hasLang misinterprets FcLangSetHasLang's FcLangResult as FcBool

Summary

• Context: LangSet.hasLang() is a wrapper around fontconfig’s FcLangSetHasLang API, used to check if a font supports a specific language (e.g., to determine if a font is an emoji font via the “und-zsye” language tag).

• Bug: hasLang() incorrectly compares the return value of FcLangSetHasLang (which returns FcLangResult) to FcTrue (a boolean constant).

• Actual vs. expected: FcLangSetHasLang returns a three-value enum (FcLangResult) indicating the quality of language match, but the code treats it as a boolean by comparing to FcTrue. This causes hasLang() to return true for partial matches (same language, different territory) when it should only return true for exact matches.

• Impact: Fonts may be incorrectly identified as supporting a language when they only support a related language variant, potentially causing incorrect font selection in the emoji presentation detection logic.

Code with bug

pub fn hasLang(self: *const LangSet, lang: [:0]

Logical proof

• Docs: https://www.freedesktop.org/software/fontconfig/fontconfig-devel/fclangsethaslang.html

• Signature:

FcLangResult FcLangSetHasLang(
    const FcLangSet *_ls_,
    const FcChar8   *_lang_

• Return values: FcLangEqual (0), FcLangDifferentTerritory (1), FcLangDifferentLang (2)

• Current code compares the enum result to FcTrue (boolean 1), yielding:

  • FcLangEqual (0) == 1 → false (exact match incorrectly returns false)

  • FcLangDifferentTerritory (1) == 1 → true (partial match incorrectly returns true)

  • FcLangDifferentLang (2) == 1 → false (mismatch correctly returns false) This demonstrates inverted behavior for exact matches and false positives for partial matches.

Recommended fix

pub fn hasLang(self: *const LangSet, lang: [:0]