Rust library to check for problematic Unicode code points.

This is a Rust port of Tim Bray's original Go implementation of RFC 9839 Unicode character validation.

This library validates Unicode code-point subsets as specified in RFC 9839.

There are three named subsets:

• Unicode Scalars - All Unicode code points except surrogates (U+D800 to U+DFFF)
• XML Characters - Valid XML 1.0 characters
• Unicode Assignables - Unicode characters excluding noncharacters

use rfc9839::{is_char_unicode_scalar, is_string_xml_chars, is_utf8_unicode_assignables};

// Check individual characters
assert!(is_char_unicode_scalar('A'));

// Check strings
assert!(is_string_xml_chars("Valid XML text\n"));

// Check UTF-8 byte slices
assert!(is_utf8_unicode_assignables(b"Hello, world!"));

This port maintains full API compatibility with the Go implementation, providing the same function names and behaviour. However, the Rust implementation leverages the language's built-in guarantees where possible:

• Unicode scalar validation: Since Rust's char type can only represent valid Unicode scalar values, the Unicode scalar validation functions use Rust's built-in validation behind the scenes. For example, is_rune_unicode_scalar simply checks whether char::from_u32() succeeds.

• String validation: Rust strings are guaranteed to be valid UTF-8 without surrogates, so is_string_unicode_scalars always returns true for any valid &str.

• UTF-8 validation: The is_utf8_unicode_scalars function leverages Rust's str::from_utf8, which already rejects surrogates and invalid UTF-8.

The XML character and Unicode assignable validation functions perform actual subset checking, as these are more restrictive than what Rust's type system guarantees.

The exported function names are descriptive of what they do.