nomicon/src/vec-dealloc.md

Deallocating

Next we should implement Drop so that we don't massively leak tons of resources. The easiest way is to just call pop until it yields None, and then deallocate our buffer. Note that calling pop is unneeded if T: !Drop. In theory we can ask Rust if T needs_drop and omit the calls to pop. However in practice LLVM is really good at removing simple side-effect free code like this, so I wouldn't bother unless you notice it's not being stripped (in this case it is).

We must not call alloc::dealloc when self.cap == 0, as in this case we haven't actually allocated any memory.

impl<T> Drop for Vec<T> {
    fn drop(&mut self) {
        if self.cap != 0 {
            while let Some(_) = self.pop() { }
            let layout = Layout::array::<T>(self.cap).unwrap();
            unsafe {
                alloc::dealloc(self.ptr.as_ptr() as *mut u8, layout);
            }
        }
    }
}