calling `impl Fn` in defining scope

[compiler-errors]

impacts multiple crates:

• shex_compact https://crater-reports.s3.amazonaws.com/pr-133502-2/try%23da0a4799770027c4c578bf48cdb390acfde09023/reg/shex_compact-0.1.60/log.txt
• wax: https://crater-reports.s3.amazonaws.com/pr-133502-2/try%23da0a4799770027c4c578bf48cdb390acfde09023/reg/wax-0.6.0/log.txt
• der-parser: https://crater-reports.s3.amazonaws.com/pr-133502-4/try%230c7fed2b7584d310ca8221267fa1b3bec4df427f/reg/arti-client-0.29.0/log.txt https://github.com/rusticata/der-parser/blob/79ec2d0cfe1e3ba2deb49ee1f68f984f4ef3a971/src/ber/serialize.rs#L161

struct ShExCompactPrinter;

struct TripleExpr;

impl ShExCompactPrinter {
    fn pp_triple_expr(&self) -> impl Fn(&TripleExpr, &ShExCompactPrinter) + '_ {
        move |te, printer| {
            printer.pp_triple_expr()(te, printer);
        }
    }
}

error[E0282]: type annotations needed
 --> /home/mgx/test.rs:8:13
  |
8 |             printer.pp_triple_expr()(te, printer);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type

error[E0618]: expected function, found `impl for<'a, 'b> Fn(&'a TripleExpr, &'b ShExCompactPrinter) + '_`
 --> /home/mgx/test.rs:8:13
  |
8 |             printer.pp_triple_expr()(te, printer);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^-------------
  |             |
  |             call expression requires function

Has to do with recursive definition of opaque.

[compiler-errors]

Same with wax:

https://crater-reports.s3.amazonaws.com/pr-133502-2/try%23da0a4799770027c4c578bf48cdb390acfde09023/reg/wax-0.6.0/log.txt

[lcnr]

fn foo() -> impl FnMut() -> usize {
    if false {
        let mut f = foo();
        f();
    }
    
    let mut state = 0;
    move || {
        state += 1;
        state
    }
}

[lcnr]

fn foo() -> impl FnOnce() {
    if false {
        let mut f = foo();
        f();
    }
    
    let string = String::new();
    move || drop(string)
}