restore move out dataflow, add report of move out errors

[mikhail-m1]

fix #45363
r? @arielb1

[arielb1]

AST borrowck only shows 1 move here, as in:

fn main() {
    let x = Box::new(1);
    if false {
        let _y = x;
    } else {
        drop(x);
    }
    x;
}

So I think you should only show a note for the first move.

[arielb1]

Also, AST borrowck prefers the "value might be uninitialized" error to the "value might be moved" one - you can get it if you add a "move" representing values being uninitialized at the start of a function to the move-propagation code, but I won't block this PR on not matching the AST error behavior - this is an edge case and both error messages point at real problems.

[arielb1]

this code doesn't account for DropAndReplace - that should be handled the same way as an assignment above, so it would be buggy in the case of e.g.

let x = Box::new(0);
let _u = x; // error shouldn't note this move
x = Box::new(1);
drop(x);
use(x);

[arielb1]

nice PR, r=me modulo comments (you only have to fix the 1-error and the DropAndReplace problems - I'm not sure defaulting to "maybe uninitialized" is actually better).

[arielb1]

Actually, could you make it so that the moving-statements pass only runs if an error occurred? This would avoid it slowing down the normal error-free path.

[arielb1]

r=me if this is green on travis

[arielb1]

@bors r+ rollup

[bors]

📌 Commit 3acb4e9 has been approved by arielb1