Auto merge of rust-lang#128234 - jcsp:retain-empty-case, r=tgross35

Optimize empty case in Vec::retain

While profiling some code that happens to call Vec::retain() in a tight loop, I noticed more runtime than expected in retain, even in a bench case where the vector was always empty.  When I wrapped my call to retain in `if !myvec.is_empty()` I saw faster execution compared with doing retain on an empty vector.

On closer inspection, Vec::retain is doing set_len(0) on itself even when the vector is empty, and then resetting the length again in BackshiftOnDrop::drop.

Unscientific screengrab of a flamegraph illustrating how we end up spending time in set_len and drop:
![image](https://github.com/user-attachments/assets/ebc72ace-84a0-4432-9b6f-1b3c96d353ba)

Author: bors

alloc/src/vec/mod.rs

@@ -1710,6 +1710,12 @@ impl<T, A: Allocator> Vec<T, A> {
         F: FnMut(&mut T) -> bool,
     {
         let original_len = self.len();
+
+        if original_len == 0 {
+            // Empty case: explicit return allows better optimization, vs letting compiler infer it
+            return;
+        }
+
         // Avoid double drop if the drop guard is not executed,
         // since we may make some holes during the process.
         unsafe { self.set_len(0) };