diff --git a/src/tools/tidy/src/lib.rs b/src/tools/tidy/src/lib.rs
index fa2274366401..022ef57503aa 100644
--- a/src/tools/tidy/src/lib.rs
+++ b/src/tools/tidy/src/lib.rs
@@ -51,6 +51,7 @@ pub mod pal;
 pub mod deps;
 pub mod ui_tests;
 pub mod unstable_book;
+pub mod libcoretest;
 
 fn filter_dirs(path: &Path) -> bool {
     let skip = [
diff --git a/src/tools/tidy/src/libcoretest.rs b/src/tools/tidy/src/libcoretest.rs
new file mode 100644
index 000000000000..ef8b55186b10
--- /dev/null
+++ b/src/tools/tidy/src/libcoretest.rs
@@ -0,0 +1,34 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Tidy check to ensure `#[test]` is not used directly inside `libcore`.
+//!
+//! `#![no_core]` libraries cannot be tested directly due to duplicating lang
+//! item. All tests must be written externally in `libcore/tests`.
+
+use std::path::Path;
+use std::fs::read_to_string;
+
+pub fn check(path: &Path, bad: &mut bool) {
+    let libcore_path = path.join("libcore");
+    super::walk(
+        &libcore_path,
+        &mut |subpath| t!(subpath.strip_prefix(&libcore_path)).starts_with("tests"),
+        &mut |subpath| {
+            if t!(read_to_string(subpath)).contains("#[test]") {
+                tidy_error!(
+                    bad,
+                    "{} contains #[test]; libcore tests must be placed inside `src/libcore/tests/`",
+                    subpath.display()
+                );
+            }
+        },
+    );
+}
diff --git a/src/tools/tidy/src/main.rs b/src/tools/tidy/src/main.rs
index 249741927956..7b86650823a4 100644
--- a/src/tools/tidy/src/main.rs
+++ b/src/tools/tidy/src/main.rs
@@ -41,6 +41,7 @@ fn main() {
     features::check(&path, &mut bad, quiet);
     pal::check(&path, &mut bad);
     unstable_book::check(&path, &mut bad);
+    libcoretest::check(&path, &mut bad);
     if !args.iter().any(|s| *s == "--no-vendor") {
         deps::check(&path, &mut bad);
     }