constify VList::new (#2293)

* Use const constructor for VList and fix EMPTY VList access

* Add comment

.gitignore

@@ -4,6 +4,7 @@ Cargo.lock
 # backup files generated by rustfmt
 **/*.rs.bk
 
-# editor config directories
+# editor config files and directories
+*.iml
 /.idea/
 /.vscode/

packages/yew/src/virtual_dom/vlist.rs

@@ -19,11 +19,7 @@ pub struct VList {
 
 impl Default for VList {
     fn default() -> Self {
-        Self {
-            children: Default::default(),
-            key: None,
-            fully_keyed: true,
-        }
+        Self::new()
     }
 }
 
@@ -88,8 +84,12 @@ impl<'s> ElementWriter<'s> {
 
 impl VList {
     /// Creates a new empty [VList] instance.
-    pub fn new() -> Self {
-        Self::default()
+    pub const fn new() -> Self {
+        Self {
+            children: Vec::new(),
+            key: None,
+            fully_keyed: true,
+        }
     }
 
     /// Creates a new [VList] instance with children.

packages/yew/src/virtual_dom/vtag.rs

@@ -8,10 +8,8 @@ use std::borrow::Cow;
 use std::cmp::PartialEq;
 use std::hint::unreachable_unchecked;
 use std::marker::PhantomData;
-use std::mem::MaybeUninit;
 use std::ops::Deref;
 use std::rc::Rc;
-use std::sync::Once;
 use wasm_bindgen::JsCast;
 use web_sys::{Element, HtmlInputElement as InputElement, HtmlTextAreaElement as TextAreaElement};
 
@@ -338,14 +336,11 @@ impl VTag {
         match &self.inner {
             VTagInner::Other { children, .. } => children,
             _ => {
-                static mut EMPTY: MaybeUninit<VList> = MaybeUninit::uninit();
-                static ONCE: Once = Once::new();
-                unsafe {
-                    ONCE.call_once(|| {
-                        EMPTY = MaybeUninit::new(VList::default());
-                    });
-                    &*EMPTY.as_ptr()
-                }
+                // This is mutable because the VList is not Sync
+                static mut EMPTY: VList = VList::new();
+
+                // SAFETY: The EMPTY value is always read-only
+                unsafe { &EMPTY }
             }
         }
     }