[fea-rs] Fix panic if same file was included twice

This wasn't something I had considered originally, but the fix is easy
and zero-overhead.

fea-rs/src/token_tree.rs

@@ -263,9 +263,7 @@ impl Node {
 
     fn update_positions_recurse(&mut self, mut pos: usize) {
         self.abs_pos = pos as _;
-        let Some(children) = Arc::get_mut(&mut self.children) else {
-            panic!("update_positions should only be called on a newly created node",);
-        };
+        let children = Arc::make_mut(&mut self.children);
 
         for child in children {
             child.update_positions(pos);

fea-rs/test-data/parse-tests/good/include_same_file_twice.PARSE_TREE

@@ -0,0 +1,51 @@
+FILE@[0; 147)
+  #@0 "# we had an issue where we would panic if the same file was included twice"
+  WS@74 "\n"
+    FeatureNode@[75; 109)
+      FeatureKw@75 "feature"
+      WS@82 " "
+      Tag@83 "nam"
+      WS@86 " "
+      {@87 "{"
+      WS@88 "\n  "
+        GposType2@[91; 102)
+          PosKw@91 "pos"
+          WS@94 " "
+          GlyphName@95 "A"
+          WS@96 " "
+          GlyphName@97 "B"
+          WS@98 " "
+            ValueRecordNode@[99; 101)
+              NUM@99 "20"
+          ;@101 ";"
+      WS@102 "\n"
+      }@103 "}"
+      WS@104 " "
+      Tag@105 "nam"
+      ;@108 ";"
+  WS@109 "\n"
+  WS@110 "\n"
+    FeatureNode@[111; 145)
+      FeatureKw@111 "feature"
+      WS@118 " "
+      Tag@119 "nam"
+      WS@122 " "
+      {@123 "{"
+      WS@124 "\n  "
+        GposType2@[127; 138)
+          PosKw@127 "pos"
+          WS@130 " "
+          GlyphName@131 "A"
+          WS@132 " "
+          GlyphName@133 "B"
+          WS@134 " "
+            ValueRecordNode@[135; 137)
+              NUM@135 "20"
+          ;@137 ";"
+      WS@138 "\n"
+      }@139 "}"
+      WS@140 " "
+      Tag@141 "nam"
+      ;@144 ";"
+  WS@145 "\n"
+  WS@146 "\n"

fea-rs/test-data/parse-tests/good/include_same_file_twice.fea

@@ -0,0 +1,3 @@
+# we had an issue where we would panic if the same file was included twice
+include(short-tag.fea);
+include(short-tag.fea);