In what circumstances might drop not be called? I know there's std::mem::forget and of course a process or thread could be forcibly terminated. Are there any other situations to be aware of?
The type may be wrapped in a ManuallyDrop, although this is essentially the same effect as std::mem::forget. And along the lines of "process or thread could be forcibly terminated", someone can run a program with panic=abort - no unwinding happens, and drops will not occur.
Leaks using (A/)Rc. Be wary of global and thread local too. Std threads can outrun the program termination when not joined, so just halt in mid execution.
static variables don't run Drop.
Ahh it seems std::thread::LocalKey lists some interesting cases.
exit will not lead to drop being called, the same goes for using system calls that replace the running process with another.
In unsafe code, destructors aren't called when you overwrite a value with ptr::write or ptr::copy (or variants like write_bytes, copy_nonoverlapping, etc.).
Misuse of other unsafe functions like Vec::set_len can cause leaks.
And there are more safe functions that intentionally leak values:
Box::leakBox::into_rawCString::into_rawRc::into_rawArc::into_raw