Crate merge_hashmap

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Provides Merge, a trait for objects that can be merged.


trait Merge {
    fn merge(&mut self, other: Self);

The Merge trait can be used to merge two objects of the same type into one. The intended use case is merging configuration from different sources, for example environment variables, multiple configuration files and command-line arguments, see the example.

This crate does not provide any Merge implementations, but Merge can be derived for structs. When deriving the Merge trait for a struct, you can provide custom merge strategies for the fields that don’t implement Merge. A merge strategy is a function with the signature fn merge<T>(left: &mut T, right: T) that merges right into left. The submodules of this crate provide strategies for the most common types, but you can also define your own strategies.


This crate has the following features:

  • derive (default): Enables the derive macro for the Merge trait using the merge_derive crate.
  • num (default): Enables the merge strategies in the num module that require the num_traits crate.
  • std (default): Enables the merge strategies in the hashmap and vec modules that require the standard library. If this feature is not set, merge is a no_std library.


use merge::Merge;

struct User {
    // Fields with the skip attribute are skipped by Merge
    pub name: &'static str,

    // The strategy attribute is used to customize the merge behavior
    #[merge(strategy = merge::option::overwrite_none)]
    pub location: Option<&'static str>,

    #[merge(strategy = merge::vec::append)]
    pub groups: Vec<&'static str>,

let defaults = User {
    name: "",
    location: Some("Internet"),
    groups: vec!["rust"],
let mut ferris = User {
    name: "Ferris",
    location: None,
    groups: vec!["mascot"],

assert_eq!(Some("Internet"), ferris.location);
assert_eq!(vec!["mascot", "rust"], ferris.groups);


Merge strategies for boolean types.
Merge strategies for hash maps.
Merge strategies for numeric types.
Merge strategies for Option
Merge strategies for types that form a total order.
Merge strategies for vectors.


A trait for objects that can be merged.

Derive Macros