wstr: statically check the size of WString

This adds the 'static_assertions' crate as a dependency of
'ruffle_wstr', but this dependency was already present in the dep tree.

Author: moulins

Cargo.lock

@@ -6,3 +6,4 @@ authors = ["Arthur Heuillard <arthur.heuillard<orange.fr>"]
 license = "MIT OR Apache-2.0"
 
 [dependencies]
+static_assertions = "1.1.0"

wstr/Cargo.toml

@@ -5,6 +5,7 @@ use core::fmt;
 use core::mem::{self, ManuallyDrop};
 use core::ops::{Deref, DerefMut};
 use core::ptr::NonNull;
+use static_assertions::assert_eq_size;
 
 use super::utils::{encode_raw_utf16, split_ascii_prefix, split_ascii_prefix_bytes, DecodeAvmUtf8};
 use super::{ptr, Units, WStr, MAX_STRING_LEN};
@@ -16,6 +17,12 @@ pub struct WString {
     capacity: u32,
 }
 
+#[cfg(target_pointer_width = "32")]
+assert_eq_size!(WString, [u8; 12]);
+
+#[cfg(target_pointer_width = "64")]
+assert_eq_size!(WString, [u8; 16]);
+
 impl WString {
     /// Creates a new empty `WString`.
     #[inline]