Consolidate subslice offset into a single SubsliceOffset function

Remove the private subsliceOffset methods from both parser.go and
errors.go. Replace them with a single exported SubsliceOffset function
in ast.go (next to the Range type it serves).

SubsliceOffset finds the byte offset by comparing element addresses:
&data[i] == &subslice[0]. This is well-defined Go pointer comparison
on elements of the same backing array.

This fixes the v2.3.0 regression (#1047) where the parser's
subsliceOffset used len(data) - len(b), which only works for suffix
slices, not arbitrary subslices like error highlights. It also removes
the reflect-based implementation from errors.go.

Fixes #1047

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

unstable/ast.go

@@ -90,6 +90,21 @@ type Range struct {
 	Length uint32
 }
 
+// SubsliceOffset returns the byte offset of subslice within data.
+// Subslice must be a subslice of data, meaning it must point into the
+// same backing array. Panics if subslice is not within data.
+func SubsliceOffset(data []byte, subslice []byte) int {
+	if len(subslice) == 0 {
+		return len(data)
+	}
+	for i := range data {
+		if &data[i] == &subslice[0] {
+			return i
+		}
+	}
+	panic("subslice is not within data")
+}
+
 // Next returns a pointer to the next node, or nil if there is no next node.
 func (n *Node) Next() *Node {
 	if n.next < 0 {