src/test/ui/hrtb/issue-30786.rs

// rust-lang/rust#30786: the use of `for<'b> &'b mut A: Stream<Item=T`
// should act as assertion that item does not borrow from its stream;
// but an earlier buggy rustc allowed `.map(|x: &_| x)` which does
// have such an item.
//
// This tests double-checks that we do not allow such behavior to leak
// through again.

// revisions: migrate nll

// Since we are testing nll (and migration) explicitly as a separate
// revisions, don't worry about the --compare-mode=nll on this test.

// ignore-compare-mode-nll
// ignore-compare-mode-polonius

//[nll]compile-flags: -Z borrowck=mir

pub trait Stream {
    type Item;
    fn next(self) -> Option<Self::Item>;
}

// Example stream
pub struct Repeat(u64);

impl<'a> Stream for &'a mut Repeat {
    type Item = &'a u64;
    fn next(self) -> Option<Self::Item> {
        Some(&self.0)
    }
}

pub struct Map<S, F> {
    stream: S,
    func: F,
}

impl<'a, A, F, T> Stream for &'a mut Map<A, F>
where &'a mut A: Stream,
      F: FnMut(<&'a mut A as Stream>::Item) -> T,
{
    type Item = T;
    fn next(self) -> Option<T> {
        match self.stream.next() {
            Some(item) => Some((self.func)(item)),
            None => None,
        }
    }
}

pub struct Filter<S, F> {
    stream: S,
    func: F,
}

impl<'a, A, F, T> Stream for &'a mut Filter<A, F>
where for<'b> &'b mut A: Stream<Item=T>, // <---- BAD
      F: FnMut(&T) -> bool,
{
    type Item = <&'a mut A as Stream>::Item;
    fn next(self) -> Option<Self::Item> {
        while let Some(item) = self.stream.next() {
            if (self.func)(&item) {
                return Some(item);
            }
        }
        None
    }
}

pub trait StreamExt where for<'b> &'b mut Self: Stream {
    fn map<F>(self, func: F) -> Map<Self, F>
    where Self: Sized,
    for<'a> &'a mut Map<Self, F>: Stream,
    {
        Map {
            func: func,
            stream: self,
        }
    }

    fn filter<F>(self, func: F) -> Filter<Self, F>
    where Self: Sized,
    for<'a> &'a mut Filter<Self, F>: Stream,
    {
        Filter {
            func: func,
            stream: self,
        }
    }

    fn count(mut self) -> usize
    where Self: Sized,
    {
        let mut count = 0;
        while let Some(_) = self.next() {
            count += 1;
        }
        count
    }
}

impl<T> StreamExt for T where for<'a> &'a mut T: Stream { }

fn main() {
    let source = Repeat(10);
    let map = source.map(|x: &_| x);
    //[migrate]~^ ERROR implementation of `Stream` is not general enough
    //[migrate]~| NOTE  `Stream` would have to be implemented for the type `&'0 mut Map
    //[migrate]~| NOTE  but `Stream` is actually implemented for the type `&'1
    let filter = map.filter(|x: &_| true);
    //[nll]~^ ERROR higher-ranked subtype error
    let count = filter.count(); // Assert that we still have a valid stream.
    //[nll]~^ ERROR higher-ranked subtype error
}