Macro shrink_a_map_field

macro_rules! shrink_a_map_field {
    ($obj:expr, $field:ident, $key_fn:expr) => { ... };
}

Shrinks a mapping field whose key is determined by the value.

For a mapping field, sometimes the key is determined by the value. To keep this constraint in shrinking, one can use shrink_a_map_field. For example,

use rs_quickcheck_util::shrink_a_map_field;
use std::collections::BTreeMap;

#[derive(Debug, Clone)]
struct T {
    m: BTreeMap<String, usize>,
}
let t = T {
    m: [
        ("1".to_string(), 1),
        ("2".to_string(), 2),
        ("3".to_string(), 3),
        ("4".to_string(), 4),
        ("5".to_string(), 5),
        ("6".to_string(), 6),
    ].into(),
};
for x in shrink_a_map_field!(t, m, |x: &usize| -> String {format!("{}", x)}) {
    for (k, v) in x.m.iter() {
        assert_eq!(k.to_string(), format!("{}", v));
    }
}

Then, both size and values of t.m will be shrinked, but the relation between keys and values are kept.