Function c_isspace 

pub const fn c_isspace(c: char) -> bool

C isspace in the “C” locale: space, \t, \n, \v, \f, \r.

Rust’s is_ascii_whitespace is the WhatWG set, which omits the vertical tab — so every port of a C strtol/strtod/sscanf/isspace skip that reached for it stopped one codepoint short of the C it cites. This is that skip’s one owner; a byte-oriented scanner passes b as char, which maps 0x80..0xFF to U+0080..U+00FF and so stays false there, as C does.