Merge pull request #1059 from neon-bindings/kv/with

feat(neon): With wrapper for TryIntoJs

crates/neon/src/types_impl/extract/mod.rs

@@ -108,7 +108,8 @@ use crate::{
     types::{JsValue, Value},
 };
 
-pub use self::error::Error;
+pub use self::{error::Error, with::With};
+
 #[cfg(feature = "serde")]
 #[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
 pub use self::json::Json;
@@ -119,6 +120,7 @@ mod json;
 mod private;
 mod try_from_js;
 mod try_into_js;
+mod with;
 
 /// Extract Rust data from a JavaScript value
 pub trait TryFromJs<'cx>

crates/neon/src/types_impl/extract/with.rs

@@ -0,0 +1,58 @@
+use crate::{
+    context::{Context, Cx},
+    result::JsResult,
+    types::extract::TryIntoJs,
+};
+
+/// Wraps a closure that will be lazily evaluated when [`TryIntoJs::try_into_js`] is
+/// called.
+///
+/// Useful for executing arbitrary code on the main thread before returning from a
+/// function exported with [`neon::export`](crate::export).
+///
+/// ## Example
+///
+/// ```
+/// # use neon::{prelude::*, types::extract::{TryIntoJs, With}};
+/// use std::time::Instant;
+///
+/// #[neon::export(task)]
+/// fn sum(nums: Vec<f64>) -> impl for<'cx> TryIntoJs<'cx> {
+///     let start = Instant::now();
+///     let sum = nums.into_iter().sum::<f64>();
+///     let log = format!("sum took {} ms", start.elapsed().as_millis());
+///
+///     With(move |cx| -> NeonResult<_> {
+///         cx.global::<JsObject>("console")?
+///             .call_method_with(cx, "log")?
+///             .arg(cx.string(log))
+///             .exec(cx)?;
+///
+///         Ok(sum)
+///     })
+/// }
+/// ```
+pub struct With<F, O>(pub F)
+where
+    // N.B.: We include additional required bounds to allow the compiler to infer the
+    // correct closure argument when using `impl for<'cx> TryIntoJs<'cx>`. Without
+    // these bounds, it would be necessary to write a more verbose signature:
+    // `With<impl for<'cx> FnOnce(&mut Cx<'cx>) -> SomeConcreteReturnType>`.
+    for<'cx> F: FnOnce(&mut Cx<'cx>) -> O;
+
+impl<'cx, F, O> TryIntoJs<'cx> for With<F, O>
+where
+    F: FnOnce(&mut Cx) -> O,
+    O: TryIntoJs<'cx>,
+{
+    type Value = O::Value;
+
+    fn try_into_js<C>(self, cx: &mut C) -> JsResult<'cx, Self::Value>
+    where
+        C: Context<'cx>,
+    {
+        (self.0)(cx.cx_mut()).try_into_js(cx)
+    }
+}
+
+impl<F, O> super::private::Sealed for With<F, O> where for<'cx> F: FnOnce(&mut Cx<'cx>) -> O {}