Remarks on perma unstable features

`rustc_private`

Overview

The rustc_private feature allows external crates to use compiler internals.

Using `rustc-private` with Official Toolchains

When using the rustc_private feature with official Rust toolchains distributed via rustup, you need to install two additional components:

rustc-dev: Provides compiler libraries
llvm-tools: Provides LLVM libraries required for linking

Installation Steps

Install both components using rustup:

rustup component add rustc-dev llvm-tools

Common Error

Without the llvm-tools component, you'll encounter linking errors like:

error: linking with `cc` failed: exit status: 1
  |
  = note: rust-lld: error: unable to find library -lLLVM-{version}

Using `rustc-private` with Custom Toolchains

For custom-built toolchains or environments not using rustup, additional configuration is typically required:

Requirements

LLVM libraries must be available in your system's library search paths
The LLVM version must match the one used to build your Rust toolchain

Troubleshooting Steps

Check LLVM installation: Verify LLVM is installed and accessible
Configure library paths: You may need to set environment variables:
```
export LD_LIBRARY_PATH=/path/to/llvm/lib:$LD_LIBRARY_PATH
```
Check version compatibility: Ensure your LLVM version is compatible with your Rust toolchain

Additional Resources

GitHub Issue #137421: Explains that rustc_private linker failures often occur because llvm-tools is not installed