[fea-rs] Allow NULL anchor without mark class in pos ligature rules#2015
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…re rules Good test: `<anchor NULL>` on a non-final ligature component followed by `ligComponent` should parse without errors (feaLib accepts this). Bad test: a non-NULL anchor without `mark @class` should still error. The good test currently fails — fea-rs rejects `<anchor NULL>` without `mark @class` even though the spec example and feaLib both allow it. Part of #2014.
In GPOS type 5 (mark-to-ligature) rules, `<anchor NULL>` denotes a ligature component with no mark attachment point. The `mark @class` that normally follows an anchor is not required in this case — the spec example shows `<anchor NULL>;` at the end of a rule, and feaLib has accepted bare `<anchor NULL>` (without mark) since its original GPOS type 5 implementation (fonttools/fonttools@0532ec85, Dec 2015). Previously, fea-rs required `mark @class` after every non-semicolon anchor, causing parse errors when `<anchor NULL>` appeared on a non-final ligature component (followed by `ligComponent` rather than `;`). This broke real-world fonts like Google Sans Arabic. The fix peeks at the anchor before consuming it: if it is NULL and no `mark` keyword follows, skip the mark-class parsing. Non-NULL anchors still require `mark @class` as before. Part of #2014.
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For reference, this is feaLib's equivalent: |
rsheeter
approved these changes
May 18, 2026
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Part of #2014.
In GPOS type 5 (mark-to-ligature) rules,
<anchor NULL>denotes a ligature component with no mark attachment point. fea-rs requiredmark @classafter every anchor, rejecting<anchor NULL>on non-final components followed byligComponent.The FEA spec grammar says
<anchor> mark <named mark class>for each attachment, but the spec's own GPOS type 5 example uses<anchor NULL>;withoutmark @class. feaLib has followed the example rather than the grammar since its original GPOS type 5 implementation (Dec 2015).The fix peeks at the anchor before consuming it: if NULL and no
markfollows, skip mark-class parsing. Non-NULL anchors withoutmarkstill error as before.