Allow 'sub X by NULL;' sequence to delete a glyph

Lib/fontTools/feaLib/ast.py

@@ -188,6 +188,21 @@ class Comment(Element):
         return self.text
 
 
+class NullGlyph(Expression):
+    """The NULL glyph, used in glyph deletion substitutions."""
+
+    def __init__(self, location=None):
+        Expression.__init__(self, location)
+        #: The name itself as a string
+
+    def glyphSet(self):
+        """The glyphs in this class as a tuple of :class:`GlyphName` objects."""
+        return ()
+
+    def asFea(self, indent=""):
+        return "NULL"
+
+
 class GlyphName(Expression):
     """A single glyph name, such as ``cedilla``."""
 
@@ -1246,8 +1261,9 @@ class MultipleSubstStatement(Statement):
                 res += " " + " ".join(map(asFea, self.suffix))
         else:
             res += asFea(self.glyph)
+        replacement = self.replacement or [ NullGlyph() ]
         res += " by "
-        res += " ".join(map(asFea, self.replacement))
+        res += " ".join(map(asFea, replacement))
         res += ";"
         return res
 

Lib/fontTools/feaLib/parser.py

@@ -314,10 +314,15 @@ class Parser(object):
                 location,
             )
 
-    def parse_glyphclass_(self, accept_glyphname):
+    def parse_glyphclass_(self, accept_glyphname, accept_null=False):
         # Parses a glyph class, either named or anonymous, or (if
-        # ``bool(accept_glyphname)``) a glyph name.
+        # ``bool(accept_glyphname)``) a glyph name. If ``bool(accept_null)`` then
+        # also accept the special NULL glyph.
         if accept_glyphname and self.next_token_type_ in (Lexer.NAME, Lexer.CID):
+            if accept_null and self.next_token_ == "NULL":
+                # If you want a glyph called NULL, you should escape it.
+                self.advance_lexer_()
+                return self.ast.NullGlyph(location=self.cur_token_location_)
             glyph = self.expect_glyph_()
             self.check_glyph_name_in_glyph_set(glyph)
             return self.ast.GlyphName(glyph, location=self.cur_token_location_)
@@ -375,7 +380,8 @@ class Parser(object):
                     self.expect_symbol_("-")
                     range_end = self.expect_cid_()
                     self.check_glyph_name_in_glyph_set(
-                        f"cid{range_start:05d}", f"cid{range_end:05d}",
+                        f"cid{range_start:05d}",
+                        f"cid{range_end:05d}",
                     )
                     glyphs.add_cid_range(
                         range_start,
@@ -804,7 +810,7 @@ class Parser(object):
         if self.next_token_ == "by":
             keyword = self.expect_keyword_("by")
             while self.next_token_ != ";":
-                gc = self.parse_glyphclass_(accept_glyphname=True)
+                gc = self.parse_glyphclass_(accept_glyphname=True, accept_null=True)
                 new.append(gc)
         elif self.next_token_ == "from":
             keyword = self.expect_keyword_("from")
@@ -837,6 +843,9 @@ class Parser(object):
 
         num_lookups = len([l for l in lookups if l is not None])
 
+        if len(new) == 1 and len(new[0].glyphSet()) == 0:
+            new = []  # Deletion
+
         # GSUB lookup type 1: Single substitution.
         # Format A: "substitute a by a.sc;"
         # Format B: "substitute [one.fitted one.oldstyle] by one;"
@@ -863,8 +872,10 @@ class Parser(object):
             not reverse
             and len(old) == 1
             and len(old[0].glyphSet()) == 1
-            and len(new) > 1
-            and max([len(n.glyphSet()) for n in new]) == 1
+            and (
+                (len(new) > 1 and max([len(n.glyphSet()) for n in new]) == 1)
+                or len(new) == 0
+            )
             and num_lookups == 0
         ):
             return self.ast.MultipleSubstStatement(

Tests/feaLib/builder_test.py

@@ -73,7 +73,7 @@ class BuilderTest(unittest.TestCase):
         LigatureSubtable AlternateSubtable MultipleSubstSubtable 
         SingleSubstSubtable aalt_chain_contextual_subst AlternateChained 
         MultipleLookupsPerGlyph MultipleLookupsPerGlyph2 GSUB_6_formats
-        GSUB_5_formats
+        GSUB_5_formats delete_glyph
     """.split()
 
     def __init__(self, methodName):

Tests/feaLib/data/delete_glyph.fea

@@ -0,0 +1,3 @@
+feature test {
+	sub a by NULL;
+} test;

Tests/feaLib/data/delete_glyph.ttx

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0">
+          <Substitution in="a" out=""/>
+        </MultipleSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>