[simongdavies]

This pull request introduces support for selecting and using an explicit guest Rust toolchain (via a rust-toolchain.toml file) when running cargo-hyperlight as a library, allowing the guest build to use a different toolchain than the host. This is required for hyperlight-js to be able to build a guest runtime with a different rust version that the host,

[Copilot]

🔵 Needs a closer look

Cross-platform toolchain switching needs human validation, with unresolved executable-selection and credential-exposure issues.

2 open issues

• Align Cargo executable selection across preparation and final build · New
• Avoid exposing inherited secrets in formatted Cargo commands · New

What changed in this PR

Adds explicit guest Rust toolchain selection for library callers, allowing host and guest builds to use different compilers.

Changes:

• Parses toolchain files and applies selected compiler executables.
• Requires preinstalled guest toolchains and Rust sources.
• Adds documentation, tests, and cross-platform CI coverage.

File	Description
tests/​toolchain_file.rs	Tests selection errors and cross-version builds.
src/​toolchain.rs	Uses effective Cargo for preparation.
src/​sysroot.rs	Uses selected tools and checks Rust sources.
src/​lib.rs	Controls source installation during preparation.
src/​command.rs	Adds the toolchain selection API.
src/​cli.rs	Uses effective Cargo for configuration queries.
src/​cargo_cmd.rs	Parses toolchain files and resolves executables.
README.md	Documents guest toolchain selection.
Cargo.toml	Adds TOML parsing dependency.
Cargo.lock	Locks new dependencies.
.github/​workflows/​ci.yml	Adds Linux and Windows toolchain-selection tests.

_{Review effort: Balanced}

[ludfjig]

lgtm but AI found this, up to you if you want to address

• rust_src_install_command copies the full environment into command overrides. On failure, checked_output prints those values, including registry tokens. Report the program, arguments, status, and stderr without environment values. Add a failing-install test with a dummy inherited token.

• GuestToolchain::from_file uses the parent environment except for RUSTUP_AUTO_INSTALL. A builder-selected RUSTUP_HOME or PATH therefore has no effect on selection. Pass the resolved environment into this helper, use its PATH to find Rustup, and apply it to every Rustup subprocess.

• get_program looks up uppercase CARGO only, so .env("cargo", guest_cargo) selects the wrong executable on Windows. Use env_keys_equal for lookup and replacement. Use get_env_value for the Cargo and RUSTUP_TOOLCHAIN reads in preparation too.

• toolchain_file runs rustup toolchain install even for installed toolchains. This can update moving channels and Rustup itself during a build. Add --no-update and --no-self-update. Set RUSTUP_AUTO_INSTALL=0 before selection in the credential-diagnostic test.