rfc9839

Validation of RFC 9839 Unicode subsets in Rust.

RFC 9839 defines three nested subsets of Unicode characters for use in text protocols:

• Unicode Scalars – all code points except UTF-16 surrogates. Every Rust char is already a scalar value; checks are included for completeness and for raw byte validation.
• XML Characters – { TAB, LF, CR } ∪ [0x20–0xD7FF] ∪ [0xE000–0xFFFD] ∪ [0x10000–0x10FFFF]. This is the XML “Char” production with legacy controls and noncharacters excluded.
• Unicode Assignables – “not problematic” characters: useful controls, printable ASCII (excluding DEL/C1), and all assigned scalars minus standardized noncharacters (…FFFE/FFFF in each plane and U+FDD0–FDEF).

Features

• Character-level APIs: is_unicode_scalar_char, is_xml_char, is_unicode_assignable_char
• String-level APIs: is_unicode_scalar, is_xml_chars, is_unicode_assignable
• Byte-level APIs: is_unicode_scalar_bytes, is_xml_chars_bytes, is_unicode_assignable_bytes
• ASCII fast-path: tight loops for ASCII data, falling back to chars() only after the first non-ASCII byte
• Zero allocations, no lookup tables

Example

use rfc9839::*;

// Scalars (always true for safe Rust strings)
assert!(is_unicode_scalar("hello 🌍"));

// XML Characters
assert!(is_xml_chars("ok\tline\n"));
assert!(!is_xml_chars("\u{0000}")); // NUL is disallowed

// Unicode Assignables
assert!(is_unicode_assignable("emoji 👍"));
assert!(!is_unicode_assignable("\u{007F}")); // DEL is excluded