unconditional_recursion does not catch indirect infinity recursion

[KisaragiEffective]

Code

fn a() {
    b()
}

fn b() {
   a()
}

fn main() {
    let _ = a();
}

Current output

none:

Desired output

warning: function cannot return without recursing
 --> <source>:1:1
  |
1 | fn a() { 
  | ^^^^^^ cannot return without recursing
2 |     b()
  |     --- recursive call site
  |
  = help: a ​`loop​` may express intention better if this is on purpose
  = note: ​`#[warn(unconditional_recursion)]​` on by default

warning: function cannot return without recursing
 --> <source>:5:1
  |
5 | fn b() { 
  | ^^^^^^ cannot return without recursing
6 |     a()
  |     --- recursive call site
  |
  = help: a ​`loop​` may express intention better if this is on purpose
  = note: ​`#[warn(unconditional_recursion)]​` on by default

warning: 2 warnings emitted

Rationale and extra context

The compiler will not emit warning from unconditional_recursion lint if the function does mutual recursion. The above case itself is obvious, but here's a bit more complicated case:

#[derive(Default)]
struct Foo {
    #[allow(dead_code)]
    b: Box<Foo>,
}

fn main() {
    let _ = Foo::default();
}

The derived implementation calls Box::<Foo>::default(), and calls Foo::default() again.

Other cases

N/A

Anything else?

1.73.0 can reproduce both case. 1.74.0-beta.4 and nightly-2023-11-02 have same issue.

[mj10021]

@rustbot claim

[clubby789]

Duplicate of #57965 I think. It would be good to look at the previous efforts to fix this to ensure any fix doesn't result in performance regressions

[lcnr]

closing this as a duplicate. #57965 has a lot of relevant info wrt to previous approaches, it's difficult to implement this in a performant way