CoreText: parse features equivalently to HarfBuzz (currently we only parse binary features)

[qwerasd205]

The CoreText shaper parses the provided font features using overly simplified assumptions about how font features are specified:

ghostty/src/font/shaper/coretext.zig

Lines 131 to 140 in 7027147

	/// Append the given feature to the list. The feature syntax is
	/// the same as Harfbuzz: "feat" enables it and "-feat" disables it.
	pub fn append(self: *FeatureList, name_raw: []const u8) !void {
	// If the name is `-name` then we are disabling the feature,
	// otherwise we are enabling it, so we need to parse this out.
	const name = if (name_raw[0] == '-') name_raw[1..] else name_raw;
	const dict = try featureDict(name, name_raw[0] != '-');
	defer dict.release();
	self.list.appendValue(macos.foundation.Dictionary, dict);
	}

The HarfBuzz shaper uses Feature.fromString which is a loose wrapper around hb_feature_from_string:

ghostty/src/font/shaper/harfbuzz.zig

Line 48 in 7027147

if (harfbuzz.Feature.fromString(name)) |feat| {

Despite the CoreText FeatureList.append's doc comments claiming to use "the same" feature syntax as HarfBuzz, this is not the case; the syntax for features passed to hb_feature_from_string can be found in the HarfBuzz docs.

The practical impact of this is that many configs that will work under HarfBuzz will not under CoreText, and it's actually impossible to specify non-boolean features (such as a specific character alternative).

I am willing to implement a parser with proper parity to the HarfBuzz one, it shouldn't be too difficult, but I figure'd I'd open this issue first to keep track of it since I might not get around to it for a couple days.

[ThorstenRhau]

https://github.com/githubnext/monaspace?tab=readme-ov-file

If I read this feature request correctly it would solve the "cv01" or first character variation (for zero) of Monaspace, right?

I am asking to make sure to not open a duplicate issue.

What I am trying to do is:

font-family = Monaspace Argon Var
font-family-bold  = Monaspace Argon Var
font-family-italic = Monaspace Radon Var
font-family-bold-italic  = Monaspace Radon Var
window-title-font-family = Monaspace Argon Var

font-variation = wght = 450
font-variation-italic = wght = 450
font-variation-bold = wght = 700
font-variation-bold-italic = wght = 700

font-feature = calt
font-feature = liga
font-feature = ss01
font-feature = ss02
font-feature = ss03
font-feature = ss04
font-feature = ss05
font-feature = ss06
font-feature = ss07
font-feature = ss08
font-feature = ss09

font-feature = cv01=1

All the configuration works except for the last line, that contains "cv01".

[qwerasd205]

If I read this feature request correctly it would solve the "cv01" or first character variation (for zero) of Monaspace, right?

Correct, this would solve that problem.