Statements generated from syntax lowering or keepNames block treeshaking

[bluwy]

When using the transform API with syntax lowering (e.g. for classes) or with keepNames (--keep-names), the code generated often look something like this:

const _Foo = class _Foo {
};
__name(_Foo, "Foo");
__publicField(_Foo, "prop", "value");
export let Foo = _Foo;

Playground link

Because the __publicField and __name aren't marked as pure (/* @__PURE__ */), even if _Foo is unused, bundlers are not able to assume it's safe to treeshake it. Should esbuild be prepending the /* @__PURE__ */ comments to __publicField and __name?

The __publicField and __name statements are holding a value on _Foo, which prevents it from being treeshaken if the exports are unused in bundlers.

---

I noticed for function expressions and keepNames, e.g. with export const foo = () => {}, esbuild is already transforming it to export const _foo = /* @__PURE__ */ __name(() => {}, "foo"). This has allowed esbuild to treeshake it if unused.

Perhaps something similar can be done for non-expressions and for the lowering code.

[hyrious]

FYI, pure annotations don't work on statements. The only way to mark the Foo as pure in your case is:

export let Foo = /* @__PURE__ */ (() => {
	const _Foo = class _Foo {
	};
	__name(_Foo, "Foo");
	__publicField(_Foo, "prop", "value");
	return _Foo;
})();

[bluwy]

Ah that pretty much makes this feature request moot then. I don't know if esbuild is up to transforming to that way to solve the treeshaking issue, or if there's some other solution to this.

[bluwy]

Going to close this for now as the issue is kinda broad and is probably worth as a better feature request to resolve the issue generically.