"error: illegal recursive type; insert an enum or struct in the cycle, if this is desired" not very helpful #17539

imbaczek · 2014-09-25T21:30:39Z

I've tried to write code that would work like this:

let mut f = some_fn;  // many other some_fns also exist

loop {
    f = f()
}

To make coding some_fn and its friends easier, I wanted to create a type:

type SomeFn = fn() -> SomeFn;

Unfortunately, this results in some type-theoretic error message that isn't really understandable by mere mortals:

<anon>:1:15: 1:29 error: illegal recursive type; insert an enum or struct in the cycle, if this is desired
<anon>:1 type SomeFn = fn() -> SomeFn;
                       ^~~~~~~~~~~~~~

It's not at all clear what 'the cycle' is and where to insert an enum or struct. Turns out that if you know what the compiler is talking about, the fix is simple:

struct SomeFn(fn() -> SomeFn);

// ergonomy suffers
fn fn1() -> SomeFn { println!("1"); SomeFn(fn2) }
fn fn2() -> SomeFn { println!("2"); SomeFn(fn1) }

fn main() {
    let mut f = SomeFn(fn1);
    f = { let SomeFn(ff) = f; ff() };
    f = { let SomeFn(ff) = f; ff() };
}

Output:

1
2

But I was very confused by the message and tried a contraption like this instead:

type SomeFn = fn() -> Option<SomeFn>;

A hint in the error message that points the programmer to the right direction would be very helpful. Ergonomy of the solution is also questionable, but at least it compiles.

The text was updated successfully, but these errors were encountered:

reem · 2014-09-26T21:59:53Z

Since the size of a fn does not depend on its output type, I doubt this is a necessary restriction in the type system.

nham · 2015-05-13T08:17:36Z

Triage: The error message seems better, but perhaps is still not ideal. Compiling this code:

type SomeFn = fn() -> SomeFn;
fn main() {}

on a recent nightly (rustc 1.1.0-nightly (4b88e8f63 2015-05-11) (built 2015-05-12)) results in:

iss_17539.rs:1:23: 1:29 error: unsupported cyclic reference between types/traits detected
iss_17539.rs:1 type SomeFn = fn() -> SomeFn;
                                     ^~~~~~
note: the cycle begins when processing `SomeFn`...
note: ...which then again requires processing `SomeFn`, completing the cycle.
error: aborting due to previous error

The "processing" language sounds a bit strange to me. Could it maybe say something like "the definition of the type SomeFn depends on SomeFn"?

steveklabnik · 2016-11-29T20:45:38Z

Triage: same as @nham reported. I agree that we could improve on 'processing'.

estebank · 2017-09-29T18:10:09Z

Current output is same as above, with new style output:

error[E0391]: unsupported cyclic reference between types/traits detected
 --> src/main.rs:1:23
  |
1 | type SomeFn = fn() -> SomeFn;
  |                       ^^^^^^ cyclic reference
  |
note: the cycle begins when processing `SomeFn`...
 --> src/main.rs:1:1
  |
1 | type SomeFn = fn() -> SomeFn;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...which then again requires processing `SomeFn`, completing the cycle.

estebank · 2018-11-07T00:15:08Z

Current output:

error[E0391]: cycle detected when processing `SomeFn`
 --> src/main.rs:1:23
  |
1 | type SomeFn = fn() -> SomeFn;
  |                       ^^^^^^
  |
  = note: ...which again requires processing `SomeFn`, completing the cycle

camelid · 2021-07-05T22:40:13Z

I have an idea on how to improve this error. @rustbot claim

… r=<try> Improve errors for recursive type aliases Fixes rust-lang#17539.

kmcallister added the A-diagnostics label Sep 27, 2014

steveklabnik added the T-compiler label Mar 9, 2017

Mark-Simulacrum added the C-enhancement label Jul 22, 2017

rustbot assigned camelid Jul 5, 2021

camelid mentioned this issue Aug 17, 2021

Improve errors for recursive type aliases #88121

Merged

bors added a commit to rust-lang-ci/rust that referenced this issue Aug 27, 2021

Auto merge of rust-lang#88121 - camelid:better-recursive-alias-error,…

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fb1a092

… r=<try> Improve errors for recursive type aliases Fixes rust-lang#17539.

bors closed this in c4f26b1 Sep 1, 2021

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rust-lang / rust

"error: illegal recursive type; insert an enum or struct in the cycle, if this is desired" not very helpful #17539

"error: illegal recursive type; insert an enum or struct in the cycle, if this is desired" not very helpful #17539

imbaczek commented Sep 25, 2014 •

edited by rustbot

reem commented Sep 26, 2014

nham commented May 13, 2015

steveklabnik commented Nov 29, 2016

estebank commented Sep 29, 2017

estebank commented Nov 7, 2018

camelid commented Jul 5, 2021

rust-lang / rust

"error: illegal recursive type; insert an enum or struct in the cycle, if this is desired" not very helpful #17539

"error: illegal recursive type; insert an enum or struct in the cycle, if this is desired" not very helpful #17539

Comments

imbaczek commented Sep 25, 2014 • edited by rustbot

reem commented Sep 26, 2014

nham commented May 13, 2015

steveklabnik commented Nov 29, 2016

estebank commented Sep 29, 2017

estebank commented Nov 7, 2018

camelid commented Jul 5, 2021

imbaczek commented Sep 25, 2014 •

edited by rustbot