fix parser error highlight offsets for non-suffix slices

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

errors_test.go

@@ -286,6 +286,27 @@ func TestDecodeError_Position(t *testing.T) {
 	}
 }
 
+func TestDecodeError_InvalidKeyStartAfterComment(t *testing.T) {
+	doc := "# comment\n= \"value\""
+
+	var out map[string]string
+	err := Unmarshal([]byte(doc), &out)
+	assert.Error(t, err)
+
+	var derr *DecodeError
+	if !errors.As(err, &derr) {
+		t.Fatal("error not in expected format")
+	}
+
+	row, col := derr.Position()
+	assert.Equal(t, 2, row)
+	assert.Equal(t, 1, col)
+	assert.Equal(t, "toml: invalid character at start of key: =", derr.Error())
+	assert.Equal(t, `1| # comment
+2| = "value"
+ | ~ invalid character at start of key: =`, derr.String())
+}
+
 func TestStrictErrorUnwrap(t *testing.T) {
 	fo := bytes.NewBufferString(`
 Missing = 1
@@ -301,73 +322,6 @@ OtherMissing = 1
 	assert.Equal(t, 2, len(strictErr.Unwrap()))
 }
 
-func TestDecodeError_PositionAfterComment(t *testing.T) {
-	// Regression test for https://github.com/pelletier/go-toml/issues/1047
-	// Error positions must be correct when the error occurs after comments or
-	// other content that was already scanned past.
-	examples := []struct {
-		desc        string
-		doc         string
-		expectedRow int
-		expectedCol int
-		expectedStr string
-	}{
-		{
-			desc:        "invalid key after comment",
-			doc:         "# comment\n= \"value\"",
-			expectedRow: 2,
-			expectedCol: 1,
-			expectedStr: "1| # comment\n2| = \"value\"\n | ~ invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key after two comments",
-			doc:         "# one\n# two\n= \"value\"",
-			expectedRow: 3,
-			expectedCol: 1,
-			expectedStr: "1| # one\n2| # two\n3| = \"value\"\n | ~ invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key after key-value pair",
-			doc:         "a = 1\n= 2",
-			expectedRow: 2,
-			expectedCol: 1,
-			expectedStr: "1| a = 1\n2| = 2\n | ~ invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key after blank line",
-			doc:         "a = 1\n\n= 2",
-			expectedRow: 3,
-			expectedCol: 1,
-			expectedStr: "1| a = 1\n2|\n3| = 2\n | ~ invalid character at start of key: =",
-		},
-	}
-
-	for _, e := range examples {
-		t.Run(e.desc, func(t *testing.T) {
-			var v interface{}
-			err := Unmarshal([]byte(e.doc), &v)
-			if err == nil {
-				t.Fatal("expected an error")
-			}
-
-			var derr *DecodeError
-			if !errors.As(err, &derr) {
-				t.Fatalf("error not a *DecodeError: %T: %v", err, err)
-			}
-
-			row, col := derr.Position()
-			if row != e.expectedRow {
-				t.Errorf("row: got %d, want %d (error: %s)", row, e.expectedRow, derr.String())
-			}
-			if col != e.expectedCol {
-				t.Errorf("col: got %d, want %d (error: %s)", col, e.expectedCol, derr.String())
-			}
-
-			assert.Equal(t, e.expectedStr, derr.String())
-		})
-	}
-}
-
 func ExampleDecodeError() {
 	doc := `name = 123__456`
 

unstable/parser.go

@@ -84,14 +84,22 @@ func (p *Parser) rangeOfToken(token, rest []byte) Range {
 }
 
 // subsliceOffset returns the byte offset of subslice b within p.data.
-// b must be a subslice of p.data (sharing the same backing array).
+// b must share the same backing array as p.data.
 func (p *Parser) subsliceOffset(b []byte) int {
 	if len(b) == 0 {
-		return 0
+		// Most callers pass suffix slices, so preserve EOF behavior.
+		return len(p.data)
 	}
+
 	dataPtr := reflect.ValueOf(p.data).Pointer()
 	subPtr := reflect.ValueOf(b).Pointer()
-	return int(subPtr - dataPtr)
+
+	offset := int(subPtr - dataPtr)
+	if offset < 0 || offset+len(b) > len(p.data) {
+		panic("subslice is not within parser input")
+	}
+
+	return offset
 }
 
 // Raw returns the slice corresponding to the bytes in the given range.