2 files changed, 87 insertions, 2 deletions

diff --git a/crates/test_utils/src/lib.rs b/crates/test_utils/src/lib.rs
index 6489033dd..35a679aea 100644
--- a/crates/test_utils/src/lib.rs
+++ b/crates/test_utils/src/lib.rs
@@ -1,5 +1,10 @@
-use std::fs;
-use std::path::{Path, PathBuf};
+#[macro_use]
+pub mod marks;
+
+use std::{
+    fs,
+    path::{Path, PathBuf}
+};
 
 use text_unit::{TextRange, TextUnit};
 use serde_json::Value;
diff --git a/crates/test_utils/src/marks.rs b/crates/test_utils/src/marks.rs
new file mode 100644
index 000000000..79ffedf69
--- /dev/null
+++ b/crates/test_utils/src/marks.rs
@@ -0,0 +1,80 @@
+//! This module implements manually tracked test coverage, which useful for
+//! quickly finding a test responsible for testing a particular bit of code.
+//!
+//! See https://matklad.github.io/2018/06/18/a-trick-for-test-maintenance.html
+//! for details, but the TL;DR is that you write your test as
+//!
+//! ```no-run
+//! #[test]
+//! fn test_foo() {
+//!     covers!(test_foo);
+//! }
+//! ```
+//!
+//! and in the code under test you write
+//!
+//! ```no-run
+//! fn foo() {
+//!     if some_condition() {
+//!         tested_by!(test_foo);
+//!     }
+//! }
+//! ```
+//!
+//! This module then checks that executing the test indeed covers the specified
+//! function. This is useful if you come back to the `foo` function ten years
+//! later and wonder where the test are: now you can grep for `test_foo`.
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+#[macro_export]
+macro_rules! tested_by {
+    ($ident:ident) => {
+        #[cfg(test)]
+        {
+            // sic! use call-site crate
+            crate::marks::$ident.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! covers {
+    ($ident:ident) => {
+        // sic! use call-site crate
+        let _checker = $crate::marks::MarkChecker::new(&crate::marks::$ident);
+    };
+}
+
+#[macro_export]
+macro_rules! mark {
+    ($ident:ident) => {
+        #[allow(bad_style)]
+        pub(crate) static $ident: std::sync::atomic::AtomicUsize =
+            std::sync::atomic::AtomicUsize::new(0);
+    };
+}
+
+pub struct MarkChecker {
+    mark: &'static AtomicUsize,
+    value_on_entry: usize,
+}
+
+impl MarkChecker {
+    pub fn new(mark: &'static AtomicUsize) -> MarkChecker {
+        let value_on_entry = mark.load(Ordering::SeqCst);
+        MarkChecker {
+            mark,
+            value_on_entry,
+        }
+    }
+}
+
+impl Drop for MarkChecker {
+    fn drop(&mut self) {
+        if std::thread::panicking() {
+            return;
+        }
+        let value_on_exit = self.mark.load(Ordering::SeqCst);
+        assert!(value_on_exit > self.value_on_entry, "mark was not hit")
+    }
+}