Move some more bootstrap logic from python to rust

Same rationale as rust-lang#76544;
it would be nice to make python entirely optional at some point.

This also removes $ROOT as an option for the build directory; I haven't been using it, and like Alex
said in rust-lang#76544 (comment) it seems like a
misfeature.

This allows running `cargo run` from src/bootstrap, although that still gives
lots of compile errors if you don't use the beta toolchain.

src/bootstrap/Cargo.toml

@@ -3,6 +3,7 @@ name = "bootstrap"
 version = "0.0.0"
 edition = "2021"
 build = "build.rs"
+default-run = "bootstrap"
 
 [lib]
 path = "lib.rs"

src/bootstrap/bootstrap.py

@@ -1267,7 +1267,7 @@ def bootstrap(help_triggered):
     build.check_vendored_status()
 
     build_dir = build.get_toml('build-dir', 'build') or 'build'
-    build.build_dir = os.path.abspath(build_dir.replace("$ROOT", build.rust_root))
+    build.build_dir = os.path.abspath(build_dir)
 
     with open(os.path.join(build.rust_root, "src", "stage0.json")) as f:
         data = json.load(f)
@@ -1302,7 +1302,6 @@ def bootstrap(help_triggered):
     env = os.environ.copy()
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
-    env["BUILD_DIR"] = build.build_dir
     env["RUSTC_BOOTSTRAP"] = '1'
     if toml_path:
         env["BOOTSTRAP_CONFIG"] = toml_path

src/bootstrap/config.rs

@@ -6,7 +6,6 @@
 use std::cmp;
 use std::collections::{HashMap, HashSet};
 use std::env;
-use std::ffi::OsString;
 use std::fmt;
 use std::fs;
 use std::path::{Path, PathBuf};
@@ -392,7 +391,6 @@ derive_merge! {
         build: Option<String>,
         host: Option<Vec<String>>,
         target: Option<Vec<String>>,
-        // This is ignored, the rust code always gets the build directory from the `BUILD_DIR` env variable
         build_dir: Option<String>,
         cargo: Option<String>,
         rustc: Option<String>,
@@ -588,18 +586,6 @@ derive_merge! {
 }
 
 impl Config {
-    fn path_from_python(var_key: &str) -> PathBuf {
-        match env::var_os(var_key) {
-            Some(var_val) => Self::normalize_python_path(var_val),
-            _ => panic!("expected '{}' to be set", var_key),
-        }
-    }
-
-    /// Normalizes paths from Python slightly. We don't trust paths from Python (#49785).
-    fn normalize_python_path(path: OsString) -> PathBuf {
-        Path::new(&path).components().collect()
-    }
-
     pub fn default_opts() -> Config {
         let mut config = Config::default();
         config.llvm_optimize = true;
@@ -625,7 +611,7 @@ impl Config {
         let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
         // Undo `src/bootstrap`
         config.src = manifest_dir.parent().unwrap().parent().unwrap().to_owned();
-        config.out = Config::path_from_python("BUILD_DIR");
+        config.out = PathBuf::from("build");
 
         config.initial_cargo = PathBuf::from(env!("CARGO"));
         config.initial_rustc = PathBuf::from(env!("RUSTC"));
@@ -655,12 +641,6 @@ impl Config {
         config.llvm_profile_use = flags.llvm_profile_use;
         config.llvm_profile_generate = flags.llvm_profile_generate;
 
-        if config.dry_run {
-            let dir = config.out.join("tmp-dry-run");
-            t!(fs::create_dir_all(&dir));
-            config.out = dir;
-        }
-
         #[cfg(test)]
         let get_toml = |_| TomlConfig::default();
         #[cfg(not(test))]
@@ -695,6 +675,19 @@ impl Config {
 
         let build = toml.build.unwrap_or_default();
 
+        set(&mut config.out, build.build_dir.map(String::into));
+        t!(fs::create_dir_all(&config.out));
+        config.out = t!(
+            config.out.canonicalize(),
+            format!("failed to canonicalize {}", config.out.display())
+        );
+
+        if config.dry_run {
+            let dir = config.out.join("tmp-dry-run");
+            t!(fs::create_dir_all(&dir));
+            config.out = dir;
+        }
+
         config.hosts = if let Some(arg_host) = flags.host {
             arg_host
         } else if let Some(file_host) = build.host {