"error: illegal recursive type"

Is the restriction on recursive types really necessary?

It makes sense with structs, because recursive structs would mean infinite-sized structs. But with types, it seems unnecessary. After a brief discussion on the Rust IRC, someone told me that this is probably a bug.

Reduced Test Case

trait A<T> {}

trait DoesAforB<T> {}

// no way to express
// struct B<T> where B<T>: A<B<T>>;

struct B<T: DoesAforB<B<T>>>;

fn main() {}

Output

rec.rs:8:23: 8:27 error: illegal recursive type; insert an enum or struct in the cycle, if this is desired
rec.rs:8 struct B<T: DoesAforB<B<T>>>;
                               ^~~~

rustc version

rustc 0.13.0-nightly (336349c93 2014-11-17 20:37:19 +0000)

Even if we decide to not allow recursion, this is NOT recursion, it's referring to the same B<T>, not B<Foo<T>>.

The use case I had in mind for this would be having B<X> implementing A<B<X>> for each X, in the crate where X is defined. So it could be specific to Xs, and it would only be possible to instantiate a B<X> if A<B<X>> is implemented.

This is more-or-less a dup of an existing issue I can't find right now. It's an artifact of how we translate bounds, which winds up forcing a DAG structure. It's in the process of being changed.

The following version of the code example in the OP works now:

trait A<T> {
    fn foo(&self) -> T;
}

struct B<T> where B<T>: A<B<T>> { t: T }

@anishathalye

@anishathalye the reduced test case produces the following error message on stable:

error: parameter `T` is never used [--explain E0392]
 --> <anon>:9:10
  |>
9 |> struct B<T: DoesAforB<B<T>>>;
  |>          ^
help: consider removing `T` or using a marker such as `std::marker::PhantomData`

error: aborting due to previous error

If we change the test case so T is actually used, then the example compiles fine. Maybe this can be closed?

ping @nikomatsakis

Marking as E-neestest to permit closing this.

@nikomatsakis

Also going to unassign @nikomatsakis since I don't think they need to do actively do anything on this issue.

Rollup merge of rust-lang#42734 - MaloJaffre:tests, r=Mark-Simulacrum

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"error: illegal recursive type" #19601

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"error: illegal recursive type" #19601

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Reduced Test Case

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rustc version

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eddyb commented on Dec 6, 2014

anishathalye commented on Dec 6, 2014

nikomatsakis commented on Jan 7, 2015

milibopp commented on Mar 24, 2015

mrhota commented on Aug 19, 2016

Mark-Simulacrum commented on May 11, 2017

Mark-Simulacrum commented on May 11, 2017

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