Compile Servo with the latest version of rust stable (#30831)

This completes the transition to compiling Servo with rust stable. Some
nightly-only features are still used when compiling the `script` and
`crown` crates, as well as for some style unit tests. These will likely
break with newer compiler versions, but `crown` can be disabled for them
conditionally. This is just the first step.

This has some caveats:

1. We need to disable setting up the special linker on Linux. The -Z
   option isn't supported with stable rust so using this is out --
   meanwhile we can't be sure that lld is installed on most systems.
2. `cargo fmt` still uses some unstable options, so we need to rely on
   the unstable toolchain just for running `fmt`. The idea is to fix this
   gradually.

components/layout_2020/display_list/stacking_context.rs

@@ -586,10 +586,15 @@ impl StackingContext {
             return;
         };
 
-        let StackingContextContent::Fragment { fragment, scroll_node_id, containing_block, .. }
-            = first_stacking_context_fragment else {
-                debug_panic!("Expected a fragment, not a stacking container");
-            };
+        let StackingContextContent::Fragment {
+            fragment,
+            scroll_node_id,
+            containing_block,
+            ..
+        } = first_stacking_context_fragment
+        else {
+            debug_panic!("Expected a fragment, not a stacking container");
+        };
         let fragment = fragment.borrow();
         let box_fragment = match &*fragment {
             Fragment::Box(box_fragment) | Fragment::Float(box_fragment) => box_fragment,

components/layout_2020/lib.rs

@@ -3,7 +3,6 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 #![deny(unsafe_code)]
-#![feature(once_cell)]
 
 mod cell;
 pub mod context;

components/layout_thread/lib.rs

@@ -1241,7 +1241,7 @@ impl LayoutThread {
         );
 
         // Flush shadow roots stylesheets if dirty.
-        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author.clone());
+        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author);
 
         let restyles = std::mem::take(&mut data.pending_restyles);
         debug!("Draining restyles: {}", restyles.len());

components/layout_thread_2020/lib.rs

@@ -913,7 +913,7 @@ impl LayoutThread {
         }
 
         // Flush shadow roots stylesheets if dirty.
-        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author.clone());
+        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author);
 
         let restyles = std::mem::take(&mut data.pending_restyles);
         debug!("Draining restyles: {}", restyles.len());

components/script/dom/gpudevice.rs

@@ -218,7 +218,7 @@ impl GPUDevice {
 
     fn handle_error(&self, scope: ErrorScopeId, error: GPUError) {
         let mut context = self.scope_context.borrow_mut();
-        if let Some(mut err_scope) = context.error_scopes.get_mut(&scope) {
+        if let Some(err_scope) = context.error_scopes.get_mut(&scope) {
             if err_scope.error.is_none() {
                 err_scope.error = Some(error);
             }
@@ -229,7 +229,7 @@ impl GPUDevice {
 
     fn try_remove_scope(&self, scope: ErrorScopeId) {
         let mut context = self.scope_context.borrow_mut();
-        let remove = if let Some(mut err_scope) = context.error_scopes.get_mut(&scope) {
+        let remove = if let Some(err_scope) = context.error_scopes.get_mut(&scope) {
             err_scope.op_count -= 1;
             if let Some(ref promise) = err_scope.promise {
                 if !promise.is_fulfilled() {
@@ -272,7 +272,7 @@ impl GPUDevice {
             .find(|meta| !meta.popped.get())
             .map(|meta| meta.id);
         scope_id.and_then(|s_id| {
-            context.error_scopes.get_mut(&s_id).map(|mut scope| {
+            context.error_scopes.get_mut(&s_id).map(|scope| {
                 scope.op_count += 1;
                 s_id
             })
@@ -1107,7 +1107,7 @@ impl GPUDeviceMethods for GPUDevice {
                 promise.reject_error(Error::Operation);
                 return promise;
             };
-        let remove = if let Some(mut err_scope) = context.error_scopes.get_mut(&scope_id) {
+        let remove = if let Some(err_scope) = context.error_scopes.get_mut(&scope_id) {
             if let Some(ref e) = err_scope.error {
                 promise.resolve_native(e);
             } else if err_scope.op_count == 0 {

components/script/dom/testbinding.rs

@@ -5,7 +5,6 @@
 // check-tidy: no specs after this line
 
 use std::borrow::ToOwned;
-use std::ptr;
 use std::ptr::NonNull;
 use std::rc::Rc;
 
@@ -1043,11 +1042,7 @@ impl TestBindingMethods for TestBinding {
 
     #[allow(unsafe_code)]
     fn CrashHard(&self) {
-        static READ_ONLY_VALUE: i32 = 0;
-        unsafe {
-            let p: *mut u32 = &READ_ONLY_VALUE as *const _ as *mut _;
-            ptr::write_volatile(p, 0xbaadc0de);
-        }
+        unsafe { std::ptr::null_mut::<i32>().write(42) }
     }
 
     fn AdvanceClock(&self, ms: i32) {

components/script/lib.rs

@@ -2,8 +2,6 @@
  * 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/. */
 
-#![feature(once_cell)]
-#![feature(plugin)]
 #![feature(register_tool)]
 #![deny(unsafe_code)]
 #![doc = "The script crate contains all matters DOM."]

components/shared/script/compositor.rs

@@ -97,7 +97,7 @@ impl ScrollTreeNode {
         &mut self,
         scroll_location: ScrollLocation,
     ) -> Option<(ExternalScrollId, LayoutVector2D)> {
-        let mut info = match self.scroll_info {
+        let info = match self.scroll_info {
             Some(ref mut data) => data,
             None => return None,
         };

components/shared/script/tests/compositor.rs

@@ -155,7 +155,7 @@ fn test_scroll_tree_chain_through_overflow_hidden() {
         .get_node_mut(&overflow_hidden_id)
         .scroll_info
         .as_mut()
-        .map(|mut info| {
+        .map(|info| {
             info.scroll_sensitivity = ScrollSensitivity::Script;
         });