Move generated bindings to script_bindings (#36323)

This is the final step of #1799, where the majority of the generated
code for the JS bindings is now compiled as part of the script_bindings
build step. The remaining pieces in script must live there because they
refer to concrete DOM types; all code in script_bindings is generic over
the
[DomTypes](https://doc.servo.org/script/dom/bindings/codegen/DomTypes/trait.DomTypes.html)
trait.

My testing with incremental builds shows me a 12 second reduction in
build times on my 2024 M4 Macbook Pro when modifying code in the script
crate after these changes. Before this PR those changes took 20 seconds
to rebuild Servo, and now they take 8 seconds.

Testing: Existing WPT tests ensure no regressions.
Fixes: #1799

---------

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

1 files changed, 142 insertions, 0 deletions

diff --git a/components/script_bindings/settings_stack.rs b/components/script_bindings/settings_stack.rs
new file mode 100644
index 00000000000..cdc8c2a029d
--- /dev/null
+++ b/components/script_bindings/settings_stack.rs
@@ -0,0 +1,142 @@
+/* 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/. */
+
+use std::marker::PhantomData;
+use std::thread;
+
+use js::jsapi::{HideScriptedCaller, UnhideScriptedCaller};
+use js::rust::Runtime;
+
+use crate::DomTypes;
+use crate::interfaces::{DomHelpers, GlobalScopeHelpers};
+use crate::root::{Dom, DomRoot};
+use crate::script_runtime::CanGc;
+
+#[derive(Debug, Eq, JSTraceable, PartialEq)]
+pub enum StackEntryKind {
+    Incumbent,
+    Entry,
+}
+
+#[cfg_attr(crown, allow(crown::unrooted_must_root))]
+#[derive(JSTraceable)]
+pub struct StackEntry<D: DomTypes> {
+    pub global: Dom<D::GlobalScope>,
+    pub kind: StackEntryKind,
+}
+
+/// RAII struct that pushes and pops entries from the script settings stack.
+pub struct GenericAutoEntryScript<D: DomTypes> {
+    global: DomRoot<D::GlobalScope>,
+    #[cfg(feature = "tracing")]
+    #[allow(dead_code)]
+    span: tracing::span::EnteredSpan,
+}
+
+impl<D: DomTypes> GenericAutoEntryScript<D> {
+    /// <https://html.spec.whatwg.org/multipage/#prepare-to-run-script>
+    pub fn new(global: &D::GlobalScope) -> Self {
+        let settings_stack = <D as DomHelpers<D>>::settings_stack();
+        settings_stack.with(|stack| {
+            trace!("Prepare to run script with {:p}", global);
+            let mut stack = stack.borrow_mut();
+            stack.push(StackEntry {
+                global: Dom::from_ref(global),
+                kind: StackEntryKind::Entry,
+            });
+            Self {
+                global: DomRoot::from_ref(global),
+                #[cfg(feature = "tracing")]
+                span: tracing::info_span!(
+                    "ScriptEvaluate",
+                    servo_profiling = true,
+                    url = global.get_url().to_string(),
+                )
+                .entered(),
+            }
+        })
+    }
+}
+
+impl<D: DomTypes> Drop for GenericAutoEntryScript<D> {
+    /// <https://html.spec.whatwg.org/multipage/#clean-up-after-running-script>
+    fn drop(&mut self) {
+        let settings_stack = <D as DomHelpers<D>>::settings_stack();
+        settings_stack.with(|stack| {
+            let mut stack = stack.borrow_mut();
+            let entry = stack.pop().unwrap();
+            assert_eq!(
+                &*entry.global as *const D::GlobalScope, &*self.global as *const D::GlobalScope,
+                "Dropped AutoEntryScript out of order."
+            );
+            assert_eq!(entry.kind, StackEntryKind::Entry);
+            trace!("Clean up after running script with {:p}", &*entry.global);
+        });
+
+        // Step 5
+        if !thread::panicking() && D::GlobalScope::incumbent().is_none() {
+            self.global.perform_a_microtask_checkpoint(CanGc::note());
+        }
+    }
+}
+
+/// RAII struct that pushes and pops entries from the script settings stack.
+pub struct GenericAutoIncumbentScript<D: DomTypes> {
+    global: usize,
+    _marker: PhantomData<D>,
+}
+
+impl<D: DomTypes> GenericAutoIncumbentScript<D> {
+    /// <https://html.spec.whatwg.org/multipage/#prepare-to-run-a-callback>
+    pub fn new(global: &D::GlobalScope) -> Self {
+        // Step 2-3.
+        unsafe {
+            let cx =
+                Runtime::get().expect("Creating a new incumbent script after runtime shutdown");
+            HideScriptedCaller(cx.as_ptr());
+        }
+        let settings_stack = <D as DomHelpers<D>>::settings_stack();
+        settings_stack.with(|stack| {
+            trace!("Prepare to run a callback with {:p}", global);
+            // Step 1.
+            let mut stack = stack.borrow_mut();
+            stack.push(StackEntry {
+                global: Dom::from_ref(global),
+                kind: StackEntryKind::Incumbent,
+            });
+            Self {
+                global: global as *const _ as usize,
+                _marker: PhantomData,
+            }
+        })
+    }
+}
+
+impl<D: DomTypes> Drop for GenericAutoIncumbentScript<D> {
+    /// <https://html.spec.whatwg.org/multipage/#clean-up-after-running-a-callback>
+    fn drop(&mut self) {
+        let settings_stack = <D as DomHelpers<D>>::settings_stack();
+        settings_stack.with(|stack| {
+            // Step 4.
+            let mut stack = stack.borrow_mut();
+            let entry = stack.pop().unwrap();
+            // Step 3.
+            assert_eq!(
+                &*entry.global as *const D::GlobalScope as usize, self.global,
+                "Dropped AutoIncumbentScript out of order."
+            );
+            assert_eq!(entry.kind, StackEntryKind::Incumbent);
+            trace!(
+                "Clean up after running a callback with {:p}",
+                &*entry.global
+            );
+        });
+        unsafe {
+            // Step 1-2.
+            if let Some(cx) = Runtime::get() {
+                UnhideScriptedCaller(cx.as_ptr());
+            }
+        }
+    }
+}