Add test for durability changes

src/function/backdate.rs

@@ -19,7 +19,7 @@ where
             // Careful: if the value became less durable than it
             // used to be, that is a "breaking change" that our
             // consumers must be aware of. Becoming *more* durable
-            // is not. See the test `constant_to_non_constant`.
+            // is not. See the test `durable_to_less_durable`.
             if revisions.durability >= old_memo.revisions.durability
                 && C::should_backdate_value(old_value, value)
             {

tests/durability.rs

@@ -0,0 +1,47 @@
+//! Tests that code using the builder's durability methods compiles.
+
+use salsa::{Database, Durability, Setter};
+use test_log::test;
+
+#[salsa::input]
+struct N {
+    value: u32,
+}
+
+#[salsa::tracked]
+fn add3(db: &dyn Database, a: N, b: N, c: N) -> u32 {
+    add(db, a, b) + c.value(db)
+}
+
+#[salsa::tracked]
+fn add(db: &dyn Database, a: N, b: N) -> u32 {
+    a.value(db) + b.value(db)
+}
+
+#[test]
+fn durable_to_less_durable() {
+    let mut db = salsa::DatabaseImpl::new();
+
+    let a = N::builder(11).value_durability(Durability::HIGH).new(&db);
+    let b = N::builder(22).value_durability(Durability::HIGH).new(&db);
+    let c = N::builder(33).value_durability(Durability::HIGH).new(&db);
+
+    // Here, `add3` invokes `add(a, b)`, which yields 33.
+    assert_eq!(add3(&db, a, b, c), 66);
+
+    a.set_value(&mut db).with_durability(Durability::LOW).to(11);
+
+    // Here, `add3` invokes `add`, which *still* yields 33, but which
+    // is no longer of high durability. Since value didn't change, we might
+    // preserve `add3` unchanged, not noticing that it is no longer
+    // of high durability.
+
+    assert_eq!(add3(&db, a, b, c), 66);
+
+    // In that case, we would not get the correct result here, when
+    // 'a' changes *again*.
+
+    a.set_value(&mut db).to(22);
+
+    assert_eq!(add3(&db, a, b, c), 77);
+}