Closure capturing itself results in unclear "mismatched types" error

[jonas-schievink]

Code:

fn main() {
    let mut r = None;
    
    let c = move || {
        let _r = r;
    };
    
    r = Some(c);
}

Results in this error:

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |       let c = move || {
  |  _____________^
5 | |         let _r = r;
6 | |     };
  | |_____^ cyclic type of infinite size

This seems a bit unclear to me. What types don't match here?

There is special code in the compiler that is supposed to give a better error message, but for some reason it's not happening in this case:

rust/src/librustc/infer/error_reporting/mod.rs

Lines 2003 to 2006 in 73f76b7

	_ => match terr {
	TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
	Error0644("closure/generator type that references itself")
	}

[jonas-schievink]

Similarly this code also doesn't seem to be run, even though it should:

rust/src/librustc/ty/error.rs

Lines 472 to 479 in 73f76b7

	// Watch out for various cases of cyclic types and try to explain.
	if ty.is_closure() || ty.is_generator() {
	db.note(
	"closures cannot capture themselves or take themselves as argument;\n\
	this error may be the result of a recent compiler bug-fix,\n\
	see https://github.com/rust-lang/rust/issues/46062 for more details",
	);
	}

[asquared31415]

This is still and issue and still provides the same mismatched types error but then proceeds to talk about the infinitely sized type instead of the mismatch.

another example