Implement support of Symbol.toPrimitive (#1020)

Co-authored-by: João Borges <rageknify@gmail.com>
Co-authored-by: tofpie <tofpie@users.noreply.github.com>

boa/src/value/mod.rs

@@ -527,11 +527,25 @@ impl Value {
         // 1. Assert: input is an ECMAScript language value. (always a value not need to check)
         // 2. If Type(input) is Object, then
         if let Value::Object(obj) = self {
+            if let Some(exotic_to_prim) =
+                obj.get_method(context, context.well_known_symbols().to_primitive_symbol())?
+            {
+                let hint = match preferred_type {
+                    PreferredType::String => "string",
+                    PreferredType::Number => "number",
+                    PreferredType::Default => "default",
+                }
+                .into();
+                let result = exotic_to_prim.call(&self, &[hint], context)?;
+                return if result.is_object() {
+                    Err(context.construct_type_error("Symbol.toPrimitive cannot return an object"))
+                } else {
+                    Ok(result)
+                };
+            }
+
             let mut hint = preferred_type;
 
-            // Skip d, e we don't support Symbols yet
-            // TODO: add when symbols are supported
-            // TODO: Add other steps.
             if hint == PreferredType::Default {
                 hint = PreferredType::Number;
             };

boa/src/value/tests.rs

@@ -619,6 +619,20 @@ fn test_accessors() {
     assert_eq!(forward(&mut context, "arr"), r#"[ "a" ]"#);
 }
 
+#[test]
+fn to_primitive() {
+    let mut context = Context::new();
+    let src = r#"
+    let a = {};
+    a[Symbol.toPrimitive] = function() {
+        return 42;
+    };
+    let primitive = a + 0;
+    "#;
+    context.eval(src).unwrap();
+    assert_eq!(forward(&mut context, "primitive"), "42");
+}
+
 /// Test cyclic conversions that previously caused stack overflows
 /// Relevant mitigations for these are in `GcObject::ordinary_to_primitive` and
 /// `GcObject::to_json`