clippy: Fix warnings in shared and config, fonts, layout, and layout_2020 components (#32674)

components/layout/display_list/items.rs

@@ -109,7 +109,7 @@ impl DisplayList {
     /// Return the bounds of this display list based on the dimensions of the root
     /// stacking context.
     pub fn bounds(&self) -> LayoutRect {
-        match self.list.get(0) {
+        match self.list.first() {
             Some(DisplayItem::PushStackingContext(item)) => item.stacking_context.bounds,
             Some(_) => unreachable!("Root element of display list not stacking context."),
             None => LayoutRect::zero(),

components/layout/flow.rs

@@ -74,8 +74,11 @@ use crate::table_wrapper::TableWrapperFlow;
 /// This marker trait indicates that a type is a struct with `#[repr(C)]` whose first field
 /// is of type `BaseFlow` or some type that also implements this trait.
 ///
-/// In other words, the memory representation of `BaseFlow` must be a prefix
-/// of the memory representation of types implementing `HasBaseFlow`.
+/// # Safety
+///
+/// The memory representation of `BaseFlow` must be a prefix of the memory representation of types
+/// implementing `HasBaseFlow`. If this isn't the case, calling [`GetBaseFlow::base`] or
+/// [`GetBaseFlow::mut_base`] could lead to memory errors.
 #[allow(unsafe_code)]
 pub unsafe trait HasBaseFlow {}
 

components/layout/layout_debug.rs

@@ -18,7 +18,7 @@ use serde_json::{to_string, to_value, Value};
 use crate::flow::GetBaseFlow;
 use crate::flow_ref::FlowRef;
 
-thread_local!(static STATE_KEY: RefCell<Option<State>> = RefCell::new(None));
+thread_local!(static STATE_KEY: RefCell<Option<State>> = const { RefCell::new(None) });
 
 #[cfg(debug_assertions)]
 static DEBUG_ID_COUNTER: AtomicUsize = AtomicUsize::new(0);

components/layout/text.rs

@@ -833,6 +833,7 @@ fn is_specific(script: Script) -> bool {
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[allow(clippy::enum_variant_names)]
 pub enum CompressionMode {
     CompressNone,
     CompressWhitespace,

components/layout/text_run.rs

@@ -21,7 +21,7 @@ use xi_unicode::LineBreakLeafIter;
 
 thread_local! {
     static INDEX_OF_FIRST_GLYPH_RUN_CACHE: Cell<Option<(*const TextRun, ByteIndex, usize)>> =
-        Cell::new(None)
+        const { Cell::new(None) }
 }
 
 /// A single "paragraph" of text in one font size and style.

components/layout/traversal.rs

@@ -187,8 +187,12 @@ where
     fn process(&mut self, node: &ConcreteThreadSafeLayoutNode);
 }
 
-#[allow(unsafe_code)]
+/// # Safety
+///
+/// This function modifies the DOM node represented by the `node` argument, so it is imperitive
+/// that no other thread is modifying the node at the same time.
 #[inline]
+#[allow(unsafe_code)]
 pub unsafe fn construct_flows_at_ancestors<'dom>(
     context: &LayoutContext,
     mut node: impl LayoutNode<'dom>,