Erroneous error note for using field after move with NLL enabled

[carado]

Hi, I noticed that the error for the following code:

#![feature(nll)]
fn main() {
    let range = 0 .. 1;
    let r = range;
    let x = range.start;
}

is this:

error[E0382]: use of moved value: `range.start`
 --> src/main.rs:5:13
  |
4 |     let r = range;
  |             ----- value moved here
5 |     let x = range.start;
  |             ^^^^^^^^^^^ value used here after move
  |
  = note: move occurs because `range.start` has type `i32`, which does not implement the `Copy` 

Notice the weird error note.

Without NLL enabled, the error becomes the more expected:

error[E0382]: use of moved value: `range.start`
 --> src/main.rs:4:9
  |
3 |     let r = range;
  |         - value moved here
4 |     let x = range.start;
  |         ^ value used here after move
  |
  = note: move occurs because `range` has type `std::ops::Range<i32>`, which does not implement the `Copy` trait

I'm running the latest nightly (as of this writing), which I got from rustup.

rustc 1.28.0-nightly (a805a2a5e 2018-06-10)
binary: rustc
commit-hash: a805a2a5ebba2802f432d79874e59c24e398f82a
commit-date: 2018-06-10
host: x86_64-unknown-linux-gnu
release: 1.28.0-nightly
LLVM version: 6.0

Thanks for your time.

[nikomatsakis]

The problem is that we are using place to get the path that we use in the note:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 128 to 132 in 6ec1b62

	err.note(&format!(
	"move occurs because {} has type `{}`, \
	which does not implement the `Copy` trait",
	note_msg, ty
	));

This corresponds to the place being accessed but not the place that was moved, which is what we actually want. The place(s) that were moved are noted in this loop here:

rust/src/librustc_mir/borrow_check/error_reporting.rs

Lines 84 to 96 in 6ec1b62

	for moi in mois {
	let move_msg = ""; //FIXME: add " (into closure)"
	let move_span = self.mir.source_info(self.move_data.moves[*moi].source).span;
	if span == move_span {
	err.span_label(
	span,
	format!("value moved{} here in previous iteration of loop", move_msg),
	);
	is_loop_move = true;
	} else {
	err.span_label(move_span, format!("value moved{} here", move_msg));
	};
	}

Here, the moi is a "move out index", which indexes into the moves vector give a MoveOut data structure:

rust/src/librustc_mir/dataflow/move_paths/mod.rs

Lines 174 to 186 in 6ec1b62

	/// `MoveOut` represents a point in a program that moves out of some
	/// L-value; i.e., "creates" uninitialized memory.
	///
	/// With respect to dataflow analysis:
	/// - Generated by moves and declaration of uninitialized variables.
	/// - Killed by assignments to the memory.
	#[derive(Copy, Clone)]
	pub struct MoveOut {
	/// path being moved
	pub path: MovePathIndex,
	/// location of move
	pub source: Location,
	}

This contains the path we want in its path field. That is a MovePathIndex which can be converted to a place through the move_data. We want to access the move_paths field:

rust/src/librustc_mir/dataflow/move_paths/mod.rs

Line 124 in 6ec1b62

pub move_paths: IndexVec<MovePathIndex, MovePath<'tcx>>,

and then use the field place:

rust/src/librustc_mir/dataflow/move_paths/mod.rs

Lines 93 to 98 in 6ec1b62

	pub struct MovePath<'tcx> {
	pub next_sibling: Option<MovePathIndex>,
	pub first_child: Option<MovePathIndex>,
	pub parent: Option<MovePathIndex>,
	pub place: Place<'tcx>,
	}

so something like move_data.move_paths[mpi].place.