How does Rust know whether to run the destructor during stack unwind?

Question

The documentation for mem::uninitialized points out why it is dangerous/unsafe to use that function: calling drop on uninitialized memory is undefined behavior.

Answer 1

First, there are drop flags - runtime information for tracking which variables have been initialized. If a variable was not assigned to, drop() will not be executed for it.

In stable, the drop flag is currently stored within the type itself. Writing uninitialized memory to it can cause undefined behavior as to whether drop() will or will not be called. This will soon be out of date information because the drop flag is moved out of the type itself in nightly.

In nightly Rust, if you assign uninitialized memory to a variable, it would be safe to assume that drop() will be executed. However, any useful implementation of drop() will operate on the value. There is no way to detect if the type is properly initialized or not within the Drop trait implementation: it could result in trying to free an invalid pointer or any other random thing, depending on the Drop implementation of the type. Assigning uninitialized memory to a type with Drop is ill-advised anyway.