Add FeatureSpec: typed OpenType feature selection

Introduce `text_primitives::FeatureSpec` (ligatures/caps/numeric) as a type-safe way to build OpenType `FontFeature` settings without parsing CSS text, with deterministic output and unit/doctests. Re-export the typed API from Parley and add ergonomic `From<FeatureSpec>` for `FontFeatures` so Parley callers can use it directly.

Author: waywardmonkeys

parley/src/lib.rs

@@ -134,6 +134,9 @@ pub use layout::Layout;
 
 pub use editing::*;
 pub use layout::*;
+pub use setting::{
+    Caps, FeatureSpec, Ligatures, Numeric, NumericFigure, NumericFraction, NumericSpacing, TriState,
+};
 pub use style::*;
 
 #[deprecated(

parley/src/setting.rs

@@ -3,4 +3,7 @@
 
 //! OpenType settings (features and variations).
 
-pub use text_primitives::{FontFeature, FontVariation, Tag};
+pub use text_primitives::{
+    Caps, FeatureSpec, FontFeature, FontVariation, Ligatures, Numeric, NumericFigure,
+    NumericFraction, NumericSpacing, Tag, TriState,
+};

parley/src/style/font.rs

@@ -1,6 +1,7 @@
 // Copyright 2021 the Parley Authors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+use crate::FeatureSpec;
 use alloc::borrow::Cow;
 
 pub use crate::setting::{FontFeature, FontVariation};
@@ -42,6 +43,22 @@ impl<'a, const N: usize> From<&'a [FontVariation; N]> for FontVariations<'a> {
 }
 
 /// Font feature settings that can be supplied as a raw source string or a parsed slice.
+///
+/// # Typed construction (recommended)
+///
+/// When you want a type-safe API (rather than parsing CSS text), you can use
+/// [`FeatureSpec`] to build a list of low-level [`FontFeature`] settings and
+/// then pass them into Parley:
+///
+/// ```
+/// use parley::{Caps, FeatureSpec, FontFeatures};
+///
+/// let features = FontFeatures::from(FeatureSpec::new().with_caps(Caps::SmallCaps));
+///
+/// // Now `features` can be used anywhere Parley accepts font features (for example, a style
+/// // builder's `push_default` / `push` methods).
+/// let _ = features;
+/// ```
 #[derive(Clone, PartialEq, Debug)]
 pub enum FontFeatures<'a> {
     /// Setting source in CSS format.
@@ -74,3 +91,9 @@ impl<'a, const N: usize> From<&'a [FontFeature; N]> for FontFeatures<'a> {
         Self::List(Cow::Borrowed(&value[..]))
     }
 }
+
+impl<'a> From<FeatureSpec> for FontFeatures<'a> {
+    fn from(value: FeatureSpec) -> Self {
+        Self::List(Cow::Owned(value.to_features()))
+    }
+}