[wip] Add bootstrap-shim and package it on nightly

- Pass `dist bootstrap-shim` explicitly in CI

 This makes it possible to run `dist bootstrap` without also building
 bootstrap-shim, and more importantly works around a bug where currently
 two steps aren't allowed to have the same path.

- Add `check::BootstrapShim`
- [wip] start unifying parsing for Config and MinimalConfig

src/bootstrap/Cargo.toml

@@ -14,6 +14,11 @@ name = "bootstrap"
 path = "bin/main.rs"
 test = false
 
+[[bin]]
+name = "bootstrap-shim"
+path = "bin/bootstrap-shim.rs"
+test = false
+
 [[bin]]
 name = "rustc"
 path = "bin/rustc.rs"

src/bootstrap/bin/bootstrap-shim.rs

@@ -0,0 +1,29 @@
+use std::{env, process::Command};
+
+use bootstrap::{t, MinimalConfig};
+
+#[path = "../../../src/tools/x/src/main.rs"]
+mod run_python;
+
+fn main() {
+    let args = env::args().skip(1).collect::<Vec<_>>();
+    let mut opts = getopts::Options::new();
+    opts.optopt("", "config", "TOML configuration file for build", "FILE");
+    let matches = t!(opts.parse(args));
+
+    // If there are no untracked changes to bootstrap, download it from CI.
+    // Otherwise, build it from source. Use python to build to avoid duplicating the code between python and rust.
+    let config = MinimalConfig::parse(t!(matches.opt_get("config")));
+    let bootstrap_bin = if let Some(commit) = last_modified_bootstrap_commit(&config) {
+        config.download_bootstrap(&commit)
+    } else {
+        return run_python::main();
+    };
+
+    let args: Vec<_> = std::env::args().skip(1).collect();
+    Command::new(bootstrap_bin).args(args).status().expect("failed to spawn bootstrap binairy");
+}
+
+fn last_modified_bootstrap_commit(config: &MinimalConfig) -> Option<String> {
+    config.last_modified_commit(&["src/bootstrap"], "download-bootstrap", true)
+}

src/bootstrap/builder.rs

@@ -656,7 +656,8 @@ impl<'a> Builder<'a> {
                 check::Rls,
                 check::RustAnalyzer,
                 check::Rustfmt,
-                check::Bootstrap
+                check::Bootstrap,
+                check::BootstrapShim,
             ),
             Kind::Test => describe!(
                 crate::toolstate::ToolStateCheck,
@@ -761,6 +762,7 @@ impl<'a> Builder<'a> {
                 dist::LlvmTools,
                 dist::RustDev,
                 dist::Bootstrap,
+                dist::BootstrapShim,
                 dist::Extended,
                 // It seems that PlainSourceTarball somehow changes how some of the tools
                 // perceive their dependencies (see #93033) which would invalidate fingerprints

src/bootstrap/check.rs

@@ -473,7 +473,9 @@ tool_check_step!(Rls, "src/tools/rls", SourceType::InTree);
 tool_check_step!(Rustfmt, "src/tools/rustfmt", SourceType::InTree);
 tool_check_step!(MiroptTestTools, "src/tools/miropt-test-tools", SourceType::InTree);
 
+// FIXME: currently  these are marked as ToolRustc, but they should be ToolBootstrap instead to avoid having to build the compiler first
 tool_check_step!(Bootstrap, "src/bootstrap", SourceType::InTree, false);
+tool_check_step!(BootstrapShim, "src/bootstrap/bin/bootstrap-shim", SourceType::InTree, false);
 
 /// Cargo's output path for the standard library in a given stage, compiled
 /// by a particular compiler for the specified target.