A weird Borrow implementation that returns a different result for each call can create a string with uninitialized bytes with join() implementation of [Borrow<str>] type.

The problem is in join_generic_copy function.

1. The borrow result is first used for the length calculation.
   

   rust/library/alloc/src/str.rs

   Lines 152 to 161 in 3ffea60

   	// compute the exact total length of the joined Vec
   	// if the `len` calculation overflows, we'll panic
   	// we would have run out of memory anyway and the rest of the function requires
   	// the entire Vec pre-allocated for safety
   	let len = sep_len
   	.checked_mul(iter.len())
   	.and_then(|n| {
   	slice.iter().map(|s| s.borrow().as_ref().len()).try_fold(n, usize::checked_add)
   	})
   	.expect("attempt to join into collection with len > usize::MAX");

2. Then, inside spezialize_for_lengths macro, the user-provided slice is borrowed again and the content is copied.
   

   rust/library/alloc/src/str.rs

   Lines 110 to 114 in 3ffea60

   	// arbitrary non-zero size fallback
   	for s in iter {
   	copy_slice_and_advance!(target, sep_bytes);
   	copy_slice_and_advance!(target, s.borrow().as_ref());
   	}

3. Finally, the length of the slice is set to the length calculated in step 1.
   

   rust/library/alloc/src/str.rs

   Line 179 in 3ffea60

   result.set_len(len);

Playground link, which demonstrates creating a non-UTF-8 string by only using safe Rust.