all: add reflect.SliceAt function

Fixes #61308

Change-Id: Ic17d737fda055a60779985d5da497745c80d5cfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/516597
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

api/next/61308.txt

@@ -0,0 +1 @@
+pkg reflect, func SliceAt(Type, unsafe.Pointer, int) Value #61308

doc/next/6-stdlib/99-minor/reflect/61308.md

@@ -0,0 +1,3 @@
+The [`SliceAt(typ Type, p unsafe.Pointer, len int)`](/pkg/reflect#SliceAt) function
+returns a Value representing a slice whose underlying array starts at p and whose
+length and capacity are len.

src/reflect/all_test.go

@@ -8548,3 +8548,44 @@ func TestValuePointerAndUnsafePointer(t *testing.T) {
 		})
 	}
 }
+
+// Test cases copied from ../../test/unsafebuiltins.go
+func TestSliceAt(t *testing.T) {
+	const maxUintptr = 1 << (8 * unsafe.Sizeof(uintptr(0)))
+	var p [10]byte
+
+	typ := TypeOf(p[0])
+
+	s := SliceAt(typ, unsafe.Pointer(&p[0]), len(p))
+	if s.Pointer() != uintptr(unsafe.Pointer(&p[0])) {
+		t.Fatalf("unexpected underlying array: %d, want: %d", s.Pointer(), uintptr(unsafe.Pointer(&p[0])))
+	}
+	if s.Len() != len(p) || s.Cap() != len(p) {
+		t.Fatalf("unexpected len or cap, len: %d, cap: %d, want: %d", s.Len(), s.Cap(), len(p))
+	}
+
+	typ = TypeOf(0)
+	if !SliceAt(typ, unsafe.Pointer((*int)(nil)), 0).IsNil() {
+		t.Fatal("nil pointer with zero length must return nil")
+	}
+
+	// nil pointer with positive length panics
+	shouldPanic("", func() { _ = SliceAt(typ, unsafe.Pointer((*int)(nil)), 1) })
+
+	// negative length
+	var neg int = -1
+	shouldPanic("", func() { _ = SliceAt(TypeOf(byte(0)), unsafe.Pointer(&p[0]), neg) })
+
+	// size overflows address space
+	n := uint64(0)
+	shouldPanic("", func() { _ = SliceAt(TypeOf(n), unsafe.Pointer(&n), maxUintptr/8) })
+	shouldPanic("", func() { _ = SliceAt(TypeOf(n), unsafe.Pointer(&n), maxUintptr/8+1) })
+
+	// sliced memory overflows address space
+	last := (*byte)(unsafe.Pointer(^uintptr(0)))
+	// This panics here, but won't panic in ../../test/unsafebuiltins.go,
+	// because unsafe.Slice(last, 1) does not escape.
+	//
+	// _ = SliceAt(typ, unsafe.Pointer(last), 1)
+	shouldPanic("", func() { _ = SliceAt(typ, unsafe.Pointer(last), 2) })
+}

src/reflect/value.go

@@ -3211,6 +3211,16 @@ func MakeSlice(typ Type, len, cap int) Value {
 	return Value{&typ.(*rtype).t, unsafe.Pointer(&s), flagIndir | flag(Slice)}
 }
 
+// SliceAt returns a [Value] representing a slice whose underlying
+// data starts at p, with length and capacity equal to n.
+//
+// This is like [unsafe.Slice].
+func SliceAt(typ Type, p unsafe.Pointer, n int) Value {
+	unsafeslice(typ.common(), p, n)
+	s := unsafeheader.Slice{Data: p, Len: n, Cap: n}
+	return Value{SliceOf(typ).common(), unsafe.Pointer(&s), flagIndir | flag(Slice)}
+}
+
 // MakeChan creates a new channel with the specified type and buffer size.
 func MakeChan(typ Type, buffer int) Value {
 	if typ.Kind() != Chan {
@@ -3978,6 +3988,9 @@ func verifyNotInHeapPtr(p uintptr) bool
 //go:noescape
 func growslice(t *abi.Type, old unsafeheader.Slice, num int) unsafeheader.Slice
 
+//go:noescape
+func unsafeslice(t *abi.Type, ptr unsafe.Pointer, len int)
+
 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.

src/runtime/unsafe.go

@@ -112,3 +112,8 @@ func panicunsafeslicenilptr1(pc uintptr) {
 	panicCheck1(pc, "unsafe.Slice: ptr is nil and len is not zero")
 	panic(errorString("unsafe.Slice: ptr is nil and len is not zero"))
 }
+
+//go:linkname reflect_unsafeslice reflect.unsafeslice
+func reflect_unsafeslice(et *_type, ptr unsafe.Pointer, len int) {
+	unsafeslice(et, ptr, len)
+}