Summary

Remove the tuple_indexing, if_let, and while_let feature gates and add them to the language.

Motivation

Tuple Indexing

This feature has proven to be quite useful for tuples and struct variants, and it allows for the removal of some unnecessary tuple accessing traits in the standard library (TupleN).

The implementation has also proven to be quite solid with very few reported internal compiler errors related to this feature.

`if let` and `while let`

This feature has also proven to be quite useful over time. Many projects are now leveraging these feature gates which is a testament to their usefulness.

Additionally, the implementation has also proven to be quite solid with very few reported internal compiler errors related to this feature.

Detailed design

Remove the if_let, while_let, and tuple_indexing feature gates.
Add these features to the language (do not require a feature gate to use them).
Deprecate the TupleN traits in std::tuple.

Drawbacks

Adding features to the language this late in the game is always somewhat of a risky business. These features, while having baked for a few weeks, haven't had much time to bake in the grand scheme of the language. These are both backwards compatible to accept, and it could be argued that this could be done later rather than sooner.

In general, the major drawbacks of this RFC are the scheduling risks and "feature bloat" worries. This RFC, however, is quite easy to implement (reducing schedule risk) and concerns two fairly minor features which are unambiguously nice to have.

Alternatives

Instead of un-feature-gating before 1.0, these features could be released after 1.0 (if at all). The TupleN traits would then be required to be deprecated for the entire 1.0 release cycle.

Unresolved questions

None at the moment.

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