Rollup merge of #93487 - yerke:yerke/fix-link-toolchain-in-setup, r=Mark-Simulacrum

Fix linking stage1 toolchain in `./x.py setup`

Closes [92319](https://github.com/rust-lang/rust/issues/92319)

Fix linking stage1 toolchain in `./x.py setup`. I guess this can be considered a follow up to https://github.com/rust-lang/rust/pull/89212 by `````@Sl1mb0.`````

We create 2 directories and 1 file that are required by rustup to [link a custom toolchain from path](5225e87a5d/src/toolchain.rs (L479-L497)).

cc `````@jyn514````` and `````@Mark-Simulacrum````` as they were active in https://github.com/rust-lang/rust/issues/89206

src/bootstrap/setup.rs

@@ -1,7 +1,9 @@
 use crate::TargetSelection;
 use crate::{t, VERSION};
+use std::env::consts::EXE_SUFFIX;
 use std::fmt::Write as _;
-use std::path::{Path, PathBuf};
+use std::fs::File;
+use std::path::{Path, PathBuf, MAIN_SEPARATOR};
 use std::process::Command;
 use std::str::FromStr;
 use std::{
@@ -109,7 +111,8 @@ pub fn setup(src_path: &Path, profile: Profile) {
     println!("`x.py` will now use the configuration at {}", include_path.display());
 
     let build = TargetSelection::from_user(&env!("BUILD_TRIPLE"));
-    let stage_path = ["build", build.rustc_target_arg(), "stage1"].join("/");
+    let stage_path =
+        ["build", build.rustc_target_arg(), "stage1"].join(&MAIN_SEPARATOR.to_string());
 
     println!();
 
@@ -171,6 +174,13 @@ fn attempt_toolchain_link(stage_path: &str) {
         return;
     }
 
+    if !ensure_stage1_toolchain_placeholder_exists(stage_path) {
+        println!(
+            "Failed to create a template for stage 1 toolchain or confirm that it already exists"
+        );
+        return;
+    }
+
     if try_link_toolchain(&stage_path[..]) {
         println!(
             "Added `stage1` rustup toolchain; try `cargo +stage1 build` on a separate rust project to run a newly-built toolchain"
@@ -219,6 +229,33 @@ fn try_link_toolchain(stage_path: &str) -> bool {
         .map_or(false, |output| output.status.success())
 }
 
+fn ensure_stage1_toolchain_placeholder_exists(stage_path: &str) -> bool {
+    let pathbuf = PathBuf::from(stage_path);
+
+    if fs::create_dir_all(pathbuf.join("lib")).is_err() {
+        return false;
+    };
+
+    let pathbuf = pathbuf.join("bin");
+    if fs::create_dir_all(&pathbuf).is_err() {
+        return false;
+    };
+
+    let pathbuf = pathbuf.join(format!("rustc{}", EXE_SUFFIX));
+
+    if pathbuf.exists() {
+        return true;
+    }
+
+    // Take care not to overwrite the file
+    let result = File::options().append(true).create(true).open(&pathbuf);
+    if result.is_err() {
+        return false;
+    }
+
+    return true;
+}
+
 // Used to get the path for `Subcommand::Setup`
 pub fn interactive_path() -> io::Result<Profile> {
     fn abbrev_all() -> impl Iterator<Item = ((String, String), Profile)> {