`cargo install` breaks: metallib path baked in at compile time points to temp directory

[rgbkrk]

Summary

When a binary using mlx-rs is installed via cargo install <crate>, it fails at runtime with:

MLX error: Failed to load the default metallib. library not found library not found library not found

The issue is that mlx-sys's CMake build defines METAL_PATH as an absolute path to the metallib inside Cargo's temp build directory:

-DMETAL_PATH="/var/folders/.../T/cargo-installXXXX/release/build/mlx-sys-.../mlx.metallib"

This path is compiled into device.cpp.o. When cargo install finishes, only the binary is copied to ~/.cargo/bin/ — the temp build dir (and the metallib in it) is deleted.

MLX's metallib search order

From device.cpp, MLX searches in order:

1. Co-located with the binary (get_binary_directory() / mlx.metallib)
2. Bundle resources
3. SwiftPM bundle
4. Compile-time METAL_PATH (the broken fallback)

Workaround

Copying the metallib next to the installed binary works because of search path #1:

cp target/release/build/mlx-sys-*/out/build/_deps/mlx-build/mlx/backend/metal/kernels/mlx.metallib ~/.cargo/bin/

Building from source (where target/ persists) also works fine because METAL_PATH still points to a valid location.

Potential fix

The build.rs in mlx-sys could copy mlx.metallib to a location that cargo install preserves. However, cargo install only copies the final binary — it doesn't preserve any other build artifacts.

Some ideas:

• Use include_bytes! to embed the metallib in the binary and write it to disk at runtime (it's ~84MB though, so this bloats the binary significantly)
• Set METAL_PATH to a well-known stable location like ~/.local/share/mlx/mlx.metallib and copy it there during build
• Document that downstream crates using mlx-rs should be installed from source

Environment

• macOS 26.3 (arm64, Apple Silicon)
• Rust 1.93.0
• mlx-rs 0.25.3 / mlx-sys 0.2.0

Reproduction

cargo install voice  # any crate that uses mlx-rs
voice "hello"        # fails with metallib error

This affects the voice TTS crate (and likely any binary crate using mlx-rs).