database/sql: Add RowsColumnScanner interface

Implementing RowsColumnScanner allows the driver
to completely control how values are scanned.

Fixes #67546

Author: jackc

src/database/sql/driver/driver.go

@@ -515,6 +515,16 @@ type RowsColumnTypePrecisionScale interface {
 	ColumnTypePrecisionScale(index int) (precision, scale int64, ok bool)
 }
 
+// RowsColumnScanner may be implemented by [Rows]. It allows the driver to control
+// how values or scanned.
+type RowsColumnScanner interface {
+	Rows
+
+	// ScanColumn copies the column in the current row into the value pointed at by
+	// dest. It returns [ErrSkip] to fall back to the normal [database/sql] scanning path.
+	ScanColumn(index int, dest any) error
+}
+
 // Tx is a transaction.
 type Tx interface {
 	Commit() error

src/database/sql/sql.go

@@ -3389,7 +3389,16 @@ func (rs *Rows) Scan(dest ...any) error {
 	}
 
 	for i, sv := range rs.lastcols {
-		err := convertAssignRows(dest[i], sv, rs)
+		err := driver.ErrSkip
+
+		if rowsColumnScanner, ok := rs.rowsi.(driver.RowsColumnScanner); ok {
+			err = rowsColumnScanner.ScanColumn(i, dest[i])
+		}
+
+		if err == driver.ErrSkip {
+			err = convertAssignRows(dest[i], sv, rs)
+		}
+
 		if err != nil {
 			rs.closemuRUnlockIfHeldByScan()
 			return fmt.Errorf(`sql: Scan error on column index %d, name %q: %w`, i, rs.rowsi.Columns()[i], err)