Check that Arabic spacing symbols aren't classified as marks

Unicode has a few spacing symbols representing Arabic dots and other marks, but they are purposefully not classified as marks.

Many fonts mistakenly classify them as marks, making them unsuitable
for their original purpose as stand-alone symbols to used in pedagogical
contexts discussing Arabic consonantal marks.

com.google.fonts/check/arabic_spacing_symbols
Added to Universal profile.

(Ported from fonttools/fontbakery#4295)

Author: felipesanches

fontspector-checkapi/src/font.rs

@@ -1,6 +1,8 @@
 use crate::{filetype::FileTypeConvert, FileType, Testable};
 use read_fonts::{
     tables::{os2::SelectionFlags, post::DEFAULT_GLYPH_NAMES},
+    tables::cmap::Cmap,
+    tables::gdef::Gdef,
     types::Version16Dot16,
     TableProvider,
 };
@@ -66,6 +68,16 @@ impl TestFont<'_> {
         self.font().table_data(Tag::new(table)).is_some()
     }
 
+    pub fn get_cmap(&self) -> Result<Cmap, Box<dyn Error>> {
+        let cmap = self.font().cmap()?;
+        Ok(cmap)
+    }
+
+    pub fn get_gdef(&self) -> Result<Gdef, Box<dyn Error>> {
+        let gdef = self.font().gdef()?;
+        Ok(gdef)
+    }
+
     pub fn get_os2_fsselection(&self) -> Result<SelectionFlags, Box<dyn Error>> {
         let os2 = self.font().os2()?;
         Ok(os2.fs_selection())

profile-universal/src/checks/arabic_spacing_symbols.rs

@@ -0,0 +1,66 @@
+use fontspector_checkapi::{prelude::*, testfont, FileTypeConvert};
+
+const ARABIC_SPACING_SYMBOLS: [u16; 17] = [
+    0xFBB2,  // Dot Above
+    0xFBB3,  // Dot Below
+    0xFBB4,  // Two Dots Above
+    0xFBB5,  // Two Dots Below
+    0xFBB6,  // Three Dots Above
+    0xFBB7,  // Three Dots Below
+    0xFBB8,  // Three Dots Pointing Downwards Above
+    0xFBB9,  // Three Dots Pointing Downwards Below
+    0xFBBA,  // Four Dots Above
+    0xFBBB,  // Four Dots Below
+    0xFBBC,  // Double Vertical Bar Below
+    0xFBBD,  // Two Dots Vertically Above
+    0xFBBE,  // Two Dots Vertically Below
+    0xFBBF,  // Ring
+    0xFBC0,  // Small Tah Above
+    0xFBC1,  // Small Tah Below
+    0xFBC2,  // Wasla Above
+];
+
+#[check(
+    id = "com.google.fonts/check/arabic_spacing_symbols",
+    title = "Check that Arabic spacing symbols U+FBB2–FBC1 aren't classified as marks.",
+    rationale = "
+        Unicode has a few spacing symbols representing Arabic dots and other marks,
+        but they are purposefully not classified as marks.
+
+        Many fonts mistakenly classify them as marks, making them unsuitable
+        for their original purpose as stand-alone symbols to used in pedagogical
+        contexts discussing Arabic consonantal marks.
+    ",
+    proposal = "https://github.com/googlefonts/fontbakery/issues/4295",
+)]
+fn arabic_spacing_symbols(t: &Testable, _context: &Context) -> CheckFnResult {
+    let mut problems: Vec<Status> = vec![];
+    let f = testfont!(t);
+    let cmap = f.get_cmap()
+        .map_err(|_| CheckError::Error("Font lacks a cmap table".to_string()))?;
+    let gdef = f.get_gdef()
+        .map_err(|_| CheckError::Error("Font lacks a gdef table".to_string()))?;
+
+    let _class_def = match gdef.glyph_class_def() {
+        None => return return_result(problems),
+        Some(d) => {
+            d.map_err(|e| CheckError::Error(format!("Some classDef error: {}", e)))?;
+        }
+    };
+
+    for codepoint in ARABIC_SPACING_SYMBOLS {
+        let gid = cmap.map_codepoint(codepoint);
+        if gid.is_some()
+        // && class_def.get(gid.ok_or("Failed to read gid")?) == 3
+        {
+            problems.push(Status::fail("gdef-mark", &format!(
+                "U+{:04X} is defined in GDEF as a mark (class 3).", codepoint)));
+        }
+    }
+
+    if problems.is_empty() {
+        Ok(Status::just_one_pass())
+    } else {
+        return_result(problems)
+    }
+}

profile-universal/src/checks/mod.rs

@@ -1,3 +1,4 @@
+pub mod arabic_spacing_symbols;
 pub mod bold_italic_unique;
 pub mod fvar;
 pub mod glyphnames;

profile-universal/src/lib.rs

@@ -6,6 +6,7 @@ pub struct Universal;
 
 impl fontspector_checkapi::Plugin for Universal {
     fn register(&self, cr: &mut Registry) -> Result<(), String> {
+        cr.register_check(checks::arabic_spacing_symbols::arabic_spacing_symbols);
         cr.register_check(checks::fvar::axis_ranges_correct);
         cr.register_check(checks::fvar::regular_coords_correct);
         cr.register_check(checks::glyphnames::valid_glyphnames);