bootstrap: read config from $RUST_BOOTSTRAP_CONFIG

This commit modifies bootstrap so that `config.toml` is read first from
`RUST_BOOTSTRAP_CONFIG`, then `--config` and finally `config.toml` in the
current directory.

This is a subjective change, intended to improve the ergnomics when
using "development shells" for rustc development (for example, using tools
such as Nix) which set environment variables to ensure a reproducible
environment (these development shells can then be version controlled). By
optionally reading `config.toml` from an environment variable, a `config.toml`
can be defined in the development shell and a path to it exposed in the
`RUST_BOOTSTRAP_CONFIG` environment variable - avoiding the need to manually
symlink the contents of this file to `config.toml` in the working
directory.

Signed-off-by: David Wood <david@davidtw.co>

0 files changed, 0 insertions, 0 deletions