Rollup merge of rust-lang#80003 - Stupremee:fix-zst-vecdeque-conversi…

…on-panic, r=dtolnay

Fix overflow when converting ZST Vec to VecDeque

```rust
let v = vec![(); 100];
let queue = VecDeque::from(v);
println!("{:?}", queue);
```
This code will currently panic with a capacity overflow.
This PR resolves this issue and makes the code run fine.

Resolves rust-lang#78532

library/alloc/src/collections/vec_deque/mod.rs

@@ -2793,8 +2793,12 @@ impl<T> From<Vec<T>> for VecDeque<T> {
             let len = other.len();
 
             // We need to extend the buf if it's not a power of two, too small
-            // or doesn't have at least one free space
-            if !buf.capacity().is_power_of_two()
+            // or doesn't have at least one free space.
+            // We check if `T` is a ZST in the first condition,
+            // because `usize::MAX` (the capacity returned by `capacity()` for ZST)
+            // is not a power of two and thus it'll always try
+            // to reserve more memory which will panic for ZST (rust-lang/rust#78532)
+            if (!buf.capacity().is_power_of_two() && mem::size_of::<T>() != 0)
                 || (buf.capacity() < (MINIMUM_CAPACITY + 1))
                 || (buf.capacity() == len)
             {

library/alloc/tests/vec_deque.rs

@@ -1728,3 +1728,10 @@ fn test_zero_sized_push() {
         }
     }
 }
+
+#[test]
+fn test_from_zero_sized_vec() {
+    let v = vec![(); 100];
+    let queue = VecDeque::from(v);
+    assert_eq!(queue.len(), 100);
+}