Add str pointer methods (take 2)

[WaffleLapkin]

This PR adds the following functions for pointers to str, similar to already existing functions for pointers to [T].

impl *const str {
    pub const fn len(self) -> usize;
    pub const fn as_ptr(self) -> *const u8;
    pub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
    where
        I: SliceIndex<str>;
}

impl *mut str {
    pub const fn len(self) -> usize;
    pub const fn as_mut_ptr(self) -> *mut u8;
    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Output
    where
        I: SliceIndex<str>;
}

impl NonNull<str> {
    pub const fn len(self) -> usize;
    pub const fn as_non_null_ptr(self) -> NonNull<u8>;
    pub const fn as_mut_ptr(self) -> *mut u8;
    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
    where
        I: SliceIndex<str>;
}

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This PR also adds const_str_ptr and mut_str_ptr lang items (they are required for *const str and *mut str impls).

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This was previously proposed in #85816. This PR includes changes proposed by the review and was created to split these changes from the addition of pointer-to-str-construction methods (#91216).

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r? @m-ou-se

[bors]

☔ The latest upstream changes (presumably #91627) made this pull request unmergeable. Please resolve the merge conflicts.

[m-ou-se]

Thanks for splitting that PR!

This looks great. I only have some small comments on the documentation:

[WaffleLapkin]

@m-ou-se I've edited the documentation as you suggested

[WaffleLapkin]

Actually... I now dought that these APIs are useful.

While &str and &[u8] are different in that &str guarantees more about the data, *const str and *const [u8] are not different at all! As discussed while editing safety comments, it's totally fine for *const str to point to non-utf8 bytes, for example.

So I'm not sure that using and operating on *const str actually gives us any advantages over using *const [u8].

[WaffleLapkin]

I'm going to close this PR per the reasoning above. Feel free to ping me, if you disagree and think that these APIs are useful.