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>

errors.go

@@ -99,7 +99,7 @@ func (e *DecodeError) Key() Key {
 //
 //nolint:funlen
 func wrapDecodeError(document []byte, de *unstable.ParserError) *DecodeError {
-	offset := subsliceOffset(document, de.Highlight)
+	offset := unstable.SubsliceOffset(document, de.Highlight)
 
 	errMessage := de.Error()
 	errLine, errColumn := positionAtEnd(document[:offset])
@@ -261,17 +261,3 @@ func positionAtEnd(b []byte) (row int, column int) {
 
 	return row, column
 }
-
-// subsliceOffset finds the byte offset of subslice within data by
-// scanning for the matching element address.
-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")
-}

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 {

unstable/parser.go

@@ -69,8 +69,8 @@ func (p *Parser) Data() []byte {
 // panics.
 func (p *Parser) Range(b []byte) Range {
 	return Range{
-		Offset: uint32(p.subsliceOffset(b)), //nolint:gosec // TOML documents are small
-		Length: uint32(len(b)),              //nolint:gosec // TOML documents are small
+		Offset: uint32(SubsliceOffset(p.data, b)), //nolint:gosec // TOML documents are small
+		Length: uint32(len(b)),                    //nolint:gosec // TOML documents are small
 	}
 }
 
@@ -82,20 +82,6 @@ func (p *Parser) rangeOfToken(token, rest []byte) Range {
 	return Range{Offset: uint32(offset), Length: uint32(len(token))} //nolint:gosec // TOML documents are small
 }
 
-// subsliceOffset finds the byte offset of subslice b within p.data
-// by scanning for the matching element address.
-func (p *Parser) subsliceOffset(b []byte) int {
-	if len(b) == 0 {
-		return len(p.data)
-	}
-	for i := range p.data {
-		if &p.data[i] == &b[0] {
-			return i
-		}
-	}
-	panic("subslice is not within parser data")
-}
-
 // Raw returns the slice corresponding to the bytes in the given range.
 func (p *Parser) Raw(raw Range) []byte {
 	return p.data[raw.Offset : raw.Offset+raw.Length]