refactor(napi): simplify `napi_value` interface (#16170)

1 files changed, 58 insertions, 0 deletions

diff --git a/ext/napi/value.rs b/ext/napi/value.rs
new file mode 100644
index 000000000..bb3966593
--- /dev/null
+++ b/ext/napi/value.rs
@@ -0,0 +1,58 @@
+// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
+
+use deno_core::v8;
+use std::mem::transmute;
+use std::ops::Deref;
+use std::os::raw::c_void;
+use std::ptr::NonNull;
+
+/// An FFI-opaque, nullable wrapper around v8::Local<v8::Value>.
+/// rusty_v8 Local handle cannot be empty but napi_value can be.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug)]
+pub struct NapiValue<'s>(
+  Option<NonNull<v8::Value>>,
+  std::marker::PhantomData<&'s ()>,
+);
+
+pub type napi_value<'s> = NapiValue<'s>;
+
+impl<'s> Deref for napi_value<'s> {
+  type Target = Option<v8::Local<'s, v8::Value>>;
+  fn deref(&self) -> &Self::Target {
+    // SAFETY: It is safe to transmute `Option<NonNull<T>>` to `Option<*const T>`.
+    //         v8::Local guarantees that *const T is not null but napi_value *can* be null.
+    unsafe { transmute::<&Self, &Self::Target>(self) }
+  }
+}
+
+impl<'s, T> From<v8::Local<'s, T>> for napi_value<'s>
+where
+  v8::Local<'s, T>: Into<v8::Local<'s, v8::Value>>,
+{
+  fn from(v: v8::Local<'s, T>) -> Self {
+    // SAFETY: It is safe to cast v8::Local<T> that implements Into<v8::Local<v8::Value>>.
+    //         `fn into(self)` transmutes anyways.
+    Self(unsafe { transmute(v) }, std::marker::PhantomData)
+  }
+}
+
+const _: () = {
+  assert!(
+    std::mem::size_of::<napi_value>() == std::mem::size_of::<*mut c_void>()
+  );
+  // Assert "nullable pointer optimization" on napi_value
+  unsafe {
+    type Src<'a> = napi_value<'a>;
+    type Dst = usize;
+    assert!(std::mem::size_of::<Src>() == std::mem::size_of::<Dst>());
+    union Transmute<'a> {
+      src: Src<'a>,
+      dst: Dst,
+    }
+    Transmute {
+      src: NapiValue(None, std::marker::PhantomData),
+    }
+    .dst
+  };
+};