Auto merge of #138532 - matthiaskrgr:rollup-mgcynqu, r=matthiaskrgr

Rollup of 5 pull requests

Successful merges:

 - #138283 (Enforce type of const param correctly in MIR typeck)
 - #138439 (feat: check ARG_MAX on Unix platforms)
 - #138502 (resolve: Avoid some unstable iteration)
 - #138514 (Remove fake borrows of refs that are converted into non-refs in `MakeByMoveBody`)
 - #138524 (Mark myself as unavailable for reviews temporarily)

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_data_structures/src/unord.rs

@@ -259,6 +259,12 @@ impl<V: Eq + Hash> UnordSet<V> {
         self.inner.is_empty()
     }
 
+    /// If the set has only one element, returns it, otherwise returns `None`.
+    #[inline]
+    pub fn get_only(&self) -> Option<&V> {
+        if self.inner.len() == 1 { self.inner.iter().next() } else { None }
+    }
+
     #[inline]
     pub fn insert(&mut self, v: V) -> bool {
         self.inner.insert(v)