Move more bindings code to script_bindings (#35578)

* Move JSContext wrapper to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move webidl constant bindings to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move CanGc to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move Dom<T> and Root<T> types to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Formatting.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Extra docs for new traits.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Fix clippy warnings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/constant.rs

@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! WebIDL constants.
+
+use std::ffi::CStr;
+
+use js::jsapi::{JSPROP_ENUMERATE, JSPROP_PERMANENT, JSPROP_READONLY};
+use js::jsval::{BooleanValue, DoubleValue, Int32Value, JSVal, NullValue, UInt32Value};
+use js::rooted;
+use js::rust::wrappers::JS_DefineProperty;
+use js::rust::HandleObject;
+
+use crate::script_runtime::JSContext;
+
+/// Representation of an IDL constant.
+#[derive(Clone)]
+pub struct ConstantSpec {
+    /// name of the constant.
+    pub name: &'static CStr,
+    /// value of the constant.
+    pub value: ConstantVal,
+}
+
+/// Representation of an IDL constant value.
+#[derive(Clone)]
+#[allow(dead_code)]
+pub enum ConstantVal {
+    /// `long` constant.
+    Int(i32),
+    /// `unsigned long` constant.
+    Uint(u32),
+    /// `double` constant.
+    Double(f64),
+    /// `boolean` constant.
+    Bool(bool),
+    /// `null` constant.
+    Null,
+}
+
+impl ConstantSpec {
+    /// Returns a `JSVal` that represents the value of this `ConstantSpec`.
+    pub fn get_value(&self) -> JSVal {
+        match self.value {
+            ConstantVal::Null => NullValue(),
+            ConstantVal::Int(i) => Int32Value(i),
+            ConstantVal::Uint(u) => UInt32Value(u),
+            ConstantVal::Double(d) => DoubleValue(d),
+            ConstantVal::Bool(b) => BooleanValue(b),
+        }
+    }
+}
+
+/// Defines constants on `obj`.
+/// Fails on JSAPI failure.
+pub fn define_constants(cx: JSContext, obj: HandleObject, constants: &[ConstantSpec]) {
+    for spec in constants {
+        rooted!(in(*cx) let value = spec.get_value());
+        unsafe {
+            assert!(JS_DefineProperty(
+                *cx,
+                obj,
+                spec.name.as_ptr(),
+                value.handle(),
+                (JSPROP_ENUMERATE | JSPROP_READONLY | JSPROP_PERMANENT) as u32
+            ));
+        }
+    }
+}