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diff --git a/tests/ui/higher-rank-trait-bounds/issue-30786.rs b/tests/ui/higher-rank-trait-bounds/issue-30786.rs
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+// 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.
+
+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 mapx<F>(self, func: F) -> Map<Self, F>
+ where
+ Self: Sized,
+ for<'a> &'a mut Map<Self, F>: Stream,
+ {
+ Map { func: func, stream: self }
+ }
+
+ fn filterx<F>(self, func: F) -> Filter<Self, F>
+ where
+ Self: Sized,
+ for<'a> &'a mut Filter<Self, F>: Stream,
+ {
+ Filter { func: func, stream: self }
+ }
+
+ fn countx(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 identity<T>(x: &T) -> &T {
+ x
+}
+
+fn variant1() {
+ let source = Repeat(10);
+
+ // Here, the call to `mapx` returns a type `T` to which `StreamExt`
+ // is not applicable, because `for<'b> &'b mut T: Stream`) doesn't hold.
+ //
+ // More concretely, the type `T` is `Map<Repeat, Closure>`, and
+ // the where clause doesn't hold because the signature of the
+ // closure gets inferred to a signature like `|&'_ Stream| -> &'_`
+ // for some specific `'_`, rather than a more generic
+ // signature.
+ //
+ // Why *exactly* we opt for this signature is a bit unclear to me,
+ // we deduce it somehow from a reuqirement that `Map: Stream` I
+ // guess.
+ let map = source.mapx(|x: &_| x);
+ let filter = map.filterx(|x: &_| true);
+ //~^ ERROR the method
+}
+
+fn variant2() {
+ let source = Repeat(10);
+
+ // Here, we use a function, which is not subject to the vagaries
+ // of closure signature inference. In this case, we get the error
+ // on `countx` as, I think, the test originally expected.
+ let map = source.mapx(identity);
+ let filter = map.filterx(|x: &_| true);
+ let count = filter.countx();
+ //~^ ERROR the method
+}
+
+fn main() {}