135 lines
3.2 KiB
Rust
135 lines
3.2 KiB
Rust
// rust-lang/rust#30786: the use of `for<'b> &'b mut A: Stream<Item=T`
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// should act as assertion that item does not borrow from its stream;
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// but an earlier buggy rustc allowed `.map(|x: &_| x)` which does
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// have such an item.
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//
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// This tests double-checks that we do not allow such behavior to leak
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// through again.
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pub trait Stream {
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type Item;
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fn next(self) -> Option<Self::Item>;
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}
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// Example stream
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pub struct Repeat(u64);
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impl<'a> Stream for &'a mut Repeat {
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type Item = &'a u64;
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fn next(self) -> Option<Self::Item> {
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Some(&self.0)
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}
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}
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pub struct Map<S, F> {
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stream: S,
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func: F,
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}
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impl<'a, A, F, T> Stream for &'a mut Map<A, F>
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where
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&'a mut A: Stream,
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F: FnMut(<&'a mut A as Stream>::Item) -> T,
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{
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type Item = T;
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fn next(self) -> Option<T> {
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match self.stream.next() {
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Some(item) => Some((self.func)(item)),
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None => None,
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}
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}
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}
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pub struct Filter<S, F> {
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stream: S,
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func: F,
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}
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impl<'a, A, F, T> Stream for &'a mut Filter<A, F>
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where
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for<'b> &'b mut A: Stream<Item = T>, // <---- BAD
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F: FnMut(&T) -> bool,
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{
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type Item = <&'a mut A as Stream>::Item;
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fn next(self) -> Option<Self::Item> {
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while let Some(item) = self.stream.next() {
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if (self.func)(&item) {
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return Some(item);
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}
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}
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None
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}
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}
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pub trait StreamExt
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where
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for<'b> &'b mut Self: Stream,
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{
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fn mapx<F>(self, func: F) -> Map<Self, F>
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where
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Self: Sized,
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for<'a> &'a mut Map<Self, F>: Stream,
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{
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Map { func: func, stream: self }
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}
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fn filterx<F>(self, func: F) -> Filter<Self, F>
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where
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Self: Sized,
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for<'a> &'a mut Filter<Self, F>: Stream,
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{
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Filter { func: func, stream: self }
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}
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fn countx(mut self) -> usize
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where
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Self: Sized,
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{
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let mut count = 0;
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while let Some(_) = self.next() {
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count += 1;
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}
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count
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}
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}
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impl<T> StreamExt for T where for<'a> &'a mut T: Stream {}
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fn identity<T>(x: &T) -> &T {
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x
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}
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fn variant1() {
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let source = Repeat(10);
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// Here, the call to `mapx` returns a type `T` to which `StreamExt`
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// is not applicable, because `for<'b> &'b mut T: Stream`) doesn't hold.
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//
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// More concretely, the type `T` is `Map<Repeat, Closure>`, and
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// the where clause doesn't hold because the signature of the
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// closure gets inferred to a signature like `|&'_ Stream| -> &'_`
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// for some specific `'_`, rather than a more generic
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// signature.
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//
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// Why *exactly* we opt for this signature is a bit unclear to me,
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// we deduce it somehow from a reuqirement that `Map: Stream` I
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// guess.
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let map = source.mapx(|x: &_| x);
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let filter = map.filterx(|x: &_| true);
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//~^ ERROR the method
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}
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fn variant2() {
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let source = Repeat(10);
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// Here, we use a function, which is not subject to the vagaries
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// of closure signature inference. In this case, we get the error
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// on `countx` as, I think, the test originally expected.
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let map = source.mapx(identity);
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let filter = map.filterx(|x: &_| true);
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let count = filter.countx();
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//~^ ERROR the method
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}
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fn main() {}
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