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>

3 files changed, 1 insertions, 23 deletions

diff --git a/components/fonts/platform/freetype/font.rs b/components/fonts/platform/freetype/font.rs
index 43885556942..827d02cca8f 100644
--- a/components/fonts/platform/freetype/font.rs
+++ b/components/fonts/platform/freetype/font.rs
@@ -75,7 +75,6 @@ pub struct PlatformFont {
     face: ReentrantMutex<FT_Face>,
     requested_face_size: Au,
     actual_face_size: Au,
-    can_do_fast_shaping: bool,
 }
 
 // FT_Face can be used in multiple threads, but from only one thread at a time.
@@ -135,15 +134,12 @@ impl PlatformFontMethods for PlatformFont {
             Some(requested_size) => (requested_size, face.set_size(requested_size)?),
             None => (Au::zero(), Au::zero()),
         };
-        let can_do_fast_shaping =
-            face.has_table(KERN) && !face.has_table(GPOS) && !face.has_table(GSUB);
 
         Ok(PlatformFont {
             face: ReentrantMutex::new(face),
             font_data: data,
             requested_face_size,
             actual_face_size,
-            can_do_fast_shaping,
         })
     }
 
@@ -215,10 +211,6 @@ impl PlatformFontMethods for PlatformFont {
         fixed_26_dot_6_to_float(delta.x) * self.unscalable_font_metrics_scale()
     }
 
-    fn can_do_fast_shaping(&self) -> bool {
-        self.can_do_fast_shaping
-    }
-
     fn glyph_h_advance(&self, glyph: GlyphId) -> Option<FractionalPixel> {
         let face = self.face.lock();
         assert!(!face.is_null());
diff --git a/components/fonts/platform/macos/font.rs b/components/fonts/platform/macos/font.rs
index 13af8ae034e..e2a68d340ce 100644
--- a/components/fonts/platform/macos/font.rs
+++ b/components/fonts/platform/macos/font.rs
@@ -26,7 +26,7 @@ use super::core_text_font_cache::CoreTextFontCache;
 use crate::{
     map_platform_values_to_style_values, FontIdentifier, FontMetrics, FontTableMethods,
     FontTableTag, FontTemplateDescriptor, FractionalPixel, GlyphId, PlatformFontMethods, CBDT,
-    COLR, GPOS, GSUB, KERN, SBIX,
+    COLR, KERN, SBIX,
 };
 
 const KERN_PAIR_LEN: usize = 6;
@@ -59,7 +59,6 @@ pub struct PlatformFont {
     /// data stays alive of the lifetime of this struct.
     _data: Arc<Vec<u8>>,
     h_kern_subtable: Option<CachedKernTable>,
-    can_do_fast_shaping: bool,
 }
 
 // From https://developer.apple.com/documentation/coretext:
@@ -189,12 +188,8 @@ impl PlatformFontMethods for PlatformFont {
             _data: data,
             ctfont: core_text_font.clone_with_font_size(size),
             h_kern_subtable: None,
-            can_do_fast_shaping: false,
         };
         handle.h_kern_subtable = handle.find_h_kern_subtable();
-        handle.can_do_fast_shaping = handle.h_kern_subtable.is_some() &&
-            handle.table_for_tag(GPOS).is_none() &&
-            handle.table_for_tag(GSUB).is_none();
         Ok(handle)
     }
 
@@ -239,10 +234,6 @@ impl PlatformFontMethods for PlatformFont {
         0.0
     }
 
-    fn can_do_fast_shaping(&self) -> bool {
-        self.can_do_fast_shaping
-    }
-
     fn glyph_h_advance(&self, glyph: GlyphId) -> Option<FractionalPixel> {
         let glyphs = [glyph as CGGlyph];
         let advance = unsafe {
diff --git a/components/fonts/platform/windows/font.rs b/components/fonts/platform/windows/font.rs
index 4974a225054..db7c94cd85a 100644
--- a/components/fonts/platform/windows/font.rs
+++ b/components/fonts/platform/windows/font.rs
@@ -211,11 +211,6 @@ impl PlatformFontMethods for PlatformFont {
         Some(f)
     }
 
-    /// Can this font do basic horizontal LTR shaping without Harfbuzz?
-    fn can_do_fast_shaping(&self) -> bool {
-        self.face.has_kerning_pairs()
-    }
-
     fn glyph_h_kerning(&self, first_glyph: GlyphId, second_glyph: GlyphId) -> FractionalPixel {
         let adjustment = self
             .face