diff --git a/build_system/build_sysroot.rs b/build_system/build_sysroot.rs
index 04cd3894adc..74bba9ed5eb 100644
--- a/build_system/build_sysroot.rs
+++ b/build_system/build_sysroot.rs
@@ -1,9 +1,9 @@
 use std::fs;
 use std::path::{Path, PathBuf};
-use std::process::{self, Command};
+use std::process::Command;
 
 use super::path::{Dirs, RelPath};
-use super::rustc_info::{get_file_name, get_rustc_version};
+use super::rustc_info::get_file_name;
 use super::utils::{
     maybe_incremental, remove_dir_if_exists, spawn_and_wait, try_hard_link, CargoProject, Compiler,
 };
@@ -158,7 +158,6 @@ impl SysrootTarget {
 }
 
 pub(crate) static STDLIB_SRC: RelPath = RelPath::BUILD.join("stdlib");
-pub(crate) static SYSROOT_RUSTC_VERSION: RelPath = STDLIB_SRC.join("rustc_version");
 pub(crate) static STANDARD_LIBRARY: CargoProject =
     CargoProject::new(&STDLIB_SRC.join("library/sysroot"), "stdlib_target");
 pub(crate) static RTSTARTUP_SYSROOT: RelPath = RelPath::BUILD.join("rtstartup");
@@ -222,24 +221,6 @@ fn build_clif_sysroot_for_triple(
     mut compiler: Compiler,
     cg_clif_dylib_path: &CodegenBackend,
 ) -> SysrootTarget {
-    match fs::read_to_string(SYSROOT_RUSTC_VERSION.to_path(dirs)) {
-        Err(e) => {
-            eprintln!("Failed to get rustc version for patched sysroot source: {}", e);
-            eprintln!("Hint: Try `./y.sh prepare` to patch the sysroot source");
-            process::exit(1);
-        }
-        Ok(source_version) => {
-            let rustc_version = get_rustc_version(&compiler.rustc);
-            if source_version != rustc_version {
-                eprintln!("The patched sysroot source is outdated");
-                eprintln!("Source version: {}", source_version.trim());
-                eprintln!("Rustc version:  {}", rustc_version.trim());
-                eprintln!("Hint: Try `./y.sh prepare` to update the patched sysroot source");
-                process::exit(1);
-            }
-        }
-    }
-
     let mut target_libs = SysrootTarget { triple: compiler.triple.clone(), libs: vec![] };
 
     if let Some(rtstartup_target_libs) = build_rtstartup(dirs, &compiler) {
@@ -302,6 +283,10 @@ fn build_clif_sysroot_for_triple(
 }
 
 fn build_rtstartup(dirs: &Dirs, compiler: &Compiler) -> Option<SysrootTarget> {
+    if !super::config::get_bool("keep_sysroot") {
+        super::prepare::prepare_stdlib(dirs, &compiler.rustc);
+    }
+
     if !compiler.triple.ends_with("windows-gnu") {
         return None;
     }
diff --git a/build_system/main.rs b/build_system/main.rs
index 06395eb141c..d51e5027cf3 100644
--- a/build_system/main.rs
+++ b/build_system/main.rs
@@ -126,6 +126,22 @@ fn main() {
         }
     }
 
+    let current_dir = std::env::current_dir().unwrap();
+    out_dir = current_dir.join(out_dir);
+
+    if command == Command::Prepare {
+        prepare::prepare(&path::Dirs {
+            source_dir: current_dir.clone(),
+            download_dir: download_dir
+                .map(|dir| current_dir.join(dir))
+                .unwrap_or_else(|| out_dir.join("download")),
+            build_dir: PathBuf::from("dummy_do_not_use"),
+            dist_dir: PathBuf::from("dummy_do_not_use"),
+            frozen,
+        });
+        process::exit(0);
+    }
+
     let rustup_toolchain_name = match (env::var("CARGO"), env::var("RUSTC"), env::var("RUSTDOC")) {
         (Ok(_), Ok(_), Ok(_)) => None,
         (Err(_), Err(_), Err(_)) => Some(rustc_info::get_toolchain_name()),
@@ -158,8 +174,6 @@ fn main() {
         .unwrap_or_else(|| bootstrap_host_compiler.triple.clone());
 
     // FIXME allow changing the location of these dirs using cli arguments
-    let current_dir = std::env::current_dir().unwrap();
-    out_dir = current_dir.join(out_dir);
     let dirs = path::Dirs {
         source_dir: current_dir.clone(),
         download_dir: download_dir
@@ -181,11 +195,6 @@ fn main() {
         std::fs::File::create(target).unwrap();
     }
 
-    if command == Command::Prepare {
-        prepare::prepare(&dirs, &bootstrap_host_compiler.rustc);
-        process::exit(0);
-    }
-
     env::set_var("RUSTC", "rustc_should_be_set_explicitly");
     env::set_var("RUSTDOC", "rustdoc_should_be_set_explicitly");
 
diff --git a/build_system/prepare.rs b/build_system/prepare.rs
index 748d92614b6..77f7175b786 100644
--- a/build_system/prepare.rs
+++ b/build_system/prepare.rs
@@ -3,24 +3,21 @@ use std::fs;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
-use super::build_sysroot::{STDLIB_SRC, SYSROOT_RUSTC_VERSION};
+use super::build_sysroot::STDLIB_SRC;
 use super::path::{Dirs, RelPath};
-use super::rustc_info::{get_default_sysroot, get_rustc_version};
+use super::rustc_info::get_default_sysroot;
 use super::utils::{
     copy_dir_recursively, git_command, remove_dir_if_exists, retry_spawn_and_wait, spawn_and_wait,
 };
 
-pub(crate) fn prepare(dirs: &Dirs, rustc: &Path) {
+pub(crate) fn prepare(dirs: &Dirs) {
     RelPath::DOWNLOAD.ensure_exists(dirs);
     super::tests::RAND_REPO.fetch(dirs);
     super::tests::REGEX_REPO.fetch(dirs);
     super::tests::PORTABLE_SIMD_REPO.fetch(dirs);
-
-    // FIXME do this on the fly?
-    prepare_stdlib(dirs, rustc);
 }
 
-fn prepare_stdlib(dirs: &Dirs, rustc: &Path) {
+pub(crate) fn prepare_stdlib(dirs: &Dirs, rustc: &Path) {
     let sysroot_src_orig = get_default_sysroot(rustc).join("lib/rustlib/src/rust");
     assert!(sysroot_src_orig.exists());
 
@@ -50,9 +47,6 @@ codegen-units = 10000
 "#,
     )
     .unwrap();
-
-    let rustc_version = get_rustc_version(rustc);
-    fs::write(SYSROOT_RUSTC_VERSION.to_path(dirs), &rustc_version).unwrap();
 }
 
 pub(crate) struct GitRepo {
diff --git a/build_system/rustc_info.rs b/build_system/rustc_info.rs
index 42cec0c6935..5b71504e90a 100644
--- a/build_system/rustc_info.rs
+++ b/build_system/rustc_info.rs
@@ -1,12 +1,6 @@
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 
-pub(crate) fn get_rustc_version(rustc: &Path) -> String {
-    let version_info =
-        Command::new(rustc).stderr(Stdio::inherit()).args(&["-V"]).output().unwrap().stdout;
-    String::from_utf8(version_info).unwrap()
-}
-
 pub(crate) fn get_host_triple(rustc: &Path) -> String {
     let version_info =
         Command::new(rustc).stderr(Stdio::inherit()).args(&["-vV"]).output().unwrap().stdout;