reflect: use Value.Len instead of conversion to slice header

This change is functionally equivalent, but reduces reliance on unsafe
features. This would allow GopherJS to avoid an additional patch to the
standard library we'd have to maintain in order to remain compatible
with Go 1.17+.

Change-Id: I4f113db0c572ec0b81ebfecf5a137145f6c8c41d
GitHub-Last-Rev: 94ebb39
GitHub-Pull-Request: #48346
Reviewed-on: https://go-review.googlesource.com/c/go/+/349469
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: nevkontakte

src/reflect/value.go

@@ -2940,8 +2940,7 @@ func (v Value) CanConvert(t Type) bool {
 	// from slice to pointer-to-array.
 	if vt.Kind() == Slice && t.Kind() == Ptr && t.Elem().Kind() == Array {
 		n := t.Elem().Len()
-		h := (*unsafeheader.Slice)(v.ptr)
-		if n > h.Len {
+		if n > v.Len() {
 			return false
 		}
 	}
@@ -3208,10 +3207,10 @@ func cvtStringRunes(v Value, t Type) Value {
 // convertOp: []T -> *[N]T
 func cvtSliceArrayPtr(v Value, t Type) Value {
 	n := t.Elem().Len()
-	h := (*unsafeheader.Slice)(v.ptr)
-	if n > h.Len {
-		panic("reflect: cannot convert slice with length " + itoa.Itoa(h.Len) + " to pointer to array with length " + itoa.Itoa(n))
+	if n > v.Len() {
+		panic("reflect: cannot convert slice with length " + itoa.Itoa(v.Len()) + " to pointer to array with length " + itoa.Itoa(n))
 	}
+	h := (*unsafeheader.Slice)(v.ptr)
 	return Value{t.common(), h.Data, v.flag&^(flagIndir|flagAddr|flagKindMask) | flag(Ptr)}
 }