Mrustc Save

Alternative rust compiler (re-implementation)

Project README

Mutabah's Rust Compiler

In-progress alternative rust compiler. Capable of building a fully-working copy of rustc, but not suitable for everyday use (due to terrible error messages).

Build Status: windows C/C++ CI


This project is a "simple" rust compiler written in C++ that is able to bootstrap a "recent" rustc, but may eventually become a full separate re-implementation.

As mrustc's primary goal is bootstrapping rustc, and as such it tends to assume that the code it's compiling is valid (and any errors in the generated code are mrustc bugs). Code generation is done by emitting a high-level assembly (currently very ugly C, but LLVM/cretone/GIMPLE/... could work) and getting an external tool (i.e. gcc) to do the heavy-lifting of optimising and machine code generation.


  • Builds working copies of rustc and cargo from a release source tarball
    • Supports (and can bootstrap) rustc 1.19.0, 1.29.0, 1.39.0, and 1.54.0
  • Supported Targets:
    • x86-64 linux GNU (fully bootstrap tested using Debian 10.9)
    • x86-64 windows MSVC (runnable executables on Windows 10, but bootstrap hasn't been fully tested)
    • x86_64 and arm64 macOS
    • (incomplete) x86 windows MSVC
  • rustc bootstrap tested and validated (1.19.0 isn't fully repeatable, but later versions are)
    • See the script for how this was done.

Getting Started

You can set the number of jobs (parallel builds) by setting the enviromment variable PARLEVEL, e.g. PARLEVEL=$(nproc) for CPU core count. This can and will break at times, as such outside of toying around, it's best to do PARLEVEL=1 or let mrustc figure it out by itself.


  • C++14-compatible compiler (tested with gcc 5.4 and gcc 6, and MSVC 2015)
  • C11 compatible C compiler (for output, see above)
  • make (for the mrustc makefiles)
  • patch (For doing minor edits to the rustc source)
  • libz-dev (used to reduce size of bytecode files, linux only - windows uses vcpkg to download it)
  • curl (for downloading the rust source, linux only)
  • cmake (at least 3.4.3, required for building llvm in rustc)
  • pkg-config (required for find crate build deps, i.e libssl)
  • python3 (required for building llvm in rustc)

Linux GNU and macOS

  • make RUSTCSRC - Downloads the rustc source tarball (1.29.0 by default)
  • make -f - Builds mrustc and minicargo, then builds libstd, libtest, finally rustc and cargo
  • make -C run_rustc - Build libstd and a "hello, world" using the above-built rustc


Similar to Linux, but you might need to

  • specify the rustc default target explicitly
  • specify the compiler
  • use gmake to run GNU make

e.g. gmake CC=cc RUSTC_TARGET=x86_64-unknown-freebsd -f


(Tested with VS2015)

  • Download and extract rustc-1.29.0-src.tar.gz to the repository root (such that the rustc-1.29.0-src directory is present)
    • NOTE: I am open to suggestions for how to automate that step
  • Open vsproject/mrustc.sln and build minicargo
  • Run vsproject/run_hello.cmd to build libstd and "hello, world", and run it
    • There are other similar scripts for building cargo and rustc. Cargo works, but rustc hasn't fully been tested (building LLVM on windows has been a challenge)

Building non-rustc code

To build your own code with mrustc, first you need to build at least libcore (and probably the full standard library). This can be done on Linux or macOS by running make -f LIBS, or on windows with build_std.cmd.

Next, run

  • minicargo -L <path_to_libstd> <crate_path> to build a cargo project.
  • or, mrustc -L <path_to_libstd> --out-dir <output_directory> <> to directly invoke mrustc.

For additional options, both programs have a --help option.

Diagnosing Issues and Reporting Bugs


Both the makefiles and minicargo write the compiler's stdout to a file in the output directory, e.g. when building output/libcore.hir it'll save to output/libcore.hir_dbg.txt. To get full debug output for a compilation run, set the environment variable MRUSTC_DEBUG to a : separated list of the passes you want to debug (pass names are printed in every log line). E.g. MRUSTC_DEBUG=Expand:Parse make -f

Bug Reports

Please try to include the following when submitting a bug report:

  • What you're trying to build
  • Your host system version (e.g. Ubuntu 17.10)
  • C/C++ compiler version
  • Revision of the mrustc repo that you're running

Support and Discussion

For problems that don't warrant opening an issue (e.g. help in running the compiler), join the IRC channel -

Current Features

  • Full compilation chain including HIR and MIR stages (outputting to C)
  • MIR optimisations (to take some load off the C compiler)
  • Optional exhaustive MIR validation (set the MRUSTC_FULL_VALIDATE environment variable)
  • Functional cargo clone (minicargo)
    • Includes build script support
  • Procedural macros (custom derive)
  • Custom target specifications
    • See docs/



  • Fix currently-failing tests (mostly in type inference)
  • Fix all known TODOs in MIR generation (still some possible leaks)


  • Propagate lifetime annotations so that MIR can include a borrow checker
  • Emit C code that is (more) human readable (uses names from the original source, reduced/no gotos)
  • Add alternate backends (e.g. LLVM IR, cretonne, ...)
  • Working parallel builds (fix races)
Open Source Agenda is not affiliated with "Mrustc" Project. README Source: thepowersgang/mrustc
Open Issues
Last Commit
3 weeks ago

Open Source Agenda Badge

Open Source Agenda Rating