MIR borrowck: wrong error for move into closure

[vramana]

# ramana at pc in ~/Documents/rust-lang/rust on git:fix-e0504 ✖︎ [18:05:33]
→ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc src/test/compile-fail/E0504.rs -Z borrowck=mir 
error[E0505]: cannot move out of `fancy_num` because it is borrowed
  --> src/test/compile-fail/E0504.rs:23:13
   |
21 |       let fancy_ref = &fancy_num;
   |                       ---------- borrow of `fancy_num` occurs here
22 | 
23 |       let x = move || {
   |  _____________^
24 | |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
25 | |     };
   | |_____^ move out of `fancy_num` occurs here

error: aborting due to previous error


# ramana at pc in ~/Documents/rust-lang/rust on git:fix-e0504 ✖︎ [18:12:36]
→ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc src/test/compile-fail/E0504.rs                 
error[E0504]: cannot move `fancy_num` into closure because it is borrowed
  --> src/test/compile-fail/E0504.rs:24:40
   |
21 |     let fancy_ref = &fancy_num;
   |                      --------- borrow of `fancy_num` occurs here
...
24 |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
   |                                        ^^^^^^^^^ move into closure occurs here

error: aborting due to previous error

cc @arielb1

I am working on this.

[vramana]

Hi,

I was little busy work stuff. I am taking a stab at this issue again today. I am using simpler case that generates less MIR code

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    // let fancy_ref = &fancy_num;

    let mut x = move || {
        fancy_num.num += 1;
    };

    x();
}

Generated MIR here

When I uncomment the line let fancy_ref = &fancy_num;, this borrow conflits with this _3 = move _1 in MIR above

       StorageLive(_2);                 // bb0[2]: scope 3 at src/main.rs:9:9: 9:14
       StorageLive(_3);                 // bb0[3]: scope 3 at src/main.rs:9:17: 11:6
       _3 = move _1;                    // bb0[4]: scope 3 at src/main.rs:9:17: 11:6
+      The line that causes the error.
       _2 = [closure@src/main.rs:9:17: 11:6] { fancy_num: move _3 }; // bb0[5]: scope 3 at src/main.rs:9:17: 11:6
                                         // closure
                                         // └ def_id: DefId(0/1:10 ~ playground[72d2]::main[0]::{{closure}}[0])

Note: _3 will be _4 in the debug log because of the extra variable fancy_ref (_2)

So the error occurs way before we enter the closure.

rust/src/librustc_mir/borrow_check/mod.rs

Lines 349 to 354 in 1699293

	self.consume_rvalue(
	ContextKind::AssignRhs.new(location),
	(rhs, span),
	location,
	flow_state,
	);

rust/src/librustc_mir/borrow_check/mod.rs

Lines 1016 to 1023 in 1699293

	// move of place: check if this is move of already borrowed path
	self.access_place(
	context,
	(place, span),
	(Deep, Write(WriteKind::Move)),
	LocalMutationIsAllowed::Yes,
	flow_state,
	);

rust/src/librustc_mir/borrow_check/mod.rs

Lines 850 to 851 in 1699293

	error_reported = true;
	this.report_move_out_while_borrowed(context, place_span, &borrow)

This call generates the error. The information we have is the place (_1) we access and the span (scope 3 at src/main.rs:9:17: 11:6) where we access it. How do I find the usages of _1 in this span?

[arielb1]

@vramana

You need to find uses of _1 within the closure. For that, you can follow down the MIR basic block until you find the closure statement, look at the closure def-id, and look for uses of the appropriate upvar within the closure's body (use a MIR visitor and visit_lvalue, they will be _0.<field#> or (*_0).<field#>, and you might also want to recurse if you encounter nested closures).

[vramana]

For that, you can follow down the MIR basic block until you find the closure statement

I don't know how to implement this in code probably because I am not too familiar with API. But I will give it a try.

[vramana]

@arielb1 There is no visit_lvalue function. Can you explain your hints in a little more detail?

[arielb1]

@vramana

Sorry for the delay. lvalue is now place, so visit_lvalue is now visit_place.

[nikomatsakis]

I wasn't able to observe this error, at least not with @vramana's minimized example:

https://play.rust-lang.org/?gist=6af331fc0c3dd1f455a2ecda1308991c&version=nightly

(I also tried removing the comment)

[nikomatsakis]

I guess this example gets an error:

#![feature(nll)]

struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let mut x = move || {
        fancy_num.num += 1;
    };

    x();
    
    drop(fancy_ref);
}

prints:

error[E0505]: cannot move out of `fancy_num` because it is borrowed
  --> src/main.rs:11:17
   |
9  |       let fancy_ref = &fancy_num;
   |                       ---------- borrow of `fancy_num` occurs here
10 | 
11 |       let mut x = move || {
   |  _________________^
12 | |         fancy_num.num += 1;
13 | |     };
   | |_____^ move out of `fancy_num` occurs here
...
17 |       drop(fancy_ref);
   |            --------- borrow later used here