Mettle

A from-scratch systems language that compiles straight to native x86-64.

Its own backend, linker, and source-level debugger. No LLVM, no VM, no managed runtime.

       

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Mettle compiles .mettle source to native x86-64. On Windows, mettle --build produces a PE executable using a built-in linker. On Linux, it produces ELF and links with the system toolchain.

Features

• Static types, pointers, structs, enums, closures, and defer/errdefer
• Direct calls to C and OS APIs; a bundled stdlib for I/O, memory, math, and more
• Compile-time memory diagnostics: use-after-free, double free, leaks, and dangling pointers caught at compile time, with no annotations. Interprocedural and zero false positives. See borrow checker.
• AVX2 auto-vectorizer that beats gcc -O3 on several kernels, plus @simd / @simd! contracts (vectorize or fail the build)
• --explain: the compiler reports every optimization decision, why loops did or didn't vectorize, and flags regressions since your last build. Suggested fixes are verified by simulation before they're printed. --explain-json for CI.
• Optimization contracts: @inline! and @noalloc fail the build with the compiler's reason if the guarantee can't be met
• Built-in source-level debugger: breakpoints, stepping, live variable read/write via --debug-hooks. No gdb, no PDB, no DWARF.
• GPU offload to NVIDIA: write kernel functions and launch with dispatch K[grid, block](args). Native CUDA/PTX backend, no nvcc. See GPU offload.
• Crash forensics: with -s, faults report what the bad address is (null field access, freed heap block, etc.); --native-heap catches use-after-free at the faulting instruction
• Optional Tracy profiling, runtime timing, and debug stack traces

Windows is the most complete target (internal PE linker, Win32 GUI via std/ui). Linux supports builds, a libc-backed stdlib, and compiler development. See known limitations.

Example

Save as hello.mettle:

import "std/io";

fn fib(n: int32) -> int64 {
  if (n <= 1) { return n; }
  var a: int64 = 0;
  var b: int64 = 1;
  var i: int32 = 2;
  while (i <= n) {
    var next = a + b;
    a = b;
    b = next;
    i = i + 1;
  }
  return b;
}

fn main() -> int32 {
  print("fib(10) = ");
  print_int(fib(10));
  newline();
  return 0;
}

mettle --build hello.mettle -o hello
./hello          # Windows: .\hello.exe

Install

Linux (x86-64)

curl -fsSL https://raw.githubusercontent.com/The-Mettle-Project/Mettle/main/install.sh | sh

Windows (x86-64), PowerShell

irm https://raw.githubusercontent.com/The-Mettle-Project/Mettle/main/install.ps1 | iex

Installs to ~/.mettle (Linux) or %LOCALAPPDATA%\Mettle (Windows) and updates user PATH. No root or admin required. Pin a release with --version v0.13.0 (Linux) or -Version v0.13.0 (Windows).

Build from source

Windows (gcc or clang):

.\build.bat          # default: gcc
.\build.bat clang

Linux / macOS:

make                 # bin/mettle + bundled stdlib/ and runtime/
make install         # optional: /usr/local/bin, stdlib, runtime

Typical release build:

./bin/mettle --build --release hello.mettle -o hello

Common flags: --release / -O, --explain, --debug-hooks, -d / -s / -g, --native-heap. Full list: mettle --help.

Documentation

• Language reference
• Borrow checker
• Control flow
• GPU offload
• Compilation
• Imports
• Runtime model
• Standard library
• C interop
• Known limitations

mettle docs prints paths to these files next to the compiler binary.

Repository layout

src/            compiler (lexer through codegen, linker, diagnostics)
stdlib/         standard library
src/runtime/    optional helper objects (crash traces, atomics, ...)
tests/          regression tests; run_tests.ps1 on Windows
examples/       benchmarks and demos
tools/          ELF tests, benchmarks, fuzz scripts
mettle-syntax/  VS Code / Cursor extension
docs/           language and tooling reference

Examples and benchmarks

Runnable samples live under examples/. Benchmark suites pair Mettle, C, and Rust:

.\tools\benchmark\run-benchmarks.ps1

Development

Windows (primary CI: full test suite):

.\build.bat
.\tests\run_tests.ps1

Linux (native ELF backend):

make -j"$(nproc)"
bash tools/test-elf-native.sh

Editor support

The mettle-syntax extension turns VS Code or Cursor into a full Mettle IDE: debugging on F5, go to definition, rename, completion, an interactive --explain dashboard, and compiler-backed diagnostics. Everything runs against the compiler in your workspace; no separate language server.

License

Apache-2.0. See LICENSE.