Function is_platform_dependent 

pub fn is_platform_dependent(c: char) -> bool

Whether a character is a 機種依存文字: encodable in CP932 (Windows-31J / Shift_JIS) as a double-byte character but outside JIS X 0208.

This is the classic “Windows 機種依存文字” set — NEC special characters, NEC-selected IBM extensions, IBM extensions, and the CP932 variant codepoints (e.g. U+FF5E full-width tilde). Single-byte CP932 characters (ASCII, half-width katakana) are not flagged: the JIS X 0208 contrast in the definition is about the double-byte graphic set, and ASCII is already in JIS X 0208 via the table.

use aozora_encoding::is_platform_dependent;
assert!(is_platform_dependent('\u{FF5E}')); // CP932 full-width tilde variant
assert!(!is_platform_dependent('亜'));       // JIS X 0208
assert!(!is_platform_dependent('a'));        // ASCII (in JIS X 0208 via the table)