diff --git a/src/pkg/io/io.go b/src/pkg/io/io.go
index 022fdb6764..e8bbad537c 100644
--- a/src/pkg/io/io.go
+++ b/src/pkg/io/io.go
@@ -63,7 +63,7 @@ var ErrNoProgress = errors.New("multiple Read calls return no data or error")
 //
 // Implementations of Read are discouraged from returning a
 // zero byte count with a nil error, and callers should treat
-// that situation as a no-op.
+// that situation as a no-op. Implementations must not retain p.
 type Reader interface {
 	Read(p []byte) (n int, err error)
 }
@@ -75,6 +75,8 @@ type Reader interface {
 // and any error encountered that caused the write to stop early.
 // Write must return a non-nil error if it returns n < len(p).
 // Write must not modify the slice data, even temporarily.
+//
+// Implementations must not retain p.
 type Writer interface {
 	Write(p []byte) (n int, err error)
 }
@@ -192,6 +194,8 @@ type WriterTo interface {
 //
 // Clients of ReadAt can execute parallel ReadAt calls on the
 // same input source.
+//
+// Implementations must not retain p.
 type ReaderAt interface {
 	ReadAt(p []byte, off int64) (n int, err error)
 }
@@ -209,6 +213,8 @@ type ReaderAt interface {
 //
 // Clients of WriteAt can execute parallel WriteAt calls on the same
 // destination if the ranges do not overlap.
+//
+// Implementations must not retain p.
 type WriterAt interface {
 	WriteAt(p []byte, off int64) (n int, err error)
 }