Add feature flags to Noir

[michaeljklein]

Problem

• It's ergonomic to include assertions and debugging helpers within the functions/methods to test, but this approach currently either requires duplication or ad-hoc abstractions to enable/disable code.
• Most dead code isn't eliminated until fairly late in compilation (?)

See description of how features work in Rust.

Happy Case

Minimal useful version:

• Support declaring features in Nargo.toml
  • [features] section
  • feature_name = ["included_feature1", "included_feature2", ..]
• Support #[cfg(feature = "feature_name")] on top-level declarations and filter out unused declarations after parsing
• Support let _: bool = cfg!(feature = "feature_name"); or similar for use inside declarations
• Support nargo CLI option: --features "feat1 feat2"

Outside original PR:

• Support all/any/not in cfg
• default feature
• Support other CLI options: --all-features, --no-default-features

Workaround

Yes

Workaround Description

Maintain multiple versions of declarations that need different features.

Additional Context

No response

Project Impact

Nice-to-have

Blocker Context

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response

[jfecher]

Wonder if this will be compatible with our current use of attributes & macros. Since we collect & typecheck functions before we apply attributes to them, if a function is #[cfg(...)]'d out of the program, we'd still issue errors for it. E.g. "duplicate function with name foo", or feature-specific functions not being available within its body, etc.