Significantly more glorious marks

2 files changed, 80 insertions, 0 deletions

diff --git a/crates/test_utils/src/lib.rs b/crates/test_utils/src/lib.rs
index b1e3c328f..4e05d464f 100644
--- a/crates/test_utils/src/lib.rs
+++ b/crates/test_utils/src/lib.rs
@@ -8,6 +8,8 @@
 
 #[macro_use]
 pub mod marks;
+#[macro_use]
+pub mod mark;
 
 use std::{
     fs,
diff --git a/crates/test_utils/src/mark.rs b/crates/test_utils/src/mark.rs
new file mode 100644
index 000000000..7c309a894
--- /dev/null
+++ b/crates/test_utils/src/mark.rs
@@ -0,0 +1,78 @@
+//! This module implements manually tracked test coverage, which is 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
+//!
+//! ```
+//! #[test]
+//! fn test_foo() {
+//!     mark::check!(test_foo);
+//! }
+//! ```
+//!
+//! and in the code under test you write
+//!
+//! ```
+//! # use test_utils::mark;
+//! # fn some_condition() -> bool { true }
+//! fn foo() {
+//!     if some_condition() {
+//!         mark::hit!(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! _hit {
+    ($ident:ident) => {{
+        #[cfg(test)]
+        {
+            extern "C" {
+                #[no_mangle]
+                static $ident: std::sync::atomic::AtomicUsize;
+            }
+            unsafe {
+                $ident.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+            }
+        }
+    }};
+}
+pub use _hit as hit;
+
+#[macro_export]
+macro_rules! _check {
+    ($ident:ident) => {
+        #[no_mangle]
+        static $ident: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+        let _checker = $crate::mark::MarkChecker::new(&$ident);
+    };
+}
+pub use _check as check;
+
+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")
+    }
+}