Make String a must_use type

Author: tesuji

library/alloc/src/string.rs

@@ -275,6 +275,7 @@ use crate::vec::Vec;
 #[derive(PartialOrd, Eq, Ord)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "string_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[must_use = "unused allocation might not be optimized out by compiler"]
 pub struct String {
     vec: Vec<u8>,
 }