Make Stacker an optional dependency (transitively)

[71]

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Describe the feature

#7719 requested for stacker to be made an optional dependency, and #7720 implemented it.

Unfortunately, Cargo features are additive so importing any package which depends on swc_ecma_parser and does not set default-features = false (such as swc_core and swc_ecma_transforms_base) will enable default features in swc_ecma_parser, and thus import stacker as a dependency.

Maybe my Cargo-fu is failing me, but AFAIK there is no workaround for this with [patch] and whatnot.

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Additional context

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[71]

EDIT: the reason why I filed this issue is that I could not build swc on macOS due to iconv not being found, which appears to be a Nix problem, not a swc problem. This issue is still valid (in that, disabling stacker cannot be done when using swc_core or swc_ecma_transforms_base), but less blocking on my end.

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I tried to add a disable_stacker feature to swc_ecma_parser, so if any dependent sets that feature we can avoid stacker with a cfg(not(feature = "disable_stack")). This works at the source-code level.

Unfortunately, by keeping stacker as a dependency (which seems inevitable; Cargo allows adding dependencies based on a feature, but not removing dependencies based on them), we still need to build that crate, and therefore the dependency on stacker remains.

I see two other possible approaches:

1. Remove stacker from default features. Easy, but may not be the best for users which expect it to be there.
2. Patch all crates in swc which depend on swc_ecma_parser to avoid the default dependency on stacker.
   • And then perhaps make swc_core/ecma_parser enable stacker by default and provide swc_core/ecma_parser_no_stacker to disable default features?