macro_state

Currently, Rust does not provide the ability to keep track of any sort of global state between macro calls out of the box.

This crate contains a series of macros that make it trivial to save and load global state (in the form of string keys and values) at compile time and from within proc macros. State that was set at compile-time can also be read directly by runtime code, if needed.

Functionality

The write_state! macro stores state in flat files that live in the target build directory for the current project. This ensures that when you do things like run cargo clean, the current state values are automatically reset as well. In other words, this crate automatically tracks with the build artifacts of whatever is using it.

After compilation, whatever values were present at compile-time are baked into the resulting binary.

Currently, we offer the following macros:

• write_state!("key", "value") - write "value" as the value for the key "key"
• read_state!("key") - returns the value for the key "key", panicking if it can't be found
• init_state!("key", "value") - if the key "key" has a value, returns it, otherwise sets it to "value" and also returns it. This can be used to quickly initialize a key/value pair that may have existing data
• has_state!("key") - returns a boolean indicating whether a value has been stored for the key "key"
• clear_state!("key") - clears any existing state value for key "key", if it exists

Non-macro analogue functions (proc_write_state, proc_read_state, etc) are provided for all of the above macros. Note that these non-macro analogues should only be called from within proc macros. They will not work if you use them outside of proc macro land!

Installation

First add macro_state as a dependency in your Cargo.toml file:

[dependencies]
macro_state = "0.1.5"

Next import the macro:

#[macro_use]
extern crate macro_state;

Usage

Now you can call write_state! and read_state! anywhere in your crate, including inside of proc macros!

write_state!("top of module", "value 1");

#[test]
fn test_write_state() {
    write_state!("top of method", "value 2");
    assert_eq!(read_state!("top of module"), "value 1");
    assert_eq!(read_state!("top of method"), "value 2");
}

After writing a call to write_state, such as write_state!("my key", "my value");, the state you wrote will now be available at the specified key for use by read_state!("my key"); calls further down in your source code. Note that all of this happens at compile-time, so make sure your source code and macro calls are laid out such that your write_state calls will be compiled before your read_state calls.