2 files changed, 18 insertions, 1 deletions
diff --git a/src/strings/builder.go b/src/strings/builder.go
index 189dadb1e7..7c9b686241 100644
--- a/src/strings/builder.go
+++ b/src/strings/builder.go
@@ -15,7 +15,11 @@ import (
 // Do not copy a non-zero Builder.
 type Builder struct {
 	addr *Builder // of receiver, to detect copies by value
-	buf  []byte
+
+	// External users should never get direct access to this buffer, since
+	// the slice at some point will be converted to a string using unsafe, also
+	// data between len(buf) and cap(buf) might be uninitialized.
+	buf []byte
 }
 
 // noescape hides a pointer from escape analysis. It is the identity function
diff --git a/src/strings/builder_test.go b/src/strings/builder_test.go
index c3c627ee7d..36fd7a77e3 100644
--- a/src/strings/builder_test.go
+++ b/src/strings/builder_test.go
@@ -385,3 +385,16 @@ func BenchmarkBuildString_ByteBuffer(b *testing.B) {
 		}
 	})
 }
+
+func TestBuilderGrowSizeclasses(t *testing.T) {
+	s := Repeat("a", 19)
+	allocs := testing.AllocsPerRun(100, func() {
+		var b Builder
+		b.Grow(18)
+		b.WriteString(s)
+		_ = b.String()
+	})
+	if allocs > 1 {
+		t.Fatalf("unexpected amount of allocations: %v, want: 1", allocs)
+	}
+}