Proposal: ctx.actions.alias / ctx.actions.derive()

[thesayyn]

Been staring at the copy_to_bin cost in rules_js and I'm increasingly convinced the real problem is Bazel not providing , not bytes — we spin up one action per file just to move things into bin, and it adds up fast. Feels like something a lot of rulesets would hit, not just us.
Rough idea: one action that aliases a files into their bin locations, instead of N separate copies. Something like:
ctx.actions.alias(src_foo, output = bin_out)No bytes copied — on RBE it's already just the source's digest showing up at the bin path, and locally/sandbox it'd hardlink the content in. One node instead of thousands.
Before I go write this up properly: is this something Bazel would be open to since it introduces a ctx.actions.symlink()  like api that does not create a filesystem indirection but a Skyframe one?

As for how this would be materialized would be up to the Sandbox implementation, whether hardlink, symlink, or custom one that could be provided by --sandbox_backend

Alternatives we considered before could not solve the problem quite;

Tried symlinks, but that breaks the NodeJS tools in weird ways, because they tend to follow symlinks.
Clonefile/reflink alternatives; this is what we use today, but action is still there and still needs to run.

Also submitted: bazelbuild/proposals#419