Monday, February 23, 2015

Creating a drop-in replacement for the Rust compiler

Many tools benefit from being a drop-in replacement for a compiler. By this, I mean that any user of the tool can use `mytool` in all the ways they would normally use `rustc` - whether manually compiling a single file or as part of a complex make project or Cargo build, etc. That could be a lot of work; rustc, like most compilers, takes a large number of command line arguments which can affect compilation in complex and interacting ways. Emulating all of this behaviour in your tool is annoying at best, especially if you are making many of the same calls into librustc that the compiler is.

The kind of things I have in mind are tools like rustdoc or a future rustfmt. These want to operate as closely as possible to real compilation, but have totally different outputs (documentation and formatted source code, respectively). Another use case is a customised compiler. Say you want to add a custom code generation phase after macro expansion, then creating a new tool should be easier than forking the compiler (and keeping it up to date as the compiler evolves).

I have gradually been trying to improve the API of librustc to make creating a drop-in tool easier to produce (many others have also helped improve these interfaces over the same time frame). It is now pretty simple to make a tool which is as close to rustc as you want it to be. In this tutorial I'll show how.

Note/warning, everything I talk about in this tutorial is internal API for rustc. It is all extremely unstable and likely to change often and in unpredictable ways. Maintaining a tool which uses these APIs will be non- trivial, although hopefully easier than maintaining one that does similar things without using them.

This tutorial starts with a very high level view of the rustc compilation process and of some of the code that drives compilation. Then I'll describe how that process can be customised. In the final section of the tutorial, I'll go through an example - stupid-stats - which shows how to build a drop-in tool.

Continue reading on GitHub...

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