diff --git a/build_system/prepare.rs b/build_system/prepare.rs
index 0302946e654..8cf51db1770 100644
--- a/build_system/prepare.rs
+++ b/build_system/prepare.rs
@@ -6,7 +6,6 @@ use std::process::Command;
 use super::build_sysroot::{BUILD_SYSROOT, ORIG_BUILD_SYSROOT, SYSROOT_RUSTC_VERSION, SYSROOT_SRC};
 use super::path::{Dirs, RelPath};
 use super::rustc_info::{get_default_sysroot, get_rustc_version};
-use super::tests::LIBCORE_TESTS_SRC;
 use super::utils::{
     copy_dir_recursively, git_command, remove_dir_if_exists, retry_spawn_and_wait, spawn_and_wait,
 };
@@ -19,7 +18,6 @@ pub(crate) fn prepare(dirs: &Dirs, rustc: &Path) {
 
     // FIXME do this on the fly?
     prepare_stdlib(dirs, rustc);
-    prepare_coretests(dirs, rustc);
 
     super::tests::RAND_REPO.patch(dirs);
     super::tests::REGEX_REPO.patch(dirs);
@@ -44,19 +42,6 @@ fn prepare_stdlib(dirs: &Dirs, rustc: &Path) {
     fs::write(SYSROOT_RUSTC_VERSION.to_path(dirs), &rustc_version).unwrap();
 }
 
-fn prepare_coretests(dirs: &Dirs, rustc: &Path) {
-    let sysroot_src_orig = get_default_sysroot(rustc).join("lib/rustlib/src/rust");
-    assert!(sysroot_src_orig.exists());
-
-    // FIXME ensure builds error out or update the copy if any of the files copied here change
-    apply_patches(
-        dirs,
-        "coretests",
-        &sysroot_src_orig.join("library/core/tests"),
-        &LIBCORE_TESTS_SRC.to_path(dirs),
-    );
-}
-
 pub(crate) struct GitRepo {
     url: GitRepoUrl,
     rev: &'static str,
@@ -263,7 +248,7 @@ fn get_patches(dirs: &Dirs, crate_name: &str) -> Vec<PathBuf> {
     patches
 }
 
-fn apply_patches(dirs: &Dirs, crate_name: &str, source_dir: &Path, target_dir: &Path) {
+pub(crate) fn apply_patches(dirs: &Dirs, crate_name: &str, source_dir: &Path, target_dir: &Path) {
     // FIXME avoid copy and patch if src, patches and target are unchanged
 
     remove_dir_if_exists(target_dir);
diff --git a/build_system/tests.rs b/build_system/tests.rs
index e60f0a4c612..ec97be2b2d9 100644
--- a/build_system/tests.rs
+++ b/build_system/tests.rs
@@ -1,12 +1,14 @@
 use super::build_sysroot;
 use super::config;
 use super::path::{Dirs, RelPath};
-use super::prepare::GitRepo;
+use super::prepare::{apply_patches, GitRepo};
+use super::rustc_info::get_default_sysroot;
 use super::utils::{spawn_and_wait, spawn_and_wait_with_input, CargoProject, Compiler};
 use super::{CodegenBackend, SysrootKind};
 use std::env;
 use std::ffi::OsStr;
 use std::fs;
+use std::path::PathBuf;
 use std::process::Command;
 
 static BUILD_EXAMPLE_OUT_DIR: RelPath = RelPath::BUILD.join("example");
@@ -125,9 +127,9 @@ pub(crate) static PORTABLE_SIMD_REPO: GitRepo = GitRepo::github(
 pub(crate) static PORTABLE_SIMD: CargoProject =
     CargoProject::new(&PORTABLE_SIMD_REPO.source_dir(), "portable_simd_target");
 
-pub(crate) static LIBCORE_TESTS_SRC: RelPath = RelPath::BUILD.join("coretests_src");
+static LIBCORE_TESTS_SRC: RelPath = RelPath::BUILD.join("coretests_src");
 
-pub(crate) static LIBCORE_TESTS: CargoProject = CargoProject::new(&LIBCORE_TESTS_SRC, "core_tests");
+static LIBCORE_TESTS: CargoProject = CargoProject::new(&LIBCORE_TESTS_SRC, "core_tests");
 
 const EXTENDED_SYSROOT_SUITE: &[TestCase] = &[
     TestCase::custom("test.rust-random/rand", &|runner| {
@@ -145,6 +147,13 @@ const EXTENDED_SYSROOT_SUITE: &[TestCase] = &[
         }
     }),
     TestCase::custom("test.libcore", &|runner| {
+        apply_patches(
+            &runner.dirs,
+            "coretests",
+            &runner.stdlib_source.join("library/core/tests"),
+            &LIBCORE_TESTS_SRC.to_path(&runner.dirs),
+        );
+
         LIBCORE_TESTS.clean(&runner.dirs);
 
         if runner.is_native {
@@ -231,6 +240,10 @@ pub(crate) fn run_tests(
     rustup_toolchain_name: Option<&str>,
     target_triple: String,
 ) {
+    let stdlib_source =
+        get_default_sysroot(&bootstrap_host_compiler.rustc).join("lib/rustlib/src/rust");
+    assert!(stdlib_source.exists());
+
     if config::get_bool("testsuite.no_sysroot") {
         let target_compiler = build_sysroot::build_sysroot(
             dirs,
@@ -247,6 +260,7 @@ pub(crate) fn run_tests(
             target_compiler,
             use_unstable_features,
             bootstrap_host_compiler.triple == target_triple,
+            stdlib_source.clone(),
         );
 
         BUILD_EXAMPLE_OUT_DIR.ensure_fresh(dirs);
@@ -277,6 +291,7 @@ pub(crate) fn run_tests(
             target_compiler,
             use_unstable_features,
             bootstrap_host_compiler.triple == target_triple,
+            stdlib_source,
         );
 
         if run_base_sysroot {
@@ -299,6 +314,7 @@ struct TestRunner {
     use_unstable_features: bool,
     dirs: Dirs,
     target_compiler: Compiler,
+    stdlib_source: PathBuf,
 }
 
 impl TestRunner {
@@ -307,6 +323,7 @@ impl TestRunner {
         mut target_compiler: Compiler,
         use_unstable_features: bool,
         is_native: bool,
+        stdlib_source: PathBuf,
     ) -> Self {
         if let Ok(rustflags) = env::var("RUSTFLAGS") {
             target_compiler.rustflags.push(' ');
@@ -327,7 +344,14 @@ impl TestRunner {
             && target_compiler.triple.contains("x86_64")
             && !target_compiler.triple.contains("windows");
 
-        Self { is_native, jit_supported, use_unstable_features, dirs, target_compiler }
+        Self {
+            is_native,
+            jit_supported,
+            use_unstable_features,
+            dirs,
+            target_compiler,
+            stdlib_source,
+        }
     }
 
     fn run_testsuite(&self, tests: &[TestCase]) {