src/test/compile-fail/unboxed-closures-recursive-fn-using-fn-mut.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 #![feature(core, fn_traits, unboxed_closures)]
  12
  13 use std::marker::PhantomData;
  14
  15 // An erroneous variant of `run-pass/unboxed_closures-infer-recursive-fn.rs`
  16 // where we attempt to perform mutation in the recursive function. This fails to compile
  17 // because it winds up requiring `FnMut` which enforces linearity.
  18
  19 struct YCombinator<F,A,R> {
  20     func: F,
  21     marker: PhantomData<(A,R)>,
  22 }
  23
  24 impl<F,A,R> YCombinator<F,A,R> {
  25     fn new(f: F) -> YCombinator<F,A,R> {
  26         YCombinator { func: f, marker: PhantomData }
  27     }
  28 }
  29
  30 impl<A,R,F : FnMut(&mut FnMut(A) -> R, A) -> R> FnMut<(A,)> for YCombinator<F,A,R> {
  31     extern "rust-call" fn call_mut(&mut self, (arg,): (A,)) -> R {
  32         (self.func)(self, arg)
  33             //~^ ERROR cannot borrow `*self` as mutable more than once at a time
  34     }
  35 }
  36
  37 impl<A,R,F : FnMut(&mut FnMut(A) -> R, A) -> R> FnOnce<(A,)> for YCombinator<F,A,R> {
  38     type Output = R;
  39     extern "rust-call" fn call_once(mut self, args: (A,)) -> R {
  40         self.call_mut(args)
  41     }
  42 }
  43
  44 fn main() {
  45     let mut counter = 0;
  46     let factorial = |recur: &mut FnMut(u32) -> u32, arg: u32| -> u32 {
  47         counter += 1;
  48         if arg == 0 {1} else {arg * recur(arg-1)}
  49     };
  50     let mut factorial: YCombinator<_,u32,u32> = YCombinator::new(factorial);
  51     let mut r = factorial(10);
  52     assert_eq!(3628800, r);
  53 }