reflect: handle String kind in Value.{Pointer,UnsafePointer}

Updates #61308

Change-Id: I92d459383c520d137787ce5c8f135d205af74e5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/516596
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/reflect/all_test.go

@@ -8512,3 +8512,39 @@ func TestClear(t *testing.T) {
 		})
 	}
 }
+
+func TestValuePointerAndUnsafePointer(t *testing.T) {
+	ptr := new(int)
+	ch := make(chan int)
+	m := make(map[int]int)
+	unsafePtr := unsafe.Pointer(ptr)
+	slice := make([]int, 1)
+	fn := func() {}
+	s := "foo"
+
+	tests := []struct {
+		name              string
+		val               Value
+		wantUnsafePointer unsafe.Pointer
+	}{
+		{"pointer", ValueOf(ptr), unsafe.Pointer(ptr)},
+		{"channel", ValueOf(ch), *(*unsafe.Pointer)(unsafe.Pointer(&ch))},
+		{"map", ValueOf(m), *(*unsafe.Pointer)(unsafe.Pointer(&m))},
+		{"unsafe.Pointer", ValueOf(unsafePtr), unsafePtr},
+		{"function", ValueOf(fn), **(**unsafe.Pointer)(unsafe.Pointer(&fn))},
+		{"slice", ValueOf(slice), unsafe.Pointer(unsafe.SliceData(slice))},
+		{"string", ValueOf(s), unsafe.Pointer(unsafe.StringData(s))},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			if got := tc.val.Pointer(); got != uintptr(tc.wantUnsafePointer) {
+				t.Errorf("unexpected uintptr result, got %#x, want %#x", got, uintptr(tc.wantUnsafePointer))
+			}
+			if got := tc.val.UnsafePointer(); got != tc.wantUnsafePointer {
+				t.Errorf("unexpected unsafe.Pointer result, got %#x, want %#x", got, tc.wantUnsafePointer)
+			}
+		})
+	}
+}

src/reflect/value.go

@@ -2184,7 +2184,7 @@ func (v Value) OverflowUint(x uint64) bool {
 // and make an exception.
 
 // Pointer returns v's value as a uintptr.
-// It panics if v's Kind is not [Chan], [Func], [Map], [Pointer], [Slice], or [UnsafePointer].
+// It panics if v's Kind is not [Chan], [Func], [Map], [Pointer], [Slice], [String], or [UnsafePointer].
 //
 // If v's Kind is [Func], the returned pointer is an underlying
 // code pointer, but not necessarily enough to identify a
@@ -2195,6 +2195,9 @@ func (v Value) OverflowUint(x uint64) bool {
 // element of the slice. If the slice is nil the returned value
 // is 0.  If the slice is empty but non-nil the return value is non-zero.
 //
+// If v's Kind is [String], the returned pointer is to the first
+// element of the underlying bytes of string.
+//
 // It's preferred to use uintptr(Value.UnsafePointer()) to get the equivalent result.
 func (v Value) Pointer() uintptr {
 	// The compiler loses track as it converts to uintptr. Force escape.
@@ -2232,9 +2235,10 @@ func (v Value) Pointer() uintptr {
 			p = *(*unsafe.Pointer)(p)
 		}
 		return uintptr(p)
-
 	case Slice:
 		return uintptr((*unsafeheader.Slice)(v.ptr).Data)
+	case String:
+		return uintptr((*unsafeheader.String)(v.ptr).Data)
 	}
 	panic(&ValueError{"reflect.Value.Pointer", v.kind()})
 }
@@ -2779,6 +2783,9 @@ func (v Value) UnsafeAddr() uintptr {
 // If v's Kind is [Slice], the returned pointer is to the first
 // element of the slice. If the slice is nil the returned value
 // is nil.  If the slice is empty but non-nil the return value is non-nil.
+//
+// If v's Kind is [String], the returned pointer is to the first
+// element of the underlying bytes of string.
 func (v Value) UnsafePointer() unsafe.Pointer {
 	k := v.kind()
 	switch k {
@@ -2812,9 +2819,10 @@ func (v Value) UnsafePointer() unsafe.Pointer {
 			p = *(*unsafe.Pointer)(p)
 		}
 		return p
-
 	case Slice:
 		return (*unsafeheader.Slice)(v.ptr).Data
+	case String:
+		return (*unsafeheader.String)(v.ptr).Data
 	}
 	panic(&ValueError{"reflect.Value.UnsafePointer", v.kind()})
 }