proc-macro-crate

Providing support for $crate in procedural macros.

• Introduction
• Example
• License

Introduction

In macro_rules! $crate is used to get the path of the crate where a macro is declared in. In procedural macros there is currently no easy way to get this path. A common hack is to import the desired crate with a know name and use this. However, with rust edition 2018 and dropping extern crate declarations from lib.rs, people start to rename crates in Cargo.toml directly. However, this breaks importing the crate, as the proc-macro developer does not know the renamed name of the crate that should be imported.

This crate provides a way to get the name of a crate, even if it renamed in Cargo.toml. For this purpose a single function crate_name is provided. This function needs to be called in the context of a proc-macro with the name of the desired crate. CARGO_MANIFEST_DIR will be used to find the current active Cargo.toml and this Cargo.toml is searched for the desired crate.

Example

use quote::quote;
use syn::Ident;
use proc_macro2::Span;
use proc_macro_crate::{crate_name, FoundCrate};

fn import_my_crate() {
    let found_crate = crate_name("my-crate").expect("my-crate is present in `Cargo.toml`");

    match found_crate {
        FoundCrate::Itself => quote!( crate::Something ),
        FoundCrate::Name(name) => {
            let ident = Ident::new(&name, Span::call_site());
            quote!( #ident::Something )
        }
    };
}

Edge cases

There are multiple edge cases when it comes to determining the correct crate. If you for example import a crate as its own dependency, like this:

[package]
name = "my_crate"

[dev-dependencies]
my_crate = { version = "0.1", features = [ "test-feature" ] }

The crate will return FoundCrate::Itself and you will not be able to find the other instance of your crate in dev-dependencies. Other similar cases are when one crate is imported multiple times:

[package]
name = "my_crate"

[dependencies]
some-crate = { version = "0.5" }
some-crate-old = { package = "some-crate", version = "0.1" }

When searching for some-crate in this Cargo.toml it will return FoundCrate::Name("some_old_crate"), aka the last definition of the crate in the Cargo.toml.

License

Licensed under either of

• Apache License, Version 2.0

• MIT license

at your option.

License: MIT OR Apache-2.0