#proc-macro #expand #macro #real-time

app croc-look

a tool to expand macros and watch them in real time

3 releases

0.1.3 Aug 11, 2022
0.1.2 Aug 10, 2022
0.1.0 Aug 10, 2022

#811 in Procedural macros

MIT and LGPL-3.0-or-later

634 lines


croc-look is a tool to make testing and debuging proc macros easier by these two features

  1. Printing the implementation specific generated code to the console
  2. Giving a real time live view of the output of the macro on rust code

croc-look allows you to narrow down your search and also provide real time view of the generated code.


croc-look requires a nightly toolchain installed, it does not need to be the default toolchain

rustup install nightly


cargo install croc-look


  1. --trait-impl or -t: Defines the trait you want to see the expansion off, this is gonna be the trait your derive macro is implementing or it could be any other trait you want to see the expansion off. For example
#[derive(Clone, Debug, MyTrait)]
struct<T> {

So the value for this flag can be either Clone, Debug or whatever trait your MyTrait derive macro is implementing

  1. --impl-for or -i: This helps you narrow down your search for a trait impl for the flag mentioned above. If you have multiple structs deriving your trait then you can do croc-look --trait-impl Clone -i <your-struct-name> and get the impl for the struct you want.

  2. --structure or -s: If you want to expand a particlar struct. This is useful when a macro is manupilating the struct itself, like adding fields, etc.

  3. --path or -p: (requies cargo expand to be installed) Use cargo expand <path> internally to narrow down code to modules. eg croc-look -p cmd -t Clone -i Context (This finds the impl Clone for Context in cmd module)

  4. --function or -f: For expanding a function.

  5. --binary or -b: To expand a cargo --bin BINARY, if not specified then --lib is used

  6. --watch or -w: This starts watching the directory/file you want to watch, this also opens up an interactive TUI which has support for live reloading changes as you do them in your proc-macro project.

How is this different from cargo expand?

cargo expand doesn't allow you to view a whole trait impl to check generics or watch particular code blocks. The motive of croc-look is to narroy arry down your search to a simple single body and reduce cluter.

You can use the --path flag to use cargo expand <path> to narrow down searches module level


Licensed under MIT license.


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