Merge #1163: M9: fall back to the Noto symbol and math fonts on Linux (#1156) (@switch87)

# Conflicts:
#	crates/text/src/behaviour_tests.rs
#	crates/text/src/fontdb.rs

crates/text/src/behaviour_tests.rs

@@ -296,6 +296,28 @@ fn thai_comes_from_a_thai_font_when_one_is_installed() {
     assert_eq!(drawn, want, "every Thai char is in a drawn cluster");
 }
 
+#[test]
+fn symbols_come_from_a_symbol_font_when_one_is_installed() {
+    // ⌖ (U+2316) and ⤒ (U+2912) are in no Latin text face. With a symbol font that has them (Apple
+    // Symbols on macOS, Segoe UI Symbol on Windows, Noto Sans Symbols 2 and Noto Sans Math on
+    // Linux) the fallback draws them instead of the missing-glyph box (#1156). Each character
+    // skips on a machine with no installed fallback family that has it.
+    let Some(f) = face() else { return };
+    let families = ["Apple Symbols", "Segoe UI Symbol", "Noto Sans Symbols", "Noto Sans Symbols 2", "Noto Sans Symbols2", "Noto Sans Math"];
+    for c in ['\u{2316}', '\u{2912}'] {
+        if f.has_char(c) || !families.iter().any(|k| find_face(k, "Regular").is_some_and(|k| k.has_char(c))) {
+            continue;
+        }
+        let t = text(&format!("A {c}"), TextAutoResize::WidthAndHeight);
+        let l = layout(&t, None, None);
+        let drawn: Vec<_> = l.glyphs.iter().filter(|g| g.char_start == 2).collect();
+        assert!(!drawn.is_empty(), "U+{:04X} has a glyph", c as u32);
+        for g in drawn {
+            assert!(g.face.has_char(c), "U+{:04X} drawn by {} {}, which lacks it", c as u32, g.face.family, g.face.style);
+        }
+    }
+}
+
 #[test]
 fn break_opportunities() {
     // (before, after, may break between, derivation)

crates/text/src/fontdb.rs

@@ -405,8 +405,10 @@ fn names(f: &skrifa::FontRef) -> (Option<String>, Option<String>) {
     (family, style)
 }
 
-/// Families tried, in order, for characters a text's own font lacks.
-const FALLBACK_FAMILIES: [&str; 25] = [
+/// Families tried, in order, for characters a text's own font lacks. The symbol fonts come last:
+/// Apple Symbols (macOS), Segoe UI Symbol (Windows) and the Noto symbol and math fonts (Linux), for
+/// signs such as ⌖ and ⤒ that no text face has.
+const FALLBACK_FAMILIES: [&str; 29] = [
     "Inter",
     "Helvetica Neue",
     "Arial",
@@ -432,6 +434,10 @@ const FALLBACK_FAMILIES: [&str; 25] = [
     "Tahoma",
     "Apple Symbols",
     "Segoe UI Symbol",
+    "Noto Sans Symbols",
+    "Noto Sans Symbols 2",
+    "Noto Sans Symbols2",
+    "Noto Sans Math",
 ];
 
 /// A system font file entry not yet loaded.

crates/text/src/fontdb_tests.rs

@@ -192,6 +192,19 @@ fn fallback_faces_follow_the_runs_style() {
     }
 }
 
+#[test]
+fn every_platform_has_a_symbol_font_in_the_fallbacks() {
+    // Symbols such as ⌖ (U+2316) and ⤒ (U+2912) are in no text face (#1156): macOS has Apple
+    // Symbols, Windows Segoe UI Symbol, and Linux the Noto symbol and math fonts (fontconfig names
+    // "Noto Sans Symbols 2"; some packages name it "Noto Sans Symbols2").
+    for family in ["Apple Symbols", "Segoe UI Symbol", "Noto Sans Symbols", "Noto Sans Symbols 2", "Noto Sans Symbols2", "Noto Sans Math"] {
+        assert!(FALLBACK_FAMILIES.contains(&family), "{family} is a fallback");
+    }
+    // Text faces come first: a symbol font only draws what they lack.
+    let at = |f: &str| FALLBACK_FAMILIES.iter().position(|x| *x == f).unwrap();
+    assert!(at("Noto Sans") < at("Noto Sans Symbols") && at("DejaVu Sans") < at("Noto Sans Math"));
+}
+
 #[cfg(all(unix, not(target_os = "macos")))]
 #[test]
 fn user_font_dirs_follow_xdg_data_home() {