update and add a few tests

compiler/rustc_ast/src/ast.rs

@@ -1814,7 +1814,7 @@ pub enum LitKind {
     /// A byte string (`b"foo"`). Not stored as a symbol because it might be
     /// non-utf8, and symbols only allow utf8 strings.
     ByteStr(Lrc<[u8]>, StrStyle),
-    /// A C String (`c"foo"`).
+    /// A C String (`c"foo"`). Guaranteed only have `\0` in the end.
     CStr(Lrc<[u8]>, StrStyle),
     /// A byte char (`b'f'`).
     Byte(u8),