std: deduplicate errno accesses

[joboet]

By marking __errno_location as #[ffi_const] and std::sys::os::errno as #[inline], this PR allows merging multiple calls to io::Error::last_os_error() into one.

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[Noratrieb]

https://github.com/bminor/glibc/blob/b0897944cc3081e019b39981790051f7ee127406/stdlib/errno.h#L37 in glibc, the function is indeed declared const, so this should be correct.
@bors r+ rollup

[bors]

📌 Commit dd4f616 has been approved by Noratrieb

It is now in the queue for this repository.

[joboet]

https://github.com/bminor/glibc/blob/b0897944cc3081e019b39981790051f7ee127406/stdlib/errno.h#L37 in glibc, the function is indeed declared const, so this should be correct.

Note that e.g. Apple's libc does not use that attribute. Still, the C standard mandates that errno

expands to a modifiable lvalue that has type int and thread storage duration [emphasis mine]

So a libc that changes the address of errno would be in clear violation of the standard (and would probably encounter errors, e.g. it's perfectly valid in C to take the address of errno and read from the resulting pointer later).