From 5cc5f11b5815131693a22d558d236e69fef532da Mon Sep 17 00:00:00 2001
From: Damian Gryski <damian@gryski.com>
Date: Sat, 25 Feb 2023 17:35:19 -0800
Subject: [PATCH] reflect: add MakeSlice()

---
 src/reflect/value.go      | 34 +++++++++++++++++++++++++++++++++-
 src/reflect/value_test.go |  7 +++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/src/reflect/value.go b/src/reflect/value.go
index d25b8bfa..dce08795 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -882,8 +882,40 @@ func (v Value) Convert(t Type) Value {
 	panic("unimplemented: (reflect.Value).Convert()")
 }
 
+//go:linkname slicePanic runtime.slicePanic
+func slicePanic()
+
 func MakeSlice(typ Type, len, cap int) Value {
-	panic("unimplemented: reflect.MakeSlice()")
+	if typ.Kind() != Slice {
+		panic("reflect.MakeSlice of non-slice type")
+	}
+
+	rtype := typ.(*rawType)
+
+	ulen := uint(len)
+	ucap := uint(cap)
+	maxSize := (^uintptr(0)) / 2
+	elementSize := rtype.elem().Size()
+	if elementSize > 1 {
+		maxSize /= uintptr(elementSize)
+	}
+	if ulen > ucap || ucap > uint(maxSize) {
+		slicePanic()
+	}
+
+	// This can't overflow because of the above checks.
+	size := uintptr(ucap) * elementSize
+
+	var slice sliceHeader
+	slice.cap = uintptr(ucap)
+	slice.len = uintptr(ulen)
+	slice.data = alloc(size, nil)
+
+	return Value{
+		typecode: rtype,
+		value:    unsafe.Pointer(&slice),
+		flags:    valueFlagExported,
+	}
 }
 
 func Zero(typ Type) Value {
diff --git a/src/reflect/value_test.go b/src/reflect/value_test.go
index de55f8d6..0c6e43d0 100644
--- a/src/reflect/value_test.go
+++ b/src/reflect/value_test.go
@@ -132,6 +132,13 @@ func TestSlice(t *testing.T) {
 			t.Errorf("s[%d]=%d, want %d", i, s[i], i*10)
 		}
 	}
+
+	refs = MakeSlice(TypeOf(s), 5, 10)
+	s = refs.Interface().([]int)
+
+	if len(s) != refs.Len() || cap(s) != refs.Cap() {
+		t.Errorf("len(s)=%v refs.Len()=%v cap(s)=%v refs.Cap()=%v", len(s), refs.Len(), cap(s), refs.Cap())
+	}
 }
 
 func equal[T comparable](a, b []T) bool {