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Fix capacity under-allocation in ESMConditions.init with user conditions #30479

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Fix capacity under-allocation in ESMConditions.init with user conditions #30479

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7 changes: 4 additions & 3 deletions src/bundler/options.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -1087,9 +1087,10 @@
var require_condition_map = ConditionsMap.init(allocator);
var style_condition_map = ConditionsMap.init(allocator);

try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
try import_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
try require_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
const addons: usize = if (allow_addons) 1 else 0;

Check notice on line 1090 in src/bundler/options.zig

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Claude / Claude Code Review

Same if-else precedence footgun remains in loadersFromTransformOptions 

Heads up: the same `if ... else 0 + ...` precedence footgun this PR fixes also exists in `loadersFromTransformOptions` in this file (lines 1575-1578). It's harmless there — the under-reserved capacity still covers the `putAssumeCapacity` calls and the remaining inserts use `getOrPutValue` which grows the map — so it's just a missed pre-allocation, but you may want to fix both occurrences together. (Pre-existing, not introduced by this PR.)

@claude claude Bot May 11, 2026

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🟣 Heads up: the same if ... else 0 + ... precedence footgun this PR fixes also exists in loadersFromTransformOptions in this file (lines 1575-1578). It's harmless there — the under-reserved capacity still covers the putAssumeCapacity calls and the remaining inserts use getOrPutValue which grows the map — so it's just a missed pre-allocation, but you may want to fix both occurrences together. (Pre-existing, not introduced by this PR.)

Extended reasoning...

What the bug is

This PR fixes a Zig operator-precedence footgun in ESMConditions.init where if (allow_addons) 1 else 0 + conditions.len parses as if (allow_addons) 1 else (0 + conditions.len). The identical pattern still exists in the same file at loadersFromTransformOptions (src/bundler/options.zig:1575-1578):

input_loaders.extensions.len +
    if (target.isBun()) default_loader_ext_bun.len else 0 +
        if (target == .browser) default_loader_ext_browser.len else 0 +
            default_loader_ext.len,

In Zig grammar, an IfExpr's else-branch greedily consumes a full expression, so each trailing + ... binds into the preceding else rather than into the outer sum. The indentation suggests four addends, but the parser sees nested if-expressions.

Step-by-step proof

Take target = .bun (so target.isBun() is true, target == .browser is false). With default_loader_ext_bun.len = 2, default_loader_ext_browser.len = 1, default_loader_ext.len = 18:

  1. The expression is extensions.len + (if (true) 2 else (0 + (if (false) 1 else (0 + 18)))).
  2. The outer if takes the then branch → 2. Everything after else (including default_loader_ext.len) is discarded.
  3. Result: extensions.len + 2.
  4. Intended result: extensions.len + 2 + 0 + 18 = extensions.len + 20.

So when target.isBun(), the capacity hint drops 18 entries. (When the target is neither bun nor browser, the nested elses do happen to sum to 0 + 0 + 18, which is accidentally correct.)

Why it doesn't crash

Unlike ESMConditions.init, this instance is benign:

  • stringHashMapFromArrays only calls putAssumeCapacity for keys = input_loaders.extensions. Since input_loaders.extensions.len is always the first addend (outside any if), the reserved capacity is always ≥ the number of putAssumeCapacity calls regardless of how the rest mis-parses.
  • The subsequent default_loader_ext / default_loader_ext_bun / default_loader_ext_browser inserts use try loaders.getOrPutValue(...), which grows the map on demand.
  • Additionally, when keys.len == 0 (no user-supplied loaders, the common case), stringHashMapFromArrays skips ensureTotalCapacity entirely, so the hint is already best-effort.

Impact

Zero correctness impact — purely a missed pre-allocation that causes one or two extra rehashes during options setup. No crash, no behavioral difference.

How to fix

Same approach as this PR — hoist the conditionals into locals so the addition is unambiguous:

const bun_exts: usize = if (target.isBun()) default_loader_ext_bun.len else 0;
const browser_exts: usize = if (target == .browser) default_loader_ext_browser.len else 0;
// ...
input_loaders.extensions.len + bun_exts + browser_exts + default_loader_ext.len,

This is pre-existing and untouched by the diff; mentioning it only because the PR's express purpose is eliminating this exact footgun in options.zig, so fixing both occurrences together seems natural. Not a blocker.

try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
try import_condition_map.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
try require_condition_map.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
try style_condition_map.ensureTotalCapacity(defaults.len + 2 + conditions.len);

import_condition_map.putAssumeCapacity("import", {});
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21 changes: 21 additions & 0 deletions test/bundler/bun-build-api.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -649,6 +649,27 @@ describe("Bun.build", () => {
expect(await html?.text()).toContain("<meta name='injected-by-plugin' content='true'>");
},
);

test.concurrent("many custom conditions does not crash", async () => {
const dir = tempDirWithFiles("bun-build-api-many-conditions", {
"entry.js": "export const x = 1;\n",
});
const conditions = Array.from({ length: 64 }, (_, i) => "cond" + i);
await using proc = Bun.spawn({
cmd: [
bunExe(),
"-e",
`const r = await Bun.build({ entrypoints: [${JSON.stringify(join(dir, "entry.js"))}], conditions: ${JSON.stringify(conditions)} }); if (!r.success) throw new AggregateError(r.logs); console.log("ok");`,
],
env: bunEnv,
stdout: "pipe",
stderr: "pipe",
});
const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
expect(stderr).toBe("");
expect(stdout).toBe("ok\n");
expect(exitCode).toBe(0);
});
});

test.concurrent("macro with nested object", async () => {
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