9 releases

0.2.1 Jan 16, 2020
0.2.0 Jan 16, 2020
0.1.6 Oct 8, 2019

#336 in Procedural macros

22 downloads per month

MIT license

152 lines


Docs Crates.io

A small library and custom derive to create a map-like struct that uses match expressions to get and insert values.

If you know your keys at compile-time, this library will likely be faster than HashMap for supported map operations.

Provides the following operations on the wrapping struct (via derive macros):

  • MyMap::get, returns Result<Option<&V>, Error>
  • MyMap::get_mut, returns Result<Option<&mut V>, Error>
  • MyMap::insert, returns Result<Option<V>, Error>, where V is the old value if one exists
  • MyMap::remove, returns Result<Option<V>, Error>
  • MyMap::values, returns an iterator over &Vs

If you know that your operations cannot fail (e.g. if your key type is an enum, and you list all variants as keys), you can add infallible = true to your derive attributes, which will unwrap the result of your map operations.


fn main() {
    pub enum A { A, B, C, D };

    #[derive(Default, fast_map::FastMap)]
    // We know this cannot fail, since we list all the `enum` variants, so we add `infallible = true`
    #[fast_map(infallible = true, keys(A::A, A::B, A::C, A::D))]
    struct Foo(fast_map::Map4<A, String>);

    let mut foo = Foo::default();

    foo.insert(A::B, "B".into());

    assert_eq!(foo.get(A::B), Some(&"B".to_string()));

    assert_eq!(foo.get(A::C), None);

    foo.insert(A::C, "C".into());

    assert_eq!(foo.values().collect::<Vec<_>>().len(), 2);



  • Add the non-erroring operations back as depending on macro attribute (infallible = true). Default is false.


  • Removed easy and strict MapLike traits. It's better to handle unknown keys explicitly, even for gets.
  • Added get_mut operation to the wrapping struct

Current version: 0.2.1

License: MIT


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