Clear out target directory if compiler has changed

src/bootstrap/builder.rs

@@ -726,7 +726,7 @@ impl<'a> Builder<'a> {
             self.clear_if_dirty(&my_out, &rustdoc);
         }
 
-        cargo.env("CARGO_TARGET_DIR", out_dir).arg(cmd).arg("-Zconfig-profile");
+        cargo.env("CARGO_TARGET_DIR", &out_dir).arg(cmd).arg("-Zconfig-profile");
 
         let profile_var = |name: &str| {
             let profile = if self.config.rust_optimize { "RELEASE" } else { "DEV" };
@@ -865,6 +865,18 @@ impl<'a> Builder<'a> {
         let sysroot = if use_snapshot { self.rustc_snapshot_sysroot() } else { &maybe_sysroot };
         let libdir = self.rustc_libdir(compiler);
 
+        // Clear the output directory if the real rustc we're using has changed;
+        // Cargo cannot detect this as it thinks rustc is bootstrap/debug/rustc.
+        //
+        // Avoid doing this during dry run as that usually means the relevant
+        // compiler is not yet linked/copied properly.
+        //
+        // Only clear out the directory if we're compiling std; otherwise, we
+        // should let Cargo take care of things for us (via depdep info)
+        if !self.config.dry_run && mode == Mode::ToolStd && cmd == "build" {
+            self.clear_if_dirty(&out_dir, &self.rustc(compiler));
+        }
+
         // Customize the compiler we're running. Specify the compiler to cargo
         // as our shim and then pass it some various options used to configure
         // how the actual compiler itself is called.

src/bootstrap/compile.rs

@@ -776,7 +776,6 @@ impl Step for Assemble {
         let bindir = sysroot.join("bin");
         t!(fs::create_dir_all(&bindir));
         let compiler = builder.rustc(target_compiler);
-        let _ = fs::remove_file(&compiler);
         builder.copy(&rustc, &compiler);
 
         target_compiler