src/test/ui/issues/issue-52126-assign-op-invariance.rs

   1 // Issue 52126: With respect to variance, the assign-op's like += were
   2 // accidentally lumped together with other binary op's. In both cases
   3 // we were coercing the LHS of the op to the expected supertype.
   4 //
   5 // The problem is that since the LHS of += is modified, we need the
   6 // parameter to be invariant with respect to the overall type, not
   7 // covariant.
   8
   9 use std::collections::HashMap;
  10 use std::ops::AddAssign;
  11
  12 pub fn main() {
  13     panics();
  14 }
  15
  16 pub struct Counter<'l> {
  17     map: HashMap<&'l str, usize>,
  18 }
  19
  20 impl<'l> AddAssign for Counter<'l>
  21 {
  22     fn add_assign(&mut self, rhs: Counter<'l>) {
  23         rhs.map.into_iter().for_each(|(key, val)| {
  24             let count = self.map.entry(key).or_insert(0);
  25             *count += val;
  26         });
  27     }
  28 }
  29
  30 /// Often crashes, if not prints invalid strings.
  31 pub fn panics() {
  32     let mut acc = Counter{map: HashMap::new()};
  33     for line in vec!["123456789".to_string(), "12345678".to_string()] {
  34         let v: Vec<&str> = line.split_whitespace().collect();
  35         //~^ ERROR `line` does not live long enough
  36         // println!("accumulator before add_assign {:?}", acc.map);
  37         let mut map = HashMap::new();
  38         for str_ref in v {
  39             let e = map.entry(str_ref);
  40             println!("entry: {:?}", e);
  41             let count = e.or_insert(0);
  42             *count += 1;
  43         }
  44         let cnt2 = Counter{map};
  45         acc += cnt2;
  46         // println!("accumulator after add_assign {:?}", acc.map);
  47         // line gets dropped here but references are kept in acc.map
  48     }
  49 }