Rework Fontconfig fallback to use cached list from font_sort

Previous implementation was querying Fontconfig using `charset` in a pattern,
which was leading to unpredictable fallbacks in some cases, since Fontconfig
was picking the font with the most coverage for a given charset, regardless of
user configuration. Moreover all fallback was based on font_match which is
extremely slow for such performance sensitive task as a fallback, so alacritty
had a hard times on vtebench's unicode-random-write.

The new approach is to use some internal fallback list from font_sort
and iterate over it to get a proper fallback font, since it matches the
following example query from `fc-match`:

`fc-match -s "monospace:pixelsize=X:style=Y"

That being said it's more intuitive for users to setup their system Fontconfig
fallback, and also most applications are doing similar things. Moreover the new
implementation uses internal caches over Fontconfig API when possible and
performs font matches only once during load of requested font with font_sort,
which leads to dramatically improved performance on already mentioned
vtebench's unicode-random-write.

Fixes #3176.
Fixes #3134.
Fixes #2657.
Fixes #1560.
Fixes #965.
Fixes #511.

1 files changed, 41 insertions, 2 deletions

diff --git a/font/src/ft/fc/char_set.rs b/font/src/ft/fc/char_set.rs
index 89554458..4bba4818 100644
--- a/font/src/ft/fc/char_set.rs
+++ b/font/src/ft/fc/char_set.rs
@@ -13,15 +13,19 @@
 // limitations under the License.
 use std::ptr::NonNull;
 
-use foreign_types::{foreign_type, ForeignTypeRef};
+use foreign_types::{foreign_type, ForeignType, ForeignTypeRef};
 
 use super::ffi::FcCharSetCreate;
-use super::ffi::{FcCharSet, FcCharSetAddChar, FcCharSetDestroy};
+use super::ffi::{
+    FcBool, FcCharSet, FcCharSetAddChar, FcCharSetCopy, FcCharSetCount, FcCharSetDestroy,
+    FcCharSetHasChar, FcCharSetMerge, FcCharSetSubtract, FcCharSetUnion,
+};
 
 foreign_type! {
     pub unsafe type CharSet {
         type CType = FcCharSet;
         fn drop = FcCharSetDestroy;
+        fn clone = FcCharSetCopy;
     }
 }
 
@@ -41,4 +45,39 @@ impl CharSetRef {
     pub fn add(&mut self, glyph: char) -> bool {
         unsafe { FcCharSetAddChar(self.as_ptr(), glyph as _) == 1 }
     }
+
+    pub fn has_char(&self, glyph: char) -> bool {
+        unsafe { FcCharSetHasChar(self.as_ptr(), glyph as _) == 1 }
+    }
+
+    pub fn count(&self) -> u32 {
+        unsafe { FcCharSetCount(self.as_ptr()) as u32 }
+    }
+
+    pub fn union(&self, other: &CharSetRef) -> CharSet {
+        unsafe {
+            let ptr = FcCharSetUnion(self.as_ptr() as _, other.as_ptr() as _);
+            CharSet::from_ptr(ptr)
+        }
+    }
+
+    pub fn subtract(&self, other: &CharSetRef) -> CharSet {
+        unsafe {
+            let ptr = FcCharSetSubtract(self.as_ptr() as _, other.as_ptr() as _);
+            CharSet::from_ptr(ptr)
+        }
+    }
+
+    pub fn merge(&self, other: &CharSetRef) -> Result<bool, ()> {
+        unsafe {
+            // Value is just an indicator whether something was added or not
+            let mut value: FcBool = 0;
+            let res = FcCharSetMerge(self.as_ptr() as _, other.as_ptr() as _, &mut value);
+            if res == 0 {
+                Err(())
+            } else {
+                Ok(value != 0)
+            }
+        }
+    }
 }