Adjust the zero check in RawVec::grow.

This was supposed to land as part of #72227. (I wish `git push` would abort when you have uncommited changes.)
rust-lang · Dylan-DPC-zz · May 16, 2020 · May 16, 2020 · May 3, 2020 · May 3, 2020
commit 9eb0399a9d506f5f421180fa00ee3eec61a3a41e
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
@@ -401,16 +401,15 @@ impl<T, A: AllocRef> RawVec<T, A> {
         needed_extra_capacity: usize,
         placement: ReallocPlacement,
     ) -> Result<(), TryReserveError> {
+        // This is ensured by the calling contexts.
+        debug_assert!(needed_extra_capacity > 0);
+
         if mem::size_of::<T>() == 0 {
             // Since we return a capacity of `usize::MAX` when `elem_size` is
             // 0, getting to here necessarily means the `RawVec` is overfull.
             return Err(CapacityOverflow);
         }
 
-        if needed_extra_capacity == 0 {
-            return Ok(());
-        }
-
         // Nothing we can really do about these checks, sadly.
         let required_cap =
             used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;