Use fallback list via dyn trait in FontSystem

src/font/fallback/macos.rs

@@ -2,8 +2,31 @@
 
 use unicode_script::Script;
 
+use super::Fallback;
+
+/// A platform-specific font fallback list, for MacOS.
+pub struct PlatformFallback;
+
+impl Fallback for PlatformFallback {
+    fn common_fallback(&self) -> &'static [&'static str] {
+        common_fallback()
+    }
+
+    fn forbidden_fallback(&self) -> &'static [&'static str] {
+        forbidden_fallback()
+    }
+
+    fn script_fallback(
+        &self,
+        script: unicode_script::Script,
+        locale: &str,
+    ) -> &'static [&'static str] {
+        script_fallback(script, locale)
+    }
+}
+
 // Fallbacks to use after any script specific fallbacks
-pub fn common_fallback() -> &'static [&'static str] {
+fn common_fallback() -> &'static [&'static str] {
     &[
         ".SF NS",
         "Menlo",
@@ -14,7 +37,7 @@ pub fn common_fallback() -> &'static [&'static str] {
 }
 
 // Fallbacks to never use
-pub fn forbidden_fallback() -> &'static [&'static str] {
+fn forbidden_fallback() -> &'static [&'static str] {
     &[".LastResort"]
 }
 
@@ -34,7 +57,7 @@ fn han_unification(locale: &str) -> &'static [&'static str] {
 }
 
 // Fallbacks to use per script
-pub fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
+fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
     //TODO: abstract style (sans/serif/monospaced)
     //TODO: pull more data from about:config font.name-list.sans-serif in Firefox
     match script {

src/font/fallback/mod.rs

@@ -25,6 +25,19 @@ mod platform;
 #[path = "windows.rs"]
 mod platform;
 
+pub trait Fallback {
+    /// Fallbacks to use after any script specific fallbacks
+    fn common_fallback(&self) -> &'static [&'static str];
+
+    /// Fallbacks to never use
+    fn forbidden_fallback(&self) -> &'static [&'static str];
+
+    /// Fallbacks to use per script
+    fn script_fallback(&self, script: Script, locale: &str) -> &'static [&'static str];
+}
+
+pub use platform::PlatformFallback;
+
 #[cfg(not(feature = "warn_on_missing_glyphs"))]
 use log::debug as missing_warn;
 #[cfg(feature = "warn_on_missing_glyphs")]
@@ -94,7 +107,7 @@ impl<'a> FontFallbackIter<'a> {
                 word
             );
         } else if !self.scripts.is_empty() && self.common_i > 0 {
-            let family = common_fallback()[self.common_i - 1];
+            let family = self.font_system.fallbacks.common_fallback()[self.common_i - 1];
             missing_warn!(
                 "Failed to find script fallback for {:?} locale '{}', used '{}': '{}'",
                 self.scripts,
@@ -262,7 +275,10 @@ impl Iterator for FontFallbackIter<'_> {
         while self.script_i.0 < self.scripts.len() {
             let script = self.scripts[self.script_i.0];
 
-            let script_families = script_fallback(script, self.font_system.locale());
+            let script_families = self
+                .font_system
+                .fallbacks
+                .script_fallback(script, self.font_system.locale());
             while self.script_i.1 < script_families.len() {
                 let script_family = script_families[self.script_i.1];
                 self.script_i.1 += 1;
@@ -285,7 +301,7 @@ impl Iterator for FontFallbackIter<'_> {
             self.script_i.1 = 0;
         }
 
-        let common_families = common_fallback();
+        let common_families = self.font_system.fallbacks.common_fallback();
         while self.common_i < common_families.len() {
             let common_family = common_families[self.common_i];
             self.common_i += 1;
@@ -301,7 +317,7 @@ impl Iterator for FontFallbackIter<'_> {
 
         //TODO: do we need to do this?
         //TODO: do not evaluate fonts more than once!
-        let forbidden_families = forbidden_fallback();
+        let forbidden_families = self.font_system.fallbacks.forbidden_fallback();
         while self.other_i < self.font_match_keys.len() {
             let id = self.font_match_keys[self.other_i].id;
             self.other_i += 1;

src/font/fallback/other.rs

@@ -2,17 +2,40 @@
 
 use unicode_script::Script;
 
+use super::Fallback;
+
+/// An empty platform-specific font fallback list.
+pub struct PlatformFallback;
+
+impl Fallback for PlatformFallback {
+    fn common_fallback(&self) -> &'static [&'static str] {
+        common_fallback()
+    }
+
+    fn forbidden_fallback(&self) -> &'static [&'static str] {
+        forbidden_fallback()
+    }
+
+    fn script_fallback(
+        &self,
+        script: unicode_script::Script,
+        locale: &str,
+    ) -> &'static [&'static str] {
+        script_fallback(script, locale)
+    }
+}
+
 // Fallbacks to use after any script specific fallbacks
-pub fn common_fallback() -> &'static [&'static str] {
+fn common_fallback() -> &'static [&'static str] {
     &[]
 }
 
 // Fallbacks to never use
-pub fn forbidden_fallback() -> &'static [&'static str] {
+fn forbidden_fallback() -> &'static [&'static str] {
     &[]
 }
 
 // Fallbacks to use per script
-pub fn script_fallback(_script: Script, _locale: &str) -> &'static [&'static str] {
+fn script_fallback(_script: Script, _locale: &str) -> &'static [&'static str] {
     &[]
 }

src/font/fallback/unix.rs

@@ -2,8 +2,31 @@
 
 use unicode_script::Script;
 
+use super::Fallback;
+
+/// A platform-specific font fallback list, for Unix.
+pub struct PlatformFallback;
+
+impl Fallback for PlatformFallback {
+    fn common_fallback(&self) -> &'static [&'static str] {
+        common_fallback()
+    }
+
+    fn forbidden_fallback(&self) -> &'static [&'static str] {
+        forbidden_fallback()
+    }
+
+    fn script_fallback(
+        &self,
+        script: unicode_script::Script,
+        locale: &str,
+    ) -> &'static [&'static str] {
+        script_fallback(script, locale)
+    }
+}
+
 // Fallbacks to use after any script specific fallbacks
-pub fn common_fallback() -> &'static [&'static str] {
+fn common_fallback() -> &'static [&'static str] {
     //TODO: abstract style (sans/serif/monospaced)
     &[
         /* Sans-serif fallbacks */
@@ -25,7 +48,7 @@ pub fn common_fallback() -> &'static [&'static str] {
 }
 
 // Fallbacks to never use
-pub fn forbidden_fallback() -> &'static [&'static str] {
+fn forbidden_fallback() -> &'static [&'static str] {
     &[]
 }
 
@@ -45,7 +68,7 @@ fn han_unification(locale: &str) -> &'static [&'static str] {
 }
 
 // Fallbacks to use per script
-pub fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
+fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
     //TODO: abstract style (sans/serif/monospaced)
     match script {
         Script::Adlam => &["Noto Sans Adlam", "Noto Sans Adlam Unjoined"],

src/font/fallback/windows.rs

@@ -2,8 +2,31 @@
 
 use unicode_script::Script;
 
+use super::Fallback;
+
+/// A platform-specific font fallback list, for Windows.
+pub struct PlatformFallback;
+
+impl Fallback for PlatformFallback {
+    fn common_fallback(&self) -> &'static [&'static str] {
+        common_fallback()
+    }
+
+    fn forbidden_fallback(&self) -> &'static [&'static str] {
+        forbidden_fallback()
+    }
+
+    fn script_fallback(
+        &self,
+        script: unicode_script::Script,
+        locale: &str,
+    ) -> &'static [&'static str] {
+        script_fallback(script, locale)
+    }
+}
+
 // Fallbacks to use after any script specific fallbacks
-pub fn common_fallback() -> &'static [&'static str] {
+fn common_fallback() -> &'static [&'static str] {
     //TODO: abstract style (sans/serif/monospaced)
     &[
         "Segoe UI",
@@ -15,7 +38,7 @@ pub fn common_fallback() -> &'static [&'static str] {
 }
 
 // Fallbacks to never use
-pub fn forbidden_fallback() -> &'static [&'static str] {
+fn forbidden_fallback() -> &'static [&'static str] {
     &[]
 }
 
@@ -36,7 +59,7 @@ fn han_unification(locale: &str) -> &'static [&'static str] {
 }
 
 // Fallbacks to use per script
-pub fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
+fn script_fallback(script: Script, locale: &str) -> &'static [&'static str] {
     //TODO: better match https://github.com/chromium/chromium/blob/master/third_party/blink/renderer/platform/fonts/win/font_fallback_win.cc#L99
     match script {
         Script::Adlam => &["Ebrima"],

src/font/system.rs

@@ -1,4 +1,5 @@
 use crate::{Attrs, Font, FontMatchAttrs, HashMap, ShapeBuffer};
+use alloc::boxed::Box;
 use alloc::collections::BTreeSet;
 use alloc::string::String;
 use alloc::sync::Arc;
@@ -10,7 +11,7 @@ use core::ops::{Deref, DerefMut};
 pub use fontdb;
 pub use rustybuzz;
 
-use super::fallback::MonospaceFallbackInfo;
+use super::fallback::{Fallback, MonospaceFallbackInfo, PlatformFallback};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct FontMatchKey {
@@ -112,6 +113,9 @@ pub struct FontSystem {
     /// Cache for shaped runs
     #[cfg(feature = "shape-run-cache")]
     pub shape_run_cache: crate::ShapeRunCache,
+
+    /// List of fallbacks
+    pub(crate) fallbacks: Box<dyn Fallback>,
 }
 
 impl fmt::Debug for FontSystem {
@@ -150,11 +154,15 @@ impl FontSystem {
         db.set_sans_serif_family("Open Sans");
         db.set_serif_family("DejaVu Serif");
 
-        Self::new_with_locale_and_db(locale, db)
+        Self::new_with_locale_and_db_and_fallback(locale, db, PlatformFallback)
     }
 
-    /// Create a new [`FontSystem`] with a pre-specified locale and font database.
+    /// Create a new [`FontSystem`] with a pre-specified locale, font database and font fallback list.
-    pub fn new_with_locale_and_db(locale: String, db: fontdb::Database) -> Self {
+    pub fn new_with_locale_and_db_and_fallback(
+        locale: String,
+        db: fontdb::Database,
+        fallbacks: impl Fallback + 'static,
+    ) -> Self {
         let mut monospace_font_ids = db
             .faces()
             .filter(|face_info| {
@@ -204,9 +212,15 @@ impl FontSystem {
             #[cfg(feature = "shape-run-cache")]
             shape_run_cache: crate::ShapeRunCache::default(),
             shape_buffer: ShapeBuffer::default(),
+            fallbacks: Box::new(fallbacks),
         }
     }
 
+    /// Create a new [`FontSystem`] with a pre-specified locale and font database.
+    pub fn new_with_locale_and_db(locale: String, db: fontdb::Database) -> Self {
+        Self::new_with_locale_and_db_and_fallback(locale, db, PlatformFallback)
+    }
+
     /// Get the locale.
     pub fn locale(&self) -> &str {
         &self.locale