Tracking moves and initialization

Part of the borrow checker's job is to track which variables are "initialized" at any given point in time -- this also requires figuring out where moves occur and tracking those.

Initialization and moves

From a user's perspective, initialization -- giving a variable some value -- and moves -- transfering ownership to another place -- might seem like distinct topics. Indeed, our borrow checker error messages often talk about them differently. But within the borrow checker, they are not nearly as separate. Roughly speaking, the borrow checker tracks the set of "initialized places" at any point in the source code. Assigning to a previously uninitialized local variable adds it to that set; moving from a local variable removes it from that set.

Consider this example:

fn foo() {
    let a: Vec<u32>;
    
    // a is not initialized yet
    
    a = vec![22];
    
    // a is initialized here
    
    std::mem::drop(a); // a is moved here
    
    // a is no longer initialized here

    let l = a.len(); //~ ERROR
}

Here you can see that a starts off as uninitialized; once it is assigned, it becomes initialized. But when drop(a) is called, that moves a into the call, and hence it becomes uninitialized again.

Subsections

To make it easier to peruse, this section is broken into a number of subsections:

  • Move paths the move path concept that we use to track which local variables (or parts of local variables, in some cases) are initialized.
  • TODO Rest not yet written =)