Provide a method to attach a database even if it's different from the current attached one

src/attach.rs

@@ -79,6 +79,38 @@ impl Attached {
         op()
     }
 
+    #[inline]
+    fn attach_allow_change<Db, R>(&self, db: &Db, op: impl FnOnce() -> R) -> R
+    where
+        Db: ?Sized + Database,
+    {
+        struct DbGuard<'s> {
+            state: &'s Attached,
+            prev: Option<NonNull<dyn Database>>,
+        }
+
+        impl<'s> DbGuard<'s> {
+            #[inline]
+            fn new(attached: &'s Attached, db: &dyn Database) -> Self {
+                let prev = attached.database.replace(Some(NonNull::from(db)));
+                Self {
+                    state: attached,
+                    prev,
+                }
+            }
+        }
+
+        impl Drop for DbGuard<'_> {
+            #[inline]
+            fn drop(&mut self) {
+                self.state.database.set(self.prev);
+            }
+        }
+
+        let _guard = DbGuard::new(self, db.as_dyn_database());
+        op()
+    }
+
     /// Access the "attached" database. Returns `None` if no database is attached.
     /// Databases are attached with `attach_database`.
     #[inline]
@@ -104,6 +136,23 @@ where
     )
 }
 
+/// Attach the database to the current thread and execute `op`.
+/// Allows a different database than currently attached. The original database
+/// will be restored on return.
+///
+/// **Note:** Switching databases can cause bugs. If you do not intend to switch
+/// databases, prefer [`attach`] which will panic if you accidentally do.
+#[inline]
+pub fn attach_allow_change<R, Db>(db: &Db, op: impl FnOnce() -> R) -> R
+where
+    Db: ?Sized + Database,
+{
+    ATTACHED.with(
+        #[inline]
+        |a| a.attach_allow_change(db, op),
+    )
+}
+
 /// Access the "attached" database. Returns `None` if no database is attached.
 /// Databases are attached with `attach_database`.
 #[inline]

src/lib.rs

@@ -67,7 +67,7 @@ pub use self::runtime::Runtime;
 pub use self::storage::{Storage, StorageHandle};
 pub use self::update::Update;
 pub use self::zalsa::IngredientIndex;
-pub use crate::attach::{attach, with_attached_database};
+pub use crate::attach::{attach, attach_allow_change, with_attached_database};
 
 pub mod prelude {
     #[cfg(feature = "accumulator")]

src/zalsa.rs

@@ -516,6 +516,9 @@ impl ErasedJar {
     pub const fn erase<I: HasJar>() -> Self {
         Self {
             kind: I::KIND,
+            // This is a false positive of the lint on beta, fixed on nightly.
+            // FIXME: Remove this when nightly stabilizes.
+            #[allow(clippy::incompatible_msrv)]
             type_id: TypeId::of::<I::Jar>,
             type_name: std::any::type_name::<I::Jar>,
             create_ingredients: <I::Jar>::create_ingredients,

tests/compile-fail/incomplete_persistence.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `NotPersistable<'_>: Serialize` is not satisfied
+error[E0277]: the trait bound `NotPersistable<'_>: serde::Serialize` is not satisfied
  --> tests/compile-fail/incomplete_persistence.rs:1:1
   |
 1 | #[salsa::tracked(persist)]
@@ -22,7 +22,7 @@ error[E0277]: the trait bound `NotPersistable<'_>: Serialize` is not satisfied
   = note: required for `(NotPersistable<'_>,)` to implement `Serialize`
   = note: this error originates in the macro `salsa::plumbing::setup_tracked_struct` which comes from the expansion of the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `NotPersistable<'_>: Deserialize<'_>` is not satisfied
+error[E0277]: the trait bound `NotPersistable<'_>: serde::Deserialize<'de>` is not satisfied
  --> tests/compile-fail/incomplete_persistence.rs:1:1
   |
 1 | #[salsa::tracked(persist)]
@@ -43,7 +43,7 @@ error[E0277]: the trait bound `NotPersistable<'_>: Deserialize<'_>` is not satis
   = note: required for `(NotPersistable<'_>,)` to implement `Deserialize<'_>`
   = note: this error originates in the macro `salsa::plumbing::setup_tracked_struct` which comes from the expansion of the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `NotPersistable<'db>: Serialize` is not satisfied
+error[E0277]: the trait bound `NotPersistable<'db>: serde::Serialize` is not satisfied
   --> tests/compile-fail/incomplete_persistence.rs:12:45
    |
 12 | fn query(_db: &dyn salsa::Database, _input: NotPersistable<'_>) {}
@@ -71,7 +71,7 @@ note: required by a bound in `query_input_is_persistable`
    | required by this bound in `query_input_is_persistable`
    = note: this error originates in the macro `salsa::plumbing::setup_tracked_fn` which comes from the expansion of the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `for<'de> NotPersistable<'db>: Deserialize<'de>` is not satisfied
+error[E0277]: the trait bound `NotPersistable<'db>: serde::Deserialize<'de>` is not satisfied
   --> tests/compile-fail/incomplete_persistence.rs:12:45
    |
 12 | fn query(_db: &dyn salsa::Database, _input: NotPersistable<'_>) {}