Use #[non_exhaustive] where appropriate

Due to the std/alloc split, it is not possible to make
`alloc::collections::TryReserveError::AllocError` non-exhaustive without
having an unstable, doc-hidden method to construct (which negates the
benefits from `#[non_exhaustive]`.

compiler/rustc_metadata/src/dynamic_lib.rs

@@ -70,13 +70,12 @@ mod dl {
         use std::sync::{Mutex, MutexGuard};
 
         pub fn lock() -> MutexGuard<'static, Guard> {
-            static LOCK: SyncLazy<Mutex<Guard>> = SyncLazy::new(|| Mutex::new(Guard { _priv: () }));
+            static LOCK: SyncLazy<Mutex<Guard>> = SyncLazy::new(|| Mutex::new(Guard));
             LOCK.lock().unwrap()
         }
 
-        pub struct Guard {
-            _priv: (),
-        }
+        #[non_exhaustive]
+        pub struct Guard;
 
         impl Guard {
             pub fn get(&mut self) -> Result<(), String> {