Avoid crashes when laying out lines containing byte order marks

This solution isn't perfect and we'll probably have layout bugs with these lines, but this prevents us from triggering undefined behavior.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

gpui/src/platform/mac/fonts.rs

@@ -23,7 +23,7 @@ use core_graphics::{
 use core_text::{line::CTLine, string_attributes::kCTFontAttributeName};
 use font_kit::{canvas::RasterizationOptions, hinting::HintingOptions, source::SystemSource};
 use parking_lot::RwLock;
-use std::{cell::RefCell, char, convert::TryFrom, ffi::c_void};
+use std::{cell::RefCell, char, cmp, convert::TryFrom, ffi::c_void};
 
 #[allow(non_upper_case_globals)]
 const kCGImageAlphaOnly: u32 = 7;
@@ -199,6 +199,7 @@ impl FontSystemState {
         let mut string = CFMutableAttributedString::new();
         {
             string.replace_str(&CFString::new(text), CFRange::init(0, 0));
+            let utf16_line_len = string.char_len() as usize;
 
             let last_run: RefCell<Option<(usize, FontId)>> = Default::default();
             let font_runs = runs
@@ -226,12 +227,16 @@ impl FontSystemState {
             for (run_len, font_id) in font_runs {
                 let utf8_end = ix_converter.utf8_ix + run_len;
                 let utf16_start = ix_converter.utf16_ix;
-                ix_converter.advance_to_utf8_ix(utf8_end);
 
-                let cf_range = CFRange::init(
-                    utf16_start as isize,
-                    (ix_converter.utf16_ix - utf16_start) as isize,
-                );
+                if utf16_start >= utf16_line_len {
+                    break;
+                }
+
+                ix_converter.advance_to_utf8_ix(utf8_end);
+                let utf16_end = cmp::min(ix_converter.utf16_ix, utf16_line_len);
+
+                let cf_range =
+                    CFRange::init(utf16_start as isize, (utf16_end - utf16_start) as isize);
                 let font = &self.fonts[font_id.0];
                 unsafe {
                     string.set_attribute(
@@ -245,6 +250,10 @@ impl FontSystemState {
                         &CFNumber::from(font_id.0 as i64),
                     );
                 }
+
+                if utf16_end == utf16_line_len {
+                    break;
+                }
             }
         }
 
@@ -483,7 +492,7 @@ mod tests {
     }
 
     #[test]
-    fn test_layout_line() {
+    fn test_wrap_line() {
         let fonts = FontSystem::new();
         let font_ids = fonts.load_family("Helvetica").unwrap();
         let font_id = fonts.select_font(&font_ids, &Default::default()).unwrap();
@@ -499,4 +508,25 @@ mod tests {
             &["aaa ααα ".len(), "aaa ααα ✋✋✋ ".len(),]
         );
     }
+
+    #[test]
+    fn test_layout_line_bom_char() {
+        let fonts = FontSystem::new();
+        let font_ids = fonts.load_family("Helvetica").unwrap();
+        let font_id = fonts.select_font(&font_ids, &Default::default()).unwrap();
+
+        let line = "\u{feff}";
+        let layout = fonts.layout_line(line, 16., &[(line.len(), font_id, Default::default())]);
+        assert_eq!(layout.len, line.len());
+        assert!(layout.runs.is_empty());
+
+        let line = "a\u{feff}b";
+        let layout = fonts.layout_line(line, 16., &[(line.len(), font_id, Default::default())]);
+        assert_eq!(layout.len, line.len());
+        assert_eq!(layout.runs.len(), 1);
+        assert_eq!(layout.runs[0].glyphs.len(), 2);
+        assert_eq!(layout.runs[0].glyphs[0].id, 68); // a
+                                                     // There's no glyph for \u{feff}
+        assert_eq!(layout.runs[0].glyphs[1].id, 69); // b
+    }
 }