chore: disallow usage of impl Trait without experimental flag

[TomAFrench]

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• No user documentation needed.
• Changes in docs/ included in this PR.
• [For Experimental Features] Changes in docs/ to be submitted in a separate PR.

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• I have tested the changes locally.
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[jfecher]

To clarify, I think it's only impl Trait in a return position that is harmful.

The way it is currently implemented, we replace the "impl Trait" with the actual type returned so:

• If a u32 is returned, you can use it as a u32, unlike in Rust / other languages.
• It is effectively just return-type inference for functions

[jfecher]

I think my past comment was a bit too strongly worded though - I didn't know we already gated these behind an experimental feature. That seems fine to me if so - sorry for the mistake.

Edit: I suppose I'm fine with removing the return value position entirely as well though - there'd likely not be anything we'd want to carry over for the actual implementation when we do do it properly. I'll leave it to your judgement.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: b9b443a	Previous: 5a4166e	Ratio
rollup-block-root-single-tx	0.003 s	0.002 s	1.50

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[aakoshh]

Huh, I didn't know it was just a warning instead of an error, like enums. Just a reminder that we can add them in Nargo.toml to allow integration tests to use it.

[jfecher]

Having every unstable feature be an error is more consistent - we should be able to have a release with the previous warning-only version as well for ease of migration