Implement TryFrom<char> for usize.

[bjoernager]

Feature gate: usize_try_from_char.

This PR implements TryFrom<char> for usize.

usize is currently the only, unsigned, integral, type to not implement TryFrom<char>. Technically, this conversion is trivial and can already be expressed with some indirection.

I think it useful to be able to describe this set of types with this interface.

[rustbot]

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[rustbot]

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[bjoernager]

@rustbot label +T-libs-api -T-libs needs-fcp

r? libs-api

[dtolnay]

Thanks!

View changes since this review

[dtolnay]

@rfcbot fcp merge

[rust-rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

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Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[teor2345]

Grammar nitpick:

Suggested change

	/// UTF-32 code point to the extent representable by pointers addresses.
	/// UTF-32 code point to the extent representable by pointer addresses.

But it might be even better to use the same wording as the usize definition:

Suggested change

	/// UTF-32 code point to the extent representable by pointers addresses.
	/// UTF-32 code point to the extent representable by pointer-sized integers.