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>

src/bootstrap/bootstrap.py

@@ -894,7 +894,7 @@ def bootstrap(help_triggered):
     build.clean = args.clean
 
     try:
-        toml_path = args.config or 'config.toml'
+        toml_path = os.getenv('RUST_BOOTSTRAP_CONFIG') or args.config or 'config.toml'
         if not os.path.exists(toml_path):
             toml_path = os.path.join(build.rust_root, toml_path)
 
@@ -947,6 +947,7 @@ def bootstrap(help_triggered):
     env["SRC"] = build.rust_root
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
+    env["BOOTSTRAP_CONFIG"] = toml_path
     env["BUILD_DIR"] = build.build_dir
     env["RUSTC_BOOTSTRAP"] = '1'
     env["CARGO"] = build.cargo()

src/bootstrap/flags.rs

@@ -3,7 +3,7 @@
 //! This module implements the command-line parsing of the build system which
 //! has various flags to configure how it's run.
 
-use std::fs;
+use std::env;
 use std::path::PathBuf;
 use std::process;
 
@@ -433,13 +433,7 @@ Arguments:
         // Get any optional paths which occur after the subcommand
         let paths = matches.free[1..].iter().map(|p| p.into()).collect::<Vec<PathBuf>>();
 
-        let cfg_file = matches.opt_str("config").map(PathBuf::from).or_else(|| {
-            if fs::metadata("config.toml").is_ok() {
-                Some(PathBuf::from("config.toml"))
-            } else {
-                None
-            }
-        });
+        let cfg_file = env::var_os("BOOTSTRAP_CONFIG").map(PathBuf::from);
 
         // All subcommands except `clean` can have an optional "Available paths" section
         if matches.opt_present("verbose") {