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WalkthroughThe parser now processes substitution revisits through normal visitation and usage accounting. Substitution avoids revisiting replaced expressions and adds a guard for side-effecting values before short-circuit operands. Switch case bodies are minified after all bodies have been visited. Regression tests and the runtime transpiler cache version are updated. ChangesSubstitution and minification
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The title clearly and concisely describes the primary change: preventing a second statement visit after inlining a single-use value. |
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Updated 8:18 AM PT - Oct 17th, 2026
✅ @robobun, your commit 493c5c119ad59f4801ade9fb8a5bd65f77511e3b passed in 🧪 To try this PR locally: bunx bun-pr 45507That installs a local version of the PR into your bun-45507 --bun |
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Status Reproduced on Bun 1.4.2, on 1.4.3-canary (367d939) and on a release build of main e4c638f. Each row is one file, run with no flag unless the row names one. Node is v26.3.0.
PR: #45507 |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked that the deferred minify_visited_stmts call in s_switch still runs inside the switch's pushed block scope (so its current_scope() direct-eval check reads the right scope), and that the removed import_record_index != u32::MAX early return in e_import_call was only reachable from the now-deleted re-visit, so no import record is minted twice. No reader of is_revisit_for_substitution remains in src/.
Extended reasoning...
The change removes the post-substitution re-visit in the JS parser's single-use symbol minifier, adds a short-circuit guard for &&/||/??, moves switch-case minification after all cases are visited, and bumps the runtime transpiler cache version; it touches no security-sensitive surface. One confirmed behavior regression (defines no longer applied through a function-local single-use alias) is posted inline, and the parser/minifier is core bundler code that warrants a maintainer's judgment on that trade-off.
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…of &&, || or ??
The single-use substitution moves a let or const into the next statement.
A literal on the left of a logical operator does not stop the walk, so a
call could move into the right side, which does not always run:
let x = fn(); let k = 0; return [k && x];
printed as `return [0 && fn()]` and fn was never called. The literal only
gets there through an earlier substitution, because the visit pass folds
`0 && x` when it is written that way.
Return Failure for the right operand of &&, || and ?? when the value has
side effects. esbuild has the same hole and no such check.
All case bodies of a switch share one block scope, but each body is its
own statement list and was minified as soon as it had been visited. The
single-use inliner then saw a let or const of an earlier case with a use
count that did not include the later cases:
switch (v) { case 3: let acc = []; acc.push(3); case 2: acc.push(2); return acc; }
// printed as
switch (v) { case 3: [].push(3); case 2: acc.push(2); return acc; }
Visit every case body first, then run the statement-list minify pass on
each of them. A use in the same case is still inlined.
This is the change of #41848.
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
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Actionable comments posted: 2
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Inline comments:
Review comments at @src/js_parser/visit/visit_stmt.rs:
- Around line 2157-2163: Shorten or remove the comment above the minification
loop in the switch-case handling code; keep the visit-before-minify loop order
unchanged.
Review comments at @test/bundler/bundler_minify.test.ts:
- Line 2193: Update both `test.each` blocks for the `minify.syntax` target
cases—covering the single-use `require` alias and the define behavior—to use
`test.concurrent.each`, so the independent target tests run concurrently.
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📒 Files selected for processing: 11
src/js_parser/p.rssrc/js_parser/visit/mod.rssrc/js_parser/visit/visit_expr.rssrc/js_parser/visit/visit_stmt.rssrc/jsc/RuntimeTranspilerCache.rstest/bundler/bundler_dynamic_import_dce.test.tstest/bundler/bundler_jsx.test.tstest/bundler/bundler_minify.test.tstest/bundler/transpiler/assign-to-import.test.tstest/bundler/transpiler/runtime-transpiler.test.tstest/bundler/transpiler/transpiler.test.js
💤 Files with no reviewable changes: 1
- src/js_parser/visit/visit_expr.rs
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… inlined After the minifier moved a single-use let or const into the next statement, substitute_single_use_symbol_in_stmt visited the whole statement a second time so that constants could fold. The visit pass is not written to run twice on a node: - e_call decided direct eval from the callee it saw on entry. The first visit had folded `(0, eval)` to `eval`, so the second visit marked an indirect eval as direct and the printer dropped the `(0, ...)` wrapper. The code then ran with the caller's local variables in reach, and every enclosing scope was marked as containing a direct eval. - A property access that the move created (`c.limit` to `config.limit`) was bound to the import on the second visit while use counts were muted, so a parameter of the same name captured it in the bundle. - An assignment target that the first visit had already rewritten (`module.exports.app = app`, `exports.count++`, an imported binding) was reported as "Invalid assignment target". - A hook call was added to its React Fast Refresh signature twice, the exports that `import().then(m => m.c)` reads were no longer tracked, and `module.require(name)` through an alias printed a `__require` that the bundle did not define. Visit a statement once, as esbuild does. Remove is_revisit_for_substitution, its guards, and the e_import check that only the second visit could reach. This removes the fold after the move that 047754d added on purpose: `let x = 1; return x + 2` prints `1 + 2`, not `3`.
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let/constinto the next statement, then visits that statement again (substitute_single_use_symbol_in_stmt,src/js_parser/p.rs). The visit pass assumes one visit per node.const r = (0, eval)(code); return r + "!"runscodeas a direct eval.const v = g(); x = von an import fails to load:error: Invalid assignment target.bun build --minify-syntaxgives that error formodule.exports.app = app, and printsreturn limit + limitwhere a parameter shadows an import.Fix
is_revisit_for_substitution. esbuild has neither.&&,||,??.switchcases after every case is visited (from minify: don't inline a single-use let out of a switch case another case still references #41848). A use in the same case still inlines.transpiler.test.js,runtime-transpiler.test.ts,bundler_minify.test.ts, then all oftest/bundler/. Self-reviewed: 22 concerns raised, 17 addressed (Notes).Background
is_direct_evaltells the printer to keepeval(x). Other calls ofevalprint as(0, eval)(x).let x = 1; return x + 2prints3. This PR reverses it.e_callguard that the report asked for. Built from source, it fixes 39 of 187 shape rows.Downsides
--minify-syntax+2 B. 25 of 1,995 package bundles differ, +125 B of 150.2 MB.defineno longer applies through a single-use alias, also withbun run --define. An aliasedrequireis not bundled.switch. Release.text: -3,072 B.Notes
Origin. A differential fuzz against Node found these. No user report exists for them. The one user-filed bug of this pass, #20398 (a single-use
const named = function () {}loses its.name), has another cause and is not changed here.What needs a maintainer's yes.
return [x + 2]prints[1 + 2]on main.!(a === b)fora !== b(25),!!!xfor!x(8), and four others (null == null, a template concat,(1 << 8) - 1). No module list changes.it.todosince mark currently known test fails as.todo#3052. This PR turns the block on, changes the four fold rows to the esbuild output, and corrects four rows that were stale on main.e_call. The fallback (that guard, a bail infold.rsfor the namespace rewrite, the&&rule and theswitchchange) was built from source and run on the same table: of the 187 rows below it matches the unminified build in 39 and main--minify-syntaxin 148. It leavesInvalid assignment targetonexports.x = v, the wrong Fast Refresh signature, the lostimport().thentree-shaking and the dangling__require.process.env.NODE_ENVdefine of a browser build:const env = process.env; return env.NODE_ENV !== "production"printedreturn !0on main under minify, and now readsprocessat run time, as the unminified build does. On main the define applied through the alias only when the root of the statement was a binary, unary or conditional expression:return env.NODE_ENVprintedreturn process.env.NODE_ENV, and an alias at module scope or with two uses was not touched. In 1,995 package bundles built with--minify-syntax --target=browser, no changed line containsprocess,env,requireorimport.meta.What else the second visit did wrong on main (each has a test in this PR):
exports.count++after a single-useconstisInvalid assignment targetunder--minify-syntax.8g6kz47xOc4=with--minify-syntax,yRI4DTW2zYY=without).const v = g(); return v + await import("./lib.js").then(m => m.c)kept every export oflib.js.const m = module; return m.require(name).value + 1printed__require(name)in an ESM bundle that does not define__require.let a1 = a0 + g(1); ... return aNin one function is visited once per declaration.transformSynctakes 287 / 1,109 / 4,448 ms at N = 4k / 8k / 16k on main and 1.9 / 4.5 / 8.3 ms here, with the same output.The
switchchange. It is the two source hunks and the test of #41848, which can close when this lands. #40791 is the same change, and its test shapes are in the table and inruntime-transpiler.test.ts. It answers the question on #41848: the pass is moved, not removed.case 0: let a = g(); return a + 1; default:still printsreturn g() + 1;. It is in this PR because the second visit hid part of that bug: without it,case 0: let a = [1]; let k = 0; k && use(a); case 1: return a;would loselet a. On main,case 3: let acc = []; acc.push(3); case 2: acc.push(2); return acc;already throwsReferenceError: acc is not defined.The
&&rule. esbuild does not have it and drops the call inlet x = fn(); let k = 0; return [k && x](0.21.5). Main drops it too when the root is not re-visited.Measurements (release builds of main e4c638f and this branch, linux x64):
--minify-syntax --target=node --packages=external), +125 B of 150,224,164 B (wc -candcmp).--minifyentry.--minify-syntax, 187 of those equal main without minify, 0 third results. In 28 of them the minified bundle ran on main and now throws as the unminified bundle does. Corpus bundles with a different module list: 0 of 1,995.&&rule 1,127 differ. Case-list fuzz (20,000 functions): main 4,505, PR 0..text: 81,109,957 B -> 81,106,885 B (size -A).perfandvalgrindare not installed here, so aperf_event_opencounter (PERF_COUNT_HW_INSTRUCTIONS, user only) took their place..pilefiles 1 -> 1, PR printsundefined!).switchchange allocates one arena vector of 24 B per case for eachswitch(873 vectors, 196 KB for typescript.js, freed with the arena).Not fixed here (the substitution itself, not the second visit):
.name: An arrow function name is erased if it's not used directly. #20398, js_parser: don't inline single-use anonymous fn/arrow/class initializers #36313let x = a.m; return xt`` callsmwith `this === a`: Keepthisundefined for call targets and template tags that a rewrite turned into a property access #40829let x = a?.b; return new x()printsnew a?.b(): Keep the parentheses of an optional chain undernewand as a template tag, and rejectnew a?.b()#41638let __proto__ = v; return { __proto__ }sets the prototype: Keep __proto__ object keys as written when printing and synthesizing object literals #40812let x = notDefined; return typeof xdoes not throwvar eval = globalThis.eval; (0, eval)(code)prints a directeval(code), as esbuild doesOpen PRs that read the removed flag: #44467, #44833, #41156, #40827, #35471, #39439 (10 lines). Each guard becomes a deletion. #44467 merges with this branch without a conflict and then does not compile (
E0609onis_revisit_for_substitution), so land this first and rebase #44467. Its testdoes not change a define value that the minifier visits againhas no subject after this PR.Deleted with the flag: the
e_importcheck onimport_record_index. Only the second visit reached it:transpose_importreturns a new node, and a debug counter in that branch stayed at 0 over 122 test files.Self-review. 22 concerns raised, 17 addressed. Not done: four that ask whether the module loader should run this pass at all (a maintainer's decision), and a debug-only visit-once check in
visit_expr_in_out(a hash insert per visited node, better as its own change).Tests. Every changed test file fails on main.
transpiler.test.jshas a table of 282 bodies that runs each body as written (new Function) and as the transpiler prints it, and compares the result and the order of calls: 98 bodies differ on main. Of the 121 files oftest/bundler/,test/js/bun/transpiler/and the transpiler cache test, the debug build passes 114. The other 7 time out in--compileand bytecode tests on this machine. The release build of this branch passes those 7 as the release build of main does, except one snapshot entry (bun build --bytecode libraries.js) that fails the same way on main here.[human-review] gate passed · iteration 1 · 11 files touched
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