"captured variable cannot escape `FnMut` closure body" doesn't say which variable escapes

[jyn514]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7118a398f76c37fd88ad80cba70e862e

pub fn run<'s, F>(_: F)
where
    F: FnMut() -> Box<dyn FnMut() + 's>, 
{
    // todo
}

fn foo(f: &mut dyn Fn()) {
    run(move || Box::new(|| {
        // dummy lines to push `f` off the screen
        //
        //
        f();
        //
        //
    }));
}

The current output is:

error: captured variable cannot escape `FnMut` closure body
  --> src/lib.rs:9:17
   |
9  |       run(move || Box::new(|| {
   |  _______________-_^
   | |               |
   | |               inferred to be a `FnMut` closure
10 | |         // dummy lines to push `f` off the screen
11 | |         //
12 | |         //
...  |
15 | |         //
16 | |     }));
   | |______^ returns a reference to a captured variable which escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape

Ideally the output should point to f on line 13 to explain that it's the variable that escapes. It would also be nice to more clearly distinguish that it's being returned from the outer closure, not the inner closure, but I'm not sure how to make that better.

[jyn514]

hmm, if I remove move it does show the variable: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d98fb97c4d07dc6f89b51ec787719c7b

error: captured variable cannot escape `FnMut` closure body
  --> src/lib.rs:9:12
   |
8  |   fn foo(f: &mut dyn FnMut()) {
   |          - variable defined here
9  |       run(|| Box::new(|| {
   |  __________-_^
   | |          |
   | |          inferred to be a `FnMut` closure
10 | |         // dummy lines to push `f` off the screen
11 | |         //
12 | |         //
13 | |         f();
   | |         - variable captured here
14 | |         //
15 | |         //
16 | |     }));
   | |______^ returns a reference to a captured variable which escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape

[clubby789]

variable captured here isn't shown in the playground or compiling with Cargo, but is when invoking rustc directly.

➜  poc git:(master) ✗ cargo check
    Checking poc v0.1.0 (/tmp/poc)
error: captured variable cannot escape `FnMut` closure body
  --> src/lib.rs:9:17
   |
9  |       run(move || Box::new(|| {
   |  _______________-_^
   | |               |
   | |               inferred to be a `FnMut` closure
10 | |         // dummy lines to push `f` off the screen
11 | |         //
12 | |         //
...  |
15 | |         //
16 | |     }));

➜  poc git:(master) ✗ rustc src/lib.rs --crate-type=lib
error: captured variable cannot escape `FnMut` closure body
  --> src/lib.rs:9:17
   |
8  |   fn foo(f: &mut dyn Fn()) {
   |          - variable defined here
9  |       run(move || Box::new(|| {
   |  _______________-_^
   | |               |
   | |               inferred to be a `FnMut` closure
10 | |         // dummy lines to push `f` off the screen
11 | |         //
12 | |         //
13 | |         f();
   | |         - variable captured here
14 | |         //
15 | |         //
16 | |     }));

[clubby789]

debug logs under cargo:

DEBUG rustc_borrowck::diagnostics::region_errors report_region_error(fr='_#5r, outlived_fr='_#1r)
DEBUG rustc_borrowck::diagnostics::region_errors report_region_error: category=Return(Normal) ObligationCause { span: src/lib.rs:9:17: 16:7 (#0), body_id: HirId(DefId(0:0 ~ poc[667e]).0), code: MiscObligation } None
DEBUG rustc_borrowck::diagnostics::region_errors report_region_error: fr_is_local=true outlived_fr_is_local=false category=Return(Normal)
DEBUG rustc_borrowck::diagnostics::region_errors report_fnmut_error: output_ty=std::boxed::Box<dyn std::ops::FnMut()>

and rustc

DEBUG rustc_borrowck::diagnostics::region_errors report_region_error(fr='_#5r, outlived_fr='_#1r)
DEBUG rustc_borrowck::diagnostics::region_errors report_region_error: category=Return(ClosureUpvar(field[0])) ObligationCause { span: src/lib.rs:9:17: 16:7 (#0), body_id: HirId(DefId(0:0 ~ lib[b3d6]).0), code: MiscObligation } None
DEBUG rustc_borrowck::diagnostics::region_errors report_region_error: fr_is_local=true outlived_fr_is_local=false category=Return(ClosureUpvar(field[0]))
DEBUG rustc_borrowck::diagnostics::region_errors report_fnmut_error: output_ty=std::boxed::Box<dyn std::ops::FnMut()>

The subdiagnostic is added here.

[jyn514]

@clubby789 I bet one of the arguments cargo passes affects it - can you pass -v to cargo and see if the rustc invocation it prints replicates the missing diagnostic?

[clubby789]

Looks like this only happens under edition=2021. 2018/15 give the subdiagnostic