Don't emit redundant empty arrays for modules with no dependencies (#1858)

[robhogan]

Summary:

Why?

This is a small optimisation for runtime speed and bundle size that avoids allocating an empty array for an argument that is not referenced by the function (module) body.

Metro-Buck already does this. This diff aligns Metro core and dev, which unblocks unforking the serialisers without introducing a regression.

Why this is non-breaking

By construction, an empty dependency map means the module has no collected dependencies, so the module body has no references to the dependency map injected by Metro.

We must still ensure there are no references not injected by Metro. Two cases:

• transformer.unstable_dependencyMapReservedName is set - in this case we enforce that there are no occurrences of the name (as a string) in the module body at all.
• Above is not set (default). In that case we use generateUid when wrapping the module to ensure we pick a name that is not already bound (and note, this is after any user/framework-supplied transforms)

In either case, the module body has no references to the argument.

One caveat to mention is that the argument is technically accessible using the arguments array-like. This is definitely not considered part of the stable API of Metro.

Note: Bundle analysers

A category of potentially-affected consumers are those which statically analyse the bundle and might expect an array at the dependency map position. Even here, we should be safe because we already have certain module types with a simpler module wrapper - e.g. wrapJson never emits a dependency map. That said, heads up @byCedric / Atlas.

Changelog: Internal

Reviewed By: huntie

Differential Revision: D113660291

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D113660291.

[meta-codesync]

This pull request has been merged in db49f2c.