ICE fictitious type in sizing_type_of()

[nikomatsakis]

This example causes an ICE:

Play: http://is.gd/pqOpJQ

trait MyIntoIterator {
  type IntoIter: Iterator;
  fn into_iter(self) -> Self::IntoIter;
}

// This should be saying `if for any lifetime, a ref to Self is MyIntoIterator`.
trait Iterable where for<'a> &'a Self: MyIntoIterator {
  fn iter<'a>(&'a self) -> <&'a Self as MyIntoIterator>::IntoIter {
    self.into_iter()
  }
}
impl<T> Iterable for T where for<'a> &'a T: MyIntoIterator {}

// Impl MyIntoIterator for &Vec<T> for all lifetimes.
impl<'a, T> MyIntoIterator for &'a Vec<T> {
  type IntoIter = ::std::slice::Iter<'a, T>;
  fn into_iter(self) -> <Self as MyIntoIterator>::IntoIter { self.iter() }
}

// Impl MyIntoIterator for &String for all lifetimes.
impl<'a> MyIntoIterator for &'a String {
  type IntoIter = ::std::str::Chars<'a>;
  fn into_iter(self) -> <Self as MyIntoIterator>::IntoIter { self.chars() }
}

fn iterate<T>(thing: T) where T: Iterable, for<'a> &'a T: MyIntoIterator {
  for x in thing.iter() {
    println!("a thing");
  }
}

fn main() {
  iterate(vec![1,2,3]);
  iterate("abc".to_string());
}

output:

error: internal compiler error: fictitious type <&'static collections::vec::Vec<i32> as MyIntoIterator>::IntoIter in sizing_type_of()
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace

[Stebalien]

Shorter example:

trait Collection where for<'a> &'a Self: IntoIterator {
    fn my_iter(&self) -> <&Self as IntoIterator>::IntoIter {
        self.into_iter()
    }
}

impl<T> Collection for [T] { }

fn main() {
    let v = [0usize];
    let _ = v.my_iter();
}

[malbarbo]

Interesting, if the where clause is moved to my_iter method its works

trait Collection {
    fn my_iter<'a>(&'a self) -> <&'a Self as IntoIterator>::IntoIter where &'a Self: IntoIterator {
        self.into_iter()
    }
}

impl<T> Collection for [T] { }

fn main() {
    let v = [0usize];
    let _ = v.my_iter();
}

[malbarbo]

It seems to be related with #23406.

[withoutboats]

Ran into this with code like the below. The IntoIterator impl is fine, only the method on Foo crashes.

pub struct Foo(Vec<i32>);

impl Foo {
    pub fn iter(&self) -> <&Self as IntoIterator>::IntoIter {
        self.into_iter()
    }
}

impl<'a> IntoIterator for &'a Foo {
    type IntoIter = <&'a Vec<i32> as IntoIterator>::IntoIter;
    type Item = <&'a Vec<i32> as IntoIterator>::Item;
    fn into_iter(self) -> Self::IntoIter {
        (&self.0).into_iter()
    }
}