diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 30fd2d7815f51..f1a9cec318fde 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -3016,6 +3016,6 @@ impl<T: ?Sized> Debug for SyncUnsafeCell<T> {
     }
 }
 
-// If you expected tests to be here, look instead at the core/tests/fmt.rs file,
+// If you expected tests to be here, look instead at coretests/tests/fmt/;
 // it's a lot easier than creating all of the rt::Piece structures here.
-// There are also tests in the alloc crate, for those that need allocations.
+// There are also tests in alloctests/tests/fmt.rs, for those that need allocations.
diff --git a/library/coretests/tests/str.rs b/library/coretests/tests/str.rs
index f5066343af20a..5e23e910f0aeb 100644
--- a/library/coretests/tests/str.rs
+++ b/library/coretests/tests/str.rs
@@ -1 +1 @@
-// All `str` tests live in library/alloc/tests/str.rs
+// All `str` tests live in library/alloctests/tests/str.rs
diff --git a/src/tools/tidy/src/unit_tests.rs b/src/tools/tidy/src/unit_tests.rs
index 708e4ef808571..90ef36d5882da 100644
--- a/src/tools/tidy/src/unit_tests.rs
+++ b/src/tools/tidy/src/unit_tests.rs
@@ -1,11 +1,13 @@
-//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside `core`.
+//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside
+//! `core` or `alloc`.
 //!
-//! `#![no_core]` libraries cannot be tested directly due to duplicating lang
-//! items. All tests and benchmarks must be written externally in `core/{tests,benches}`.
+//! `core` and `alloc` cannot be tested directly due to duplicating lang items.
+//! All tests and benchmarks must be written externally in
+//! `{coretests,alloctests}/{tests,benches}`.
 //!
-//! Outside of core tests and benchmarks should be outlined into separate files
-//! named `tests.rs` or `benches.rs`, or directories named `tests` or `benches` unconfigured
-//! during normal build.
+//! Outside of `core` and `alloc`, tests and benchmarks should be outlined into
+//! separate files named `tests.rs` or `benches.rs`, or directories named
+//! `tests` or `benches` unconfigured during normal build.
 
 use std::path::Path;
 
@@ -14,40 +16,51 @@ use crate::walk::{filter_dirs, walk};
 pub fn check(root_path: &Path, bad: &mut bool) {
     let core = root_path.join("core");
     let core_copy = core.clone();
-    let core_tests = core.join("tests");
-    let core_benches = core.join("benches");
-    let is_core = move |path: &Path| {
-        path.starts_with(&core)
-            && !(path.starts_with(&core_tests) || path.starts_with(&core_benches))
-    };
+    let is_core = move |path: &Path| path.starts_with(&core);
+    let alloc = root_path.join("alloc");
+    let alloc_copy = alloc.clone();
+    let is_alloc = move |path: &Path| path.starts_with(&alloc);
 
     let skip = move |path: &Path, is_dir| {
         let file_name = path.file_name().unwrap_or_default();
         if is_dir {
             filter_dirs(path)
                 || path.ends_with("src/doc")
-                || (file_name == "tests" || file_name == "benches") && !is_core(path)
+                || (file_name == "tests" || file_name == "benches")
+                    && !is_core(path)
+                    && !is_alloc(path)
         } else {
             let extension = path.extension().unwrap_or_default();
             extension != "rs"
-                || (file_name == "tests.rs" || file_name == "benches.rs") && !is_core(path)
-                // UI tests with different names
-                || path.ends_with("src/thread/local/dynamic_tests.rs")
-                || path.ends_with("src/sync/mpsc/sync_tests.rs")
+                || (file_name == "tests.rs" || file_name == "benches.rs")
+                    && !is_core(path)
+                    && !is_alloc(path)
+                // Tests which use non-public internals and, as such, need to
+                // have the types in the same crate as the tests themselves. See
+                // the comment in alloctests/lib.rs.
+                || path.ends_with("library/alloc/src/collections/btree/borrow/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/map/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/node/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/set/tests.rs")
+                || path.ends_with("library/alloc/src/collections/linked_list/tests.rs")
+                || path.ends_with("library/alloc/src/collections/vec_deque/tests.rs")
+                || path.ends_with("library/alloc/src/raw_vec/tests.rs")
         }
     };
 
     walk(root_path, skip, &mut |entry, contents| {
         let path = entry.path();
         let is_core = path.starts_with(&core_copy);
+        let is_alloc = path.starts_with(&alloc_copy);
         for (i, line) in contents.lines().enumerate() {
             let line = line.trim();
             let is_test = || line.contains("#[test]") && !line.contains("`#[test]");
             let is_bench = || line.contains("#[bench]") && !line.contains("`#[bench]");
             if !line.starts_with("//") && (is_test() || is_bench()) {
                 let explanation = if is_core {
-                    "core unit tests and benchmarks must be placed into \
-                         `core/tests` or `core/benches`"
+                    "`core` unit tests and benchmarks must be placed into `coretests`"
+                } else if is_alloc {
+                    "`alloc` unit tests and benchmarks must be placed into `alloctests`"
                 } else {
                     "unit tests and benchmarks must be placed into \
                          separate files or directories named \