compiler,reflect: add support for [...]T -> []T in reflect

2 files changed, 41 insertions, 4 deletions

diff --git a/src/reflect/type.go b/src/reflect/type.go
index bd717af0d..39659cc62 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -29,6 +29,7 @@
 //     ptrTo        *typeStruct
 //     elem         *typeStruct // element type of the array
 //     arrayLen     uintptr     // length of the array (this is part of the type)
+//     slicePtr     *typeStruct // pointer to []T type
 // - map types (this is still missing the key and element types)
 //     meta         uint8
 //     nmethods     uint16 (0)
@@ -427,6 +428,7 @@ type arrayType struct {
 	ptrTo     *rawType
 	elem      *rawType
 	arrayLen  uintptr
+	slicePtr  *rawType
 }
 
 type mapType struct {
diff --git a/src/reflect/value.go b/src/reflect/value.go
index 2499fca78..dc81fe66a 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -555,8 +555,26 @@ func (v Value) Slice(i, j int) Value {
 		}
 
 	case Array:
-		// TODO(dgryski): can't do this yet because the resulting value needs type slice of v.elem(), not array of v.elem().
-		// need to be able to look up this "new" type so pointer equality of types still works
+		v.checkAddressable()
+		buf, length := buflen(v)
+		i, j := uintptr(i), uintptr(j)
+		if j < i || length < j {
+			slicePanic()
+		}
+
+		elemSize := v.typecode.underlying().elem().Size()
+
+		var hdr sliceHeader
+		hdr.len = j - i
+		hdr.cap = length - i
+		hdr.data = unsafe.Add(buf, i*elemSize)
+
+		sliceType := (*arrayType)(unsafe.Pointer(v.typecode.underlying())).slicePtr
+		return Value{
+			typecode: sliceType,
+			value:    unsafe.Pointer(&hdr),
+			flags:    v.flags,
+		}
 
 	case String:
 		i, j := uintptr(i), uintptr(j)
@@ -604,9 +622,26 @@ func (v Value) Slice3(i, j, k int) Value {
 		}
 
 	case Array:
-		// TODO(dgryski): can't do this yet because the resulting value needs type v.elem(), not array of v.elem().
-		// need to be able to look up this "new" type so pointer equality of types still works
+		v.checkAddressable()
+		buf, length := buflen(v)
+		i, j, k := uintptr(i), uintptr(j), uintptr(k)
+		if j < i || k < j || length < k {
+			slicePanic()
+		}
 
+		elemSize := v.typecode.underlying().elem().Size()
+
+		var hdr sliceHeader
+		hdr.len = j - i
+		hdr.cap = k - i
+		hdr.data = unsafe.Add(buf, i*elemSize)
+
+		sliceType := (*arrayType)(unsafe.Pointer(v.typecode.underlying())).slicePtr
+		return Value{
+			typecode: sliceType,
+			value:    unsafe.Pointer(&hdr),
+			flags:    v.flags,
+		}
 	}
 
 	panic("unimplemented: (reflect.Value).Slice3()")