1 unstable release

0.1.0 Sep 4, 2020

#370 in Procedural macros


648 lines


This crate provides the ability to write procedural macros directly in your code, instead of having to use another crate.

Repo - crates.io - documentation


use inline_proc::inline_proc;

mod example {
        edition: "2018",
        clippy: true,
        dependencies: {
            "quote": "1",
        exports: (
            bang_macros: {
                "def_func": "define_function",

    pub fn define_function(_: proc_macro::TokenStream) -> proc_macro::TokenStream {
            fn macro_function() {
                println!("Hello from a proc macro!");


// => Hello from a proc macro!

How It Works

inline-proc takes your code and puts it in a crate with the path {temporary directory}/inline-proc-crates/{package name}-{significant package version}-{module name}. For example, if you call inline-proc on Linux from the module my_module in my-nice-crate which has version 0.7.3, a temporary crate will be created in /tmp/inline-proc-crates/my-nice-crate-0.7-my_module.

It then compiles this crate as a dylib with Cargo and translates all the outputted errors into errors from the proc macro, so it appears identical to writing the code inline. Note that proc macros cannot currently emit warnings on stable, so you will have to use nightly if you want that.

It outputs macro_rules! macros that expand to invocations of the private inline_proc::invoke_inline_macro! macro. This macro takes in the path of a dylib generated by the inline_proc attribute macro, the name of the macro that is inside that dylib, the type of macro that it is (bang/derive/attribute) and the input to the macro. It opens up the dylib and calls the macro, returning its result.

Using the generated macros

The macros generated by #[inline_proc] can be used directly:

use inline_proc::inline_proc;

mod direct_usage {
        edition: "2018",
        dependencies: {},
        exports: (
            bang_macros: { "my_bang_macro": "my_bang_macro" },
            derives: { "MyDeriveMacro": "my_derive_macro" },
            attributes: { "my_attribute_macro": "my_attribute_macro" },
    use proc_macro::TokenStream;

    pub fn my_bang_macro(_input: TokenStream) -> TokenStream {
    pub fn my_derive_macro(_item: TokenStream) -> TokenStream {
    pub fn my_attribute_macro(_attr: TokenStream, _item: TokenStream) -> TokenStream {

my_bang_macro!(input tokens);
MyDeriveMacro!(struct InnerItem;);
my_attribute_macro!((attribute tokens) struct InnerItem;);

This works fine for bang macros, but is not so good for attribute or derive macros. So this crate provides the attribute and derive macros #[inline_attr] and InlineDerive; they can be used like this:

use inline_proc::{InlineDerive, inline_attr};

struct InnerItem;

#[inline_attr[my_attribute_macro(attribute tokens)]]
struct InnerItem;

They expand to the same code as above.

Exporting the macros

In order to export your macro, you will first have to change your macro definition to: "macro_name": ( function: "macro_function", export: true ). This will do three things:

  1. Label the generated macro_rules! with #[macro_export] and #[doc(hidden)].
  2. Have the macro take a path to inline_proc::invoke_inline_macro.
  3. Suffix the macro's name with _inner.

You then create a wrapper around it like so:

// At the crate root

pub use inline_proc::invoke_inline_macro;

// Where your #[inline_proc] is

/// This macro does XYZ.
macro_rules! my_macro {
    ($($tt:tt)*) => {
        $crate::my_macro_inner!($crate::invoke_inline_macro, $($tt)*);

This level of indirection is necessary as proc macros don't have a way of getting the current crate like MBEs do ($crate), so you have to supply it via the MBE method.


This approach comes with several caveats over regular proc macros:

  • Slower compilation speeds as a second Cargo instance has to be invoked.
  • Not able to use TOML to define dependencies.
  • Exporting macros is a pain.
  • The macros can only be defined in one file.
  • Errors are a lot less helpful. This is improved a bit by Nightly, but still isn't is good as native proc macro errors.
  • Derive helper attributes are not supported. The InlineDerive macro does reserve the helper helper attribute, so you can for example replace #[my_helper] with #[helper[my_helper]].

License: MIT OR Apache-2.0


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