Skip update fallback if return type has non-static lifetime

components/salsa-macros/src/tracked_fn.rs

@@ -91,6 +91,12 @@ impl Macro {
             ty
         });
         let output_ty = self.output_ty(&db_lt, &item)?;
+        let mut has_lifetime_visitor = HasLifetimeVisitor {
+            has_lifetime: false,
+        };
+        syn::visit::visit_type(&mut has_lifetime_visitor, &output_ty);
+        let has_lifetime = has_lifetime_visitor.has_lifetime;
+
         let (cycle_recovery_fn, cycle_recovery_initial, cycle_recovery_strategy) =
             self.cycle_recovery()?;
         let is_specifiable = self.args.specify.is_some();
@@ -184,11 +190,23 @@ impl Macro {
             UpdateDispatch::<#output_ty>::maybe_update
         };
         let assert_return_type_is_update = if requires_update {
-            quote! {
-                #[allow(clippy::all, warnings)]
-                fn _assert_return_type_is_update<#db_lt>()  {
-                    use #zalsa::{UpdateFallback, UpdateDispatch};
-                    #maybe_update_path;
+            if has_lifetime {
+                // don't import the `UpdateFallback` if we have non-static
+                // lifetimes to avoid confusing errors
+                quote! {
+                    #[allow(clippy::all, warnings)]
+                    fn _assert_return_type_is_update<#db_lt>()  {
+                        fn _assert_is_update<T: #zalsa::Update>() { }
+                        _assert_is_update::<#output_ty>();
+                    }
+                }
+            } else {
+                quote! {
+                    #[allow(clippy::all, warnings)]
+                    fn _assert_return_type_is_update<#db_lt>()  {
+                        use #zalsa::{UpdateFallback, UpdateDispatch};
+                        #maybe_update_path;
+                    }
                 }
             }
         } else {
@@ -307,6 +325,19 @@ impl syn::visit_mut::VisitMut for ToDbLifetimeVisitor {
     }
 }
 
+struct HasLifetimeVisitor {
+    has_lifetime: bool,
+}
+
+impl<'ast> syn::visit::Visit<'ast> for HasLifetimeVisitor {
+    fn visit_lifetime(&mut self, l: &'ast syn::Lifetime) {
+        // We don't consider `'static` to be a lifetime in this context.
+        if l.ident != "static" {
+            self.has_lifetime = true;
+        }
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Hash)]
 enum FunctionType {
     Constant,

src/update.rs

@@ -494,6 +494,7 @@ fallback_impl! { compact_str::CompactString, }
 
 macro_rules! tuple_impl {
     ($($t:ident),*; $($u:ident),*) => {
+        #[diagnostic::do_not_recommend]
         unsafe impl<$($t),*> Update for ($($t,)*)
         where
             $($t: Update,)*

tests/compile-fail/tracked_fn_return_ref.stderr

@@ -29,26 +29,62 @@ warning: elided lifetime has a name
 43 | ) -> ContainsRef<'_> {
    |                  ^^ this elided lifetime gets resolved as `'db`
 
-error: lifetime may not live long enough
+error[E0277]: the trait bound `&'db str: Update` is not satisfied
   --> tests/compile-fail/tracked_fn_return_ref.rs:15:67
    |
 15 | fn tracked_fn_return_ref<'db>(db: &'db dyn Db, input: MyInput) -> &'db str {
-   |                          --- lifetime `'db` defined here          ^ requires that `'db` must outlive `'static`
+   |                                                                   ^^^^^^^^ the trait `Update` is not implemented for `&'db str`
+   |
+   = help: the trait `Update` is implemented for `String`
+note: required by a bound in `<tracked_fn_return_ref::Configuration_ as salsa::function::Configuration>::execute::_assert_return_type_is_update::_assert_is_update`
+  --> tests/compile-fail/tracked_fn_return_ref.rs:14:1
+   |
+14 | #[salsa::tracked]
+   | ^^^^^^^^^^^^^^^^^ required by this bound in `_assert_is_update`
+   = note: this error originates in the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: lifetime may not live long enough
+error[E0277]: the trait bound `ContainsRef<'db>: Update` is not satisfied
   --> tests/compile-fail/tracked_fn_return_ref.rs:23:6
    |
-20 | fn tracked_fn_return_struct_containing_ref<'db>(
-   |                                            --- lifetime `'db` defined here
-...
 23 | ) -> ContainsRef<'db> {
-   |      ^^^^^^^^^^^ requires that `'db` must outlive `'static`
+   |      ^^^^^^^^^^^^^^^^ the trait `Update` is not implemented for `ContainsRef<'db>`
+   |
+   = help: the following other types implement trait `Update`:
+             Arc<T>
+             BTreeMap<K, V>
+             BTreeSet<K>
+             Box<T>
+             Box<[T]>
+             HashMap<K, V, S>
+             HashSet<K, S>
+             Infallible
+           and $N others
+note: required by a bound in `<tracked_fn_return_struct_containing_ref::Configuration_ as salsa::function::Configuration>::execute::_assert_return_type_is_update::_assert_is_update`
+  --> tests/compile-fail/tracked_fn_return_ref.rs:19:1
+   |
+19 | #[salsa::tracked]
+   | ^^^^^^^^^^^^^^^^^ required by this bound in `_assert_is_update`
+   = note: this error originates in the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: lifetime may not live long enough
+error[E0277]: the trait bound `ContainsRef<'db>: Update` is not satisfied
   --> tests/compile-fail/tracked_fn_return_ref.rs:43:6
    |
-40 | fn tracked_fn_return_struct_containing_ref_elided_explicit<'db>(
-   |                                                            --- lifetime `'db` defined here
-...
 43 | ) -> ContainsRef<'_> {
-   |      ^^^^^^^^^^^ requires that `'db` must outlive `'static`
+   |      ^^^^^^^^^^^^^^^ the trait `Update` is not implemented for `ContainsRef<'db>`
+   |
+   = help: the following other types implement trait `Update`:
+             Arc<T>
+             BTreeMap<K, V>
+             BTreeSet<K>
+             Box<T>
+             Box<[T]>
+             HashMap<K, V, S>
+             HashSet<K, S>
+             Infallible
+           and $N others
+note: required by a bound in `<tracked_fn_return_struct_containing_ref_elided_explicit::Configuration_ as salsa::function::Configuration>::execute::_assert_return_type_is_update::_assert_is_update`
+  --> tests/compile-fail/tracked_fn_return_ref.rs:39:1
+   |
+39 | #[salsa::tracked]
+   | ^^^^^^^^^^^^^^^^^ required by this bound in `_assert_is_update`
+   = note: this error originates in the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/tracked_fn_return_static.rs

@@ -0,0 +1,27 @@
+//! Test that a `tracked` function can return a
+//! non-`Update` type, so long as it has `'static` lifetime.
+
+use salsa::Database;
+
+#[salsa::input]
+struct Input {}
+
+#[derive(Clone, PartialEq)]
+struct NotUpdate<'a> {
+    _marker: std::marker::PhantomData<&'a ()>,
+}
+
+#[salsa::tracked]
+fn test(_db: &dyn salsa::Database, _: Input) -> NotUpdate<'static> {
+    NotUpdate {
+        _marker: std::marker::PhantomData,
+    }
+}
+
+#[test]
+fn invoke() {
+    salsa::DatabaseImpl::new().attach(|db| {
+        let input = Input::new(db);
+        test(db, input);
+    })
+}