ACP: Add `FromByteStr` trait with blanket impl `FromStr`

Proposal

Problem statement

Many data forms that can be parsed from a string representation do not need UTF-8. Here, FromStr is unnecessarily restrictive because a byte slice &[u8] cannot be parsed directly. Instead, a UTF-8 &str must be constructed to use .parse().

This is inconvenient when working with any raw buffers where one cannot assume that str::from_utf8 will be successful, nor is there any reason to incur the UTF-8 verification overhead. An example is IP addresses, for which there is an unstable from_bytes function: https://github.com/rust-lang/rust/issues/101035

Motivating examples or use cases

Any input data where UTF-8 cannot be guaranteed: stdin, file paths, data from Read, network packets, no_std without UTF tables, any data read one byte at a time, etc.

Any output data that doesn't require specific knowledge of UTF-8: integers, floating point, IP/socket addresses, MAC addresses, UUIDs, etc.

Solution sketch

Add a trait that mirrors FromStr but works with &[u8] byte slices:

// Likely located in core::slice
pub trait FromByteStr: Sized {
    type Err;

    // Required method
    fn from_byte_str(bytes: &[u8]) -> Result<Self, Self::Err>;
}

This will get a corresponding parse on &[u8]

impl<[u8]> {
    pub fn parse<F>(&self) -> Result<F, <F as FromByteStr>::Err>
        where F: FromByteStr
    { /* ... */ }
}

Since &str is necessarily represented as &[u8], we can provide a blanket impl so no types need to be duplicated:

impl<T> FromStr for T where T: FromByteStr {
    type Err = T::Err;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        s.as_bytes().parse()
    }
}

If this is done, almost all types in std that implement FromStr will be able to switch to a FromByteStr implementation:

• integer and floating point types
• NonZeroX
• IpAddr and SocketAddr (https://github.com/rust-lang/rust/issues/101035)

Alternatives

• Just use TryFrom - this was decided against in the case of IpAddr, see https://github.com/rust-lang/rust/pull/94890#issuecomment-1065970151
• Name this FromBytes::from_bytes: from_byte_str is proposed instead to make this clear that this parses text-encoded data, as opposed to binary serialization (https://github.com/rust-lang/libs-team/issues/287#issuecomment-1781752776 and https://github.com/rust-lang/libs-team/issues/287#issuecomment-1788184920)
• Name this FromAscii and place it in std::ascii: if this name were selected, users may expect this to parse something like [ascii::Char; N] rather than &[u8]. I don't think we want this since &[u8] -> &[ascii::Char] requires a validation step, and most implementations should be able to just raise an error if input is invalid.

Open Questions

• Should the return type and possibly Err be able to reference the source bytes? (https://github.com/rust-lang/libs-team/issues/287#issuecomment-1781861525 and its followup)
• Should we name the associated type Error rather than Err? This isn't consistent with FromStr but is more consistent with TryFrom and TryInto, as well as the rest of the ecosystem (where Err is usually only Result::Err, Error is an error type)

Links and related work

• IpAddr::from_bytes https://github.com/rust-lang/rust/issues/101035 and its pre-implementation discussion https://github.com/rust-lang/rust/pull/94890
• ASCII character type discussion https://github.com/rust-lang/libs-team/issues/179

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading the proposed FromByteStr API, the existing FromStr behavior, and the referenced IpAddr::from_bytes discussion. Review the open questions about borrowed errors and naming; done means the API design is resolved by the libs-api team and an implementation scope is agreed.