Like this:

  fn leak() {
      // Create a 1KiB heap-allocated vector
      let b = Box::new(vec![0u8; 1024]);
      // Turn it into a raw pointer
      let p = Box::into_raw(b);
      // Then leak the pointer
  }

Obviously that's kind of blatant, but there are more subtle ways to leak memory. Memory leaks aren't considered unsafe, so even though they're undesirable the compiler doesn't guarantee you won't have any.

Reference cycles when using Rc<T> are a big one, but generally it's pretty hard to cause leaks by accident. I've only run into one instance of leaking memory outside of unsafe code, and that was caused by a library issue.

The same way that memory leaks are possible in Java: rather than a technical bug (you forgot to `free` some buffer), instead you have a semantical bug (you're holding on to a pointer to the data after you're done with it, and that keeps the data alive).

Granted, the ownership/borrowing semantics of rust make this a lot harder, but anything that uses Rc/Arc can easily fall prey to it — you can use those to create a reference cycle.

Memory leaks are not only possible but they are officially supported. The most obvious being `std::mem::forget` and `Box::leak`. Of course the user of these functions should usually ensure that drop is eventually called for all initialized data but there's no way to enforce this.

If you mean unintentional leaks then that is a harder problem. Others have noted ARC and RC leaks but also thread locals may (or may not) leak[0].

[0]: https://doc.rust-lang.org/std/thread/struct.LocalKey.html#pl...

Because the standard library has reference counting with no static checking to avoid cycles, and it was decided to also have safe mem::forget since it can be (mostly) emulated with the former.

It has no such static checking because it was deemed to reduce expressiveness, while not impacting memory safety.

The documentation for std::mem::forget says this:

> Rust's safety guarantees do not include a guarantee that destructors will always run. For example, a program can create a reference cycle using Rc, or call process::exit to exit without running destructors. Thus, allowing mem::forget from safe code does not fundamentally change Rust's safety guarantees.

https://doc.rust-lang.org/std/mem/fn.forget.html