note: cat_expr Errd during dtor check

[oli-obk]

I tried to get a minimal example, now i just ended up with completely senseless code, that still gives the error.

use std::ops::IndexMut;

trait Combiner : IndexMut<usize> {
    fn combine<P>(&mut self, mut predicate : P) where
        P: FnMut(&[<Self as IndexMut<usize>>::Output]) -> <Self as IndexMut<usize>>::Output;
}

impl<T> Combiner for Vec<T> {
    fn combine<P>(&mut self, mut predicate : P) where
        P: FnMut(&[<Self as IndexMut<usize>>::Output]) -> <Self as IndexMut<usize>>::Output {
        for _ in 0..self.len() {
            let elem = predicate(&self[0..]);
            *self = vec![elem];
        }
    }
}

fn main() {}

playpen says:

<anon>:11:13: 11:14 note: cat_expr Errd during dtor check
<anon>:11         for _ in 0..self.len() {
                      ^
<anon>:11:32: 14:10 note: cat_expr Errd during dtor check
<anon>:11         for _ in 0..self.len() {
<anon>:12             let elem = predicate(&self[0..]);
<anon>:13             *self = vec![elem];
<anon>:14         }
<anon>:12:24: 12:45 note: cat_expr Errd during dtor check
<anon>:12             let elem = predicate(&self[0..]);
                                 ^~~~~~~~~~~~~~~~~~~~~
<anon>:13:26: 13:30 note: cat_expr Errd during dtor check
<anon>:13             *self = vec![elem];
                                   ^~~~
<std macros>:1:1: 7:33 note: in expansion of vec!
<anon>:13:21: 13:32 note: expansion site

if you remove the for loop, only the last two cat_expr notes are left. adding more code just ends up with more of the notes.

[pnkfelix]

this is pretty much my fault. :)

In all the cases that I have seen where it arises, it is due to a type error (or a type inference failure at at least) somewhere in the code.

Of course the notes are incredibly annoying. I will try to fix.

[pnkfelix]

And of course the concrete example given here is one where there is no type error. At least, not one exposed by compiling this code. (I have not yet managed to show Combiner being instantiated concretely without causing some other type error to arise.)

[pnkfelix]

(but there may be a mix of bugs causing the original input here to pass. In particular, the code in the description is doing <Self as IndexMut>::Output, but that associated type does not exist. One needs <Self as Index>::Output. See also: #21259 )

[pnkfelix]

cc #22321