => console.log(hi)) was transpiled to log(() => console.log("hi")), changing what Function.prototype.toString() returns.

When the serialized function was eval()'d in another module, the inlined literal meant the eval succeeded when it should have thrown ReferenceError, since the original variable hi doesn't exist in the eval scope.

Fix

Save and clear the const_values map when entering function/arrow bodies (in visitFunc and e_arrow), and restore it when leaving. This prevents outer-scope const values from being inlined into function bodies. Const values declared inside a function body are still inlined within that function.

The fix is gated on !p.options.bundle so the bundler path (where users explicitly opt into minification via minifySyntax) is unaffected.

Test plan

• bun bd test test/regression/issue/12710.test.ts — 5/5 pass
• USE_SYSTEM_BUN=1 bun test test/regression/issue/12710.test.ts — 1 fail (confirms test catches the bug)
• bun bd test test/bundler/esbuild/default.test.ts — 150 pass (was 149 before fix due to ConstDeclRemovedIfReferencedBeforeAllUses)
• bun bd test test/bundler/bundler_minify.test.ts — 32 pass
• bun bd test test/bundler/bundler_string.test.ts — 59 pass
• bun bd test test/bundler/esbuild/ts.test.ts — 57 pass

Closes #12710

🤖 Generated with Claude Code

[robobun]

^{Updated 8:12 AM PT - Feb 19th, 2026}

❌ Your commit dc3ba54f has 4 failures in Build #37589 (All Failures):

• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 14 aarch64
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 13 aarch64
• test/regression/issue/24364.test.ts - 1 failing on 🪟 11 aarch64
• test/regression/issue/11297/11297.test.ts - 1 failing on 🪟 2019 x64
• test/regression/issue/11297/11297.test.ts - 1 failing on 🪟 11 aarch64
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🪟 11 aarch64
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🪟 2019 x64-baseline
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🪟 2019 x64
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🐧 3.23 x64-baseline
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🍎 13 x64
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🍎 13 aarch64
• test/js/third_party/mongodb/mongodb.test.ts - 1 failing on 🍎 14 aarch64

---

🧪   To try this PR locally:

bunx bun-pr 27160

That installs a local version of the PR into your bun-27160 executable, so you can run:

bun-27160 --bun

[coderabbitai]

Warning

Rate limit exceeded

@robobun has exceeded the limit for the number of commits that can be reviewed per hour. Please wait 5 minutes and 28 seconds before requesting another review.

⌛ How to resolve this issue?

After the wait time has elapsed, a review can be triggered using the @coderabbitai review command as a PR comment. Alternatively, push new commits to this PR.

We recommend that you space out your commits to avoid hitting the rate limit.

🚦 How do rate limits work?

CodeRabbit enforces hourly rate limits for each developer per organization.

Our paid plans have higher rate limits than the trial, open-source and free plans. In all cases, we re-allow further reviews after a brief timeout.

Please see our FAQ for further information.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[claude]

✅ dc3ba — Looks good!

Reviewed 3 files across src/ast/ and test/regression/: Prevents the transpiler from inlining outer-scope const values into function and arrow function bodies when not bundling, preserving Function.prototype.toString() behavior for code that relies on eval() of function source text.

[Jarred-Sumner]

Nooo