[fea-rs] Support inline 'by Null' in contextual sub

fea-rs/src/compile/compile_ctx.rs

@@ -700,7 +700,7 @@ impl<'a, F: FeatureProvider, V: VariationInfo> CompilationCtx<'a, F, V> {
             } else {
                 let target = input.items().next().unwrap().target();
                 let arity = rule.replacements().count();
-                if arity == 1 {
+                if arity == 1 && rule.null().is_none() {
                     let replacement = rule.replacements().next().unwrap();
                     if let Some((target, replacement)) =
                         self.validate_single_sub_inputs(&target, Some(&replacement))
@@ -721,10 +721,13 @@ impl<'a, F: FeatureProvider, V: VariationInfo> CompilationCtx<'a, F, V> {
                             "Inline multiple substitution must have a single glyph target",
                         );
                     }
-                    let replacements = rule
-                        .replacement_glyphs()
-                        .map(|g| self.resolve_glyph(&g))
-                        .collect();
+                    let replacements = if rule.null().is_some() {
+                        Vec::new()
+                    } else {
+                        rule.replacement_glyphs()
+                            .map(|g| self.resolve_glyph(&g))
+                            .collect()
+                    };
                     if let Some(target_id) = self.resolve_glyph_or_class(&target).iter().next() {
                         let lookup = self.ensure_current_lookup_type(Kind::GsubType6);
                         Some(

fea-rs/src/parse/grammar/gsub.rs

@@ -126,7 +126,9 @@ fn finish_chain_rule(parser: &mut Parser, recovery: TokenSet) -> AstKind {
 
     // now we may be done, or we may have a single inline rule
     if parser.eat(Kind::ByKw) {
-        if glyph::eat_glyph_name_like(parser) {
+        if parser.eat(Kind::NullKw) {
+            // allowed, continue down to 'expect_semi'
+        } else if glyph::eat_glyph_name_like(parser) {
             while glyph::eat_glyph_name_like(parser) {
                 continue;
             }
@@ -145,6 +147,9 @@ fn finish_chain_rule(parser: &mut Parser, recovery: TokenSet) -> AstKind {
     }
 
     if parser.expect_semi() {
+        // this looks like a valid contextual rule, but we will need to
+        // reparse it. This happens automatically when the node is added to
+        // the `AstSink`
         AstKind::GsubNodeNeedsRewrite
     } else {
         AstKind::GsubNode

fea-rs/src/token_tree/rewrite.rs

@@ -168,6 +168,9 @@ pub(crate) fn reparse_contextual_sub_rule(rewriter: &mut ReparseCtx) -> Kind {
     if rewriter.matches(0, Kind::ByKw) {
         rewriter.in_node(Kind::InlineSubNode, |rewriter| {
             rewriter.expect(Kind::ByKw);
+            if rewriter.eat(Kind::NullKw) {
+                return;
+            }
             expect_glyph_or_glyph_class(rewriter);
             while at_glyph_or_glyph_class(rewriter.nth_kind(0)) {
                 eat_glyph_or_glyph_class(rewriter);

fea-rs/src/token_tree/typed.rs

@@ -890,6 +890,10 @@ impl InlineSubRule {
     pub(crate) fn replacements(&self) -> impl Iterator<Item = GlyphOrClass> + '_ {
         self.iter().filter_map(GlyphOrClass::cast)
     }
+
+    pub(crate) fn null(&self) -> Option<Null> {
+        self.iter().find_map(Null::cast)
+    }
 }
 
 impl Gpos1 {

fea-rs/test-data/compile-tests/mini-latin/good/contextual_null_sub.fea

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

fea-rs/test-data/compile-tests/mini-latin/good/contextual_null_sub.ttx

@@ -0,0 +1,69 @@
+<?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=2 -->
+      <Lookup index="0">
+        <LookupType value="6"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="1">
+          <Coverage>
+            <Glyph value="c"/>
+          </Coverage>
+          <!-- ChainSubRuleSetCount=1 -->
+          <ChainSubRuleSet index="0">
+            <!-- ChainSubRuleCount=1 -->
+            <ChainSubRule index="0">
+              <!-- BacktrackGlyphCount=2 -->
+              <Backtrack index="0" value="b"/>
+              <Backtrack index="1" value="a"/>
+              <!-- InputGlyphCount=1 -->
+              <!-- LookAheadGlyphCount=0 -->
+              <!-- SubstCount=1 -->
+              <SubstLookupRecord index="0">
+                <SequenceIndex value="0"/>
+                <LookupListIndex value="1"/>
+              </SubstLookupRecord>
+            </ChainSubRule>
+          </ChainSubRuleSet>
+        </ChainContextSubst>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0">
+          <Substitution in="c" out=""/>
+        </MultipleSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>