Bug 1589738 - nsACString::as_str_unchecked should actually check in debug builds. r=nika

Today I reviewed wrong usage of this API.

It'd be good if it crashed in debug builds when misused.

Differential Revision: https://phabricator.services.mozilla.com/D49778

--HG--
extra : moz-landing-system : lando

xpcom/rust/nsstring/src/lib.rs

@@ -1086,8 +1086,13 @@ define_string_types! {
 }
 
 impl nsACString {
+    #[inline]
     pub unsafe fn as_str_unchecked(&self) -> &str {
-        str::from_utf8_unchecked(self)
+        if cfg!(debug_assertions) {
+            str::from_utf8(self).expect("Should be utf-8")
+        } else {
+            str::from_utf8_unchecked(self)
+        }
     }
 }