NLL ICE: unexpected region for local data ReFree

[simonvandel]

The code in https://github.com/simonvandel/fcbf/tree/nll-ice produces the following ICE:

error: internal compiler error: unexpected region for local data ReFree(DefId(0/0:13 ~ fcbf[25b8]::x[0]), BrNamed(crate0:DefIndex(1:12), 'x))
  --> src/main.rs:23:8
   |
23 |     f(&f_y);
   |        ^^^

When compiled with:

cargo +nightly check

NLL is enabled in the top of main.rs with #![feature(nll)]. Without NLL, a normal borrow check error is presented.

Meta

rustc --version --verbose:
rustc 1.27.0-nightly (91db9dc 2018-05-04)
binary: rustc
commit-hash: 91db9dc
commit-date: 2018-05-04
host: x86_64-unknown-linux-gnu
release: 1.27.0-nightly
LLVM version: 6.0

[pnkfelix]

triage: it would be good to reduce this down to a standalone example. (In part, that may help us identify if this is a duplicate bug or not.)

Assigning to self to do reduction.

[nikomatsakis]

I'm "nominating" this issue as something to fix for Edition Preview 2 -- specifically, we should first try and reduce it to a standalone example. I would do this by checking out the project above and then removing the ndarray and trying to "stub out" the dependencies locally, making dummy structs etc as needed.

[shepmaster]

I will attempt to reduce the problem, we'll see if I can go any further than that!

[shepmaster]

With rustc 1.28.0-nightly (ae46aefd5 2018-06-16) I do not see an ICE, but I do see these errors:

warning: unused variable: `cacher`
  --> src/main.rs:31:37
   |
31 | fn symmetric_uncertainty_cached<'a>(cacher: &mut MyCacher, x: &'a Col<'a>, y: &'a Col<'a>) -> ValueType {
   |                                     ^^^^^^ help: consider using `_cacher` instead
   |
   = note: #[warn(unused_variables)] on by default

error[E0597]: `x` does not live long enough
   --> src/main.rs:106:26
    |
106 |     let cols: Vec<Col> = x.gencolumns()
    |                          ^ borrowed value does not live long enough
...
172 | }
    | - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0597]: `s_list` does not live long enough
   --> src/main.rs:130:27
    |
130 |     while let Some(f_p) = s_list.get(cur_feature_idx) {
    |                           ^^^^^^ borrowed value does not live long enough
...
172 | }
    | - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0597]: `s_list` does not live long enough
   --> src/main.rs:132:26
    |
132 |         let unexplored = s_list.iter().skip(cur_feature_idx + 1).filter(|f_q| {
    |                          ^^^^^^ borrowed value does not live long enough
...
172 | }
    | - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0597]: `f_y` does not live long enough
   --> src/main.rs:132:73
    |
132 |           let unexplored = s_list.iter().skip(cur_feature_idx + 1).filter(|f_q| {
    |  _________________________________________________________________________^
133 | |             assert_ne!(f_p.col.idx, f_q.col.idx);
134 | |             let su_p_q = symmetric_uncertainty_cached(&mut cacher,
135 | |                     &f_q.col,
...   |
141 | |             f_q_su_target > su_p_q
142 | |         });
    | |_________^ borrowed value does not live long enough
...
172 |   }
    |   - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0597]: `c` does not live long enough
   --> src/main.rs:118:64
    |
118 |         let su_i_c = symmetric_uncertainty_cached(&mut cacher, &c, &f_y);
    |                                                                ^^ borrowed value does not live long enough
...
172 | }
    | - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0597]: `f_y` does not live long enough
   --> src/main.rs:118:68
    |
118 |         let su_i_c = symmetric_uncertainty_cached(&mut cacher, &c, &f_y);
    |                                                                    ^^^^ borrowed value does not live long enough
...
172 | }
    | - borrowed value only lives until here
    |
note: borrowed value must be valid for the lifetime 'x as defined on the function body at 91:1...
   --> src/main.rs:91:1
    |
91  | / pub fn fcbf<'x>(
92  | |     x: ArrayView<'x, ValueType, Ix2>,
93  | |     y: ArrayView<'x, ValueType, Ix1>,
94  | |     threshold: f64,
...   |
171 | |     x.select(Axis(1), &feature_views)
172 | | }
    | |_^

error[E0505]: cannot move out of `c` because it is borrowed
   --> src/main.rs:120:38
    |
118 |         let su_i_c = symmetric_uncertainty_cached(&mut cacher, &c, &f_y);
    |                                                                -- borrow of `c` occurs here
119 |         if su_i_c >= threshold {
120 |             s_list.push(Feature{col: c, su_target: su_i_c});
    |                                      ^
    |                                      |
    |                                      move out of `c` occurs here
    |                                      borrow later used here

error[E0502]: cannot borrow `s_list` as mutable because it is also borrowed as immutable
   --> src/main.rs:159:5
    |
130 |     while let Some(f_p) = s_list.get(cur_feature_idx) {
    |                           ------ immutable borrow occurs here
...
159 |     s_list.sort_by(|a, b| {
    |     ^^^^^^
    |     |
    |     mutable borrow occurs here
    |     borrow later used here

error[E0506]: cannot assign to `s_list` because it is borrowed
   --> src/main.rs:153:9
    |
130 |     while let Some(f_p) = s_list.get(cur_feature_idx) {
    |                           ------
    |                           |
    |                           borrow of `s_list` occurs here
    |                           borrow later used here
...
153 |         s_list = new_s_list.clone();
    |         ^^^^^^ assignment to borrowed `s_list` occurs here

[shepmaster]

I see the same behavior with rustc 1.28.0-nightly (e3bf634e0 2018-06-28)