Implement TryFromJs for Either<L, R> (#3822)

* Implement TryFromJs for Either<L, R>

* Add explicit feature in Cargo.toml

core/engine/Cargo.toml

@@ -14,6 +14,7 @@ rust-version.workspace = true
 [features]
 profiler = ["boa_profiler/profiler"]
 deser = ["boa_interner/serde", "boa_ast/serde"]
+either = ["dep:either"]
 
 # Enables the `Intl` builtin object and bundles a default ICU4X data provider.
 # Prefer this over `intl` if you just want to enable `Intl` without dealing with the
@@ -104,6 +105,7 @@ intrusive-collections = "0.9.6"
 cfg-if = "1.0.0"
 time.workspace = true
 hashbrown.workspace = true
+either = { version = "1", optional = true }
 
 # intl deps
 boa_icu_provider = { workspace = true, features = ["std"], optional = true }

core/engine/src/value/conversions/either.rs

@@ -0,0 +1,44 @@
+//! Implementation of [`TryFromJs`] for [`Either`].
+//!
+//! This will try to deserialize for the [`Either::Left`] type
+//! first, and if it fails will try the [`Either::Right`] type.
+//!
+//! Upon failure of both, the second failure will be returned.
+#![cfg(feature = "either")]
+
+use crate::value::TryFromJs;
+use boa_engine::{Context, JsResult, JsValue};
+use either::Either;
+
+impl<L, R> TryFromJs for Either<L, R>
+where
+    L: TryFromJs,
+    R: TryFromJs,
+{
+    #[inline]
+    fn try_from_js(value: &JsValue, context: &mut Context) -> JsResult<Self> {
+        L::try_from_js(value, context)
+            .map(Self::Left)
+            .or_else(|_| R::try_from_js(value, context).map(Self::Right))
+    }
+}
+
+#[test]
+fn either() {
+    let v = JsValue::Integer(123);
+    let mut context = Context::default();
+
+    assert_eq!(
+        Either::<i32, i32>::try_from_js(&v, &mut context),
+        Ok(Either::Left(123))
+    );
+    assert_eq!(
+        Either::<i32, String>::try_from_js(&v, &mut context),
+        Ok(Either::Left(123))
+    );
+    assert_eq!(
+        Either::<String, i32>::try_from_js(&v, &mut context),
+        Ok(Either::Right(123))
+    );
+    assert!(Either::<String, String>::try_from_js(&v, &mut context).is_err());
+}

core/engine/src/value/conversions/mod.rs

@@ -4,6 +4,7 @@ use crate::js_string;
 
 use super::{JsBigInt, JsObject, JsString, JsSymbol, JsValue, Profiler};
 
+mod either;
 mod serde_json;
 pub(super) mod try_from_js;