fonts: Make fast shaping determination platform-independent (#33540)

This makes the determination of whether or not to use fast shaping
platform independent. Previously it was less stringent for Windows,
leading to using it in cases where a font had a GSUB or GPOS table --
which broke proper shaping.

In addition, the test is made platform independent and expanded to be
more complete.

Finally, comments are added indicating that "fast shaping" will be
removed.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/fonts/tests/font.rs

@@ -0,0 +1,109 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::fs::File;
+use std::io::Read;
+use std::path::PathBuf;
+use std::sync::Arc;
+
+use app_units::Au;
+use euclid::num::Zero;
+use fonts::platform::font::PlatformFont;
+use fonts::{
+    Font, FontData, FontDescriptor, FontIdentifier, FontTemplate, PlatformFontMethods,
+    ShapingFlags, ShapingOptions,
+};
+use servo_url::ServoUrl;
+use style::properties::longhands::font_variant_caps::computed_value::T as FontVariantCaps;
+use style::values::computed::{FontStretch, FontStyle, FontWeight};
+use unicode_script::Script;
+
+fn make_font(path: PathBuf) -> Font {
+    let identifier = FontIdentifier::Web(ServoUrl::from_file_path(path.clone()).unwrap());
+    let file = File::open(path).unwrap();
+    let data = Arc::new(FontData::from_bytes(
+        file.bytes().map(Result::unwrap).collect(),
+    ));
+    let platform_font =
+        PlatformFont::new_from_data(identifier.clone(), data.as_arc().clone(), 0, None).unwrap();
+
+    let template = FontTemplate {
+        identifier,
+        descriptor: platform_font.descriptor(),
+        stylesheet: None,
+    };
+    let descriptor = FontDescriptor {
+        weight: FontWeight::normal(),
+        stretch: FontStretch::hundred(),
+        style: FontStyle::normal(),
+        variant: FontVariantCaps::Normal,
+        pt_size: Au::from_px(24),
+    };
+    Font::new(
+        Arc::new(atomic_refcell::AtomicRefCell::new(template)),
+        descriptor,
+        data,
+        None,
+    )
+    .unwrap()
+}
+
+#[test]
+fn test_font_can_do_fast_shaping() {
+    let dejavu_sans = make_font(
+        [
+            env!("CARGO_MANIFEST_DIR"),
+            "tests",
+            "support",
+            "dejavu-fonts-ttf-2.37",
+            "ttf",
+            "DejaVuSans.ttf",
+        ]
+        .iter()
+        .collect(),
+    );
+
+    let dejavu_sans_fast_shapeable = make_font(
+        [
+            env!("CARGO_MANIFEST_DIR"),
+            "tests",
+            "support",
+            "dejavu-fonts-ttf-2.37",
+            "ttf",
+            "DejaVuSansNoGSUBNoGPOS.ttf",
+        ]
+        .iter()
+        .collect(),
+    );
+
+    // Fast shaping requires a font with a kern table and no GPOS or GSUB tables.
+    let shaping_options = ShapingOptions {
+        letter_spacing: None,
+        word_spacing: Au::zero(),
+        script: Script::Latin,
+        flags: ShapingFlags::empty(),
+    };
+    assert!(!dejavu_sans.can_do_fast_shaping("WAVE", &shaping_options));
+    assert!(dejavu_sans_fast_shapeable.can_do_fast_shaping("WAVE", &shaping_options));
+
+    // Non-Latin script should never have fast shaping.
+    let shaping_options = ShapingOptions {
+        letter_spacing: None,
+        word_spacing: Au::zero(),
+        script: Script::Cherokee,
+        flags: ShapingFlags::empty(),
+    };
+    assert!(!dejavu_sans.can_do_fast_shaping("WAVE", &shaping_options));
+    assert!(!dejavu_sans_fast_shapeable.can_do_fast_shaping("WAVE", &shaping_options));
+
+    // Right-to-left text should never use fast shaping.
+    let shaping_options = ShapingOptions {
+        letter_spacing: None,
+        word_spacing: Au::zero(),
+        script: Script::Latin,
+        flags: ShapingFlags::RTL_FLAG,
+    };
+    assert!(!dejavu_sans.can_do_fast_shaping("WAVE", &shaping_options));
+    assert!(!dejavu_sans_fast_shapeable.can_do_fast_shaping("WAVE", &shaping_options));
+}