perf(external): drop the runtime import from chunks that never wrap

[IWANABETHATGUY]

Stacked on #10516 — review that one first.

A chunk that does not wrap an external still emitted a useless import of the runtime chunk:

Object.defineProperty(exports, Symbol.toStringTag, { value: "Module" });
require("./rolldown-runtime.js");        // nothing is used from it
let node_https = require("node:https");
exports.foo = node_https.request;

Why it happened

Not an empty cross-chunk import. The chunk really did import __toESM, so it rendered const require_rolldown_runtime = require(...); DCE then dropped the unused binding and kept the side-effectful call. The import existed because patch_module_dependencies set reads_external_as_esm from the bundle-wide record, so a module reading only a name off the external still demanded the helper and put it in its chunk's depended symbols.

The fix

Derive that edge from the same observer set the renderers use, so a module demands __toESM only when it is an observer. Both decisions now come from one source, which is what makes narrowing safe: the chunk that wraps is exactly the chunk that depends on the runtime.

Under-supplying is the dangerous direction — a chunk that wraps without the edge references a helper it never imported and finalization panics — so an observer that is not included keeps the previous bundle-wide behaviour, matching rendering's own unattributable fallback. Chunks do not exist yet at that point, so is_included stands in for "will have a chunk".

This only became safe once #10516 made rendering observer-attributed. Narrowing this edge before that reintroduces the panic, which is why it is a separate commit rather than part of the fix below it.

Result

external_interop_only_reachable_via_entry_export shows the payoff — with only entry-a demanding the helper, the runtime stops being shared and folds into entry-a, so the separate runtime chunk disappears entirely:

// entry-a.js
let node_https = require("node:https");
node_https = __toESM(node_https);

// entry-b.js
let node_https = require("node:https");
exports.foo = node_https.request;

named-user in reexport_default_import_of_external_multi_chunk loses its bare require the same way; that fixture's _test.mjs still asserts the values executably.

Those two snapshots are the only output changes. The fixture comment and internal doc previously stated this edge had to stay bundle-wide with the bare require as an accepted cost — true when written, so both are updated.

[IWANABETHATGUY]

• perf(external): drop the runtime import from chunks that never wrap #10517 👈 (View in Graphite)
• fix(external): keep __toESM when the importing module is tree-shaken #10516
• main

---

How to use the Graphite Merge Queue

Add the label graphite: merge-when-ready to this PR to add it to the merge queue.

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[netlify]

✅ Deploy Preview for rolldown-rs canceled.

Name	Link
🔨 Latest commit	5c0bfcf
🔍 Latest deploy log	https://app.netlify.com/projects/rolldown-rs/deploys/6a6ca3a7186c7d0007b1db37

[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks
⏩ 10 skipped benchmarks¹

---

_{Comparing perf/external-toesm-runtime-edge (5c0bfcf) with main (e273daa)}

Footnotes

1. 10 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[hyfdev]

The narrowing keeps the two decisions in agreement: a chunk that wraps an external — via an observer, an unattributable observer, or a kept default/namespace import — depends on the runtime, and a named-only chunk drops both the wrap and the import together. The same invariant holds on the dead-import and entry-export paths.

Two notes on the documented safety argument below; neither blocks merge.

— Review by @hyfdev, assisted by deepseek-v4-flash

[hyfdev]

The observer-scoped edge matches rendering for the attributable cases this PR targets: named-only chunks drop both the wrap and the bare runtime import, while observer chunks keep __toESM with a live runtime edge. The unattributable !is_included fallback keeps the dangerous under-supply direction conservative, and the two snapshot deltas plus the multi-chunk executable assert match that contract.

The open notes on the dual wrap/edge channels and the untested safety paths still stand. One small leftover in the same function, outside the changed hunk: the entry-export block still says the recorded interop is bundle-wide; under the new predicate a named-only entry export neither wraps nor demands the helper.

— Review by @hyfdev, assisted by Grok 4.5