encoding/gob: update decgen to generate current dec_helpers

I edited dec_helpers.go without realizing that it is a generated file.
Fix the generator to generate the current version (which generates
a small comment change).

Change-Id: I70e3bc78eb0728d23c08972611218f288dc1d29c
Reviewed-on: https://go-review.googlesource.com/c/go/+/479117
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Andrew Ekstedt <andrew.ekstedt@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/encoding/gob/dec_helpers.go

@@ -359,6 +359,7 @@ func decStringSlice(state *decoderState, v reflect.Value, length int, ovfl error
 			errorf("decoding string array or slice: length exceeds input size (%d elements)", length)
 		}
 		if i >= len(slice) {
+			// This is a slice that we only partially allocated.
 			growSlice(v, &slice, length)
 		}
 		u := state.decodeUint()

src/encoding/gob/decgen.go

@@ -180,6 +180,7 @@ func main() {
 		fmt.Fprintf(&b, arrayHelper, t.lower, t.upper)
 		fmt.Fprintf(&b, sliceHelper, t.lower, t.upper, t.decoder)
 	}
+	fmt.Fprintf(&b, trailer)
 	source, err := format.Source(b.Bytes())
 	if err != nil {
 		log.Fatal("source format error:", err)
@@ -236,8 +237,29 @@ func dec%[2]sSlice(state *decoderState, v reflect.Value, length int, ovfl error)
 		if state.b.Len() == 0 {
 			errorf("decoding %[1]s array or slice: length exceeds input size (%%d elements)", length)
 		}
+		if i >= len(slice) {
+			// This is a slice that we only partially allocated.
+			growSlice(v, &slice, length)
+		}
 		%[3]s
 	}
 	return true
 }
 `
+
+const trailer = `
+// growSlice is called for a slice that we only partially allocated,
+// to grow it up to length.
+func growSlice[E any](v reflect.Value, ps *[]E, length int) {
+	var zero E
+	s := *ps
+	s = append(s, zero)
+	cp := cap(s)
+	if cp > length {
+		cp = length
+	}
+	s = s[:cp]
+	v.Set(reflect.ValueOf(s))
+	*ps = s
+}
+`