tests/ui/unboxed-closures/unboxed-closures-recursive-fn-using-fn-mut.rs

   1 #![feature(fn_traits, unboxed_closures)]
   2
   3 use std::marker::PhantomData;
   4
   5 // An erroneous variant of `run-pass/unboxed_closures-infer-recursive-fn.rs`
   6 // where we attempt to perform mutation in the recursive function. This fails to compile
   7 // because it winds up requiring `FnMut` which enforces linearity.
   8
   9 struct YCombinator<F,A,R> {
  10     func: F,
  11     marker: PhantomData<(A,R)>,
  12 }
  13
  14 impl<F,A,R> YCombinator<F,A,R> {
  15     fn new(f: F) -> YCombinator<F,A,R> {
  16         YCombinator { func: f, marker: PhantomData }
  17     }
  18 }
  19
  20 impl<A,R,F : FnMut(&mut dyn FnMut(A) -> R, A) -> R> FnMut<(A,)> for YCombinator<F,A,R> {
  21     extern "rust-call" fn call_mut(&mut self, (arg,): (A,)) -> R {
  22         (self.func)(self, arg)
  23             //~^ ERROR cannot borrow `*self` as mutable more than once at a time
  24     }
  25 }
  26
  27 impl<A,R,F : FnMut(&mut dyn FnMut(A) -> R, A) -> R> FnOnce<(A,)> for YCombinator<F,A,R> {
  28     type Output = R;
  29     extern "rust-call" fn call_once(mut self, args: (A,)) -> R {
  30         self.call_mut(args)
  31     }
  32 }
  33
  34 fn main() {
  35     let mut counter = 0;
  36     let factorial = |recur: &mut dyn FnMut(u32) -> u32, arg: u32| -> u32 {
  37         counter += 1;
  38         if arg == 0 {1} else {arg * recur(arg-1)}
  39     };
  40     let mut factorial: YCombinator<_,u32,u32> = YCombinator::new(factorial);
  41     let mut r = factorial(10);
  42     assert_eq!(3628800, r);
  43 }