Use *fallback* trick for tracked-fn Update constraint

Cargo.toml

@@ -10,6 +10,7 @@ rust-version = "1.76"
 
 [dependencies]
 arc-swap = "1"
+compact_str = { version = "0.8", optional = true }
 crossbeam = "0.8"
 dashmap = { version = "6", features = ["raw-api"] }
 hashlink = "0.9"

components/salsa-macro-rules/src/setup_tracked_fn.rs

@@ -55,6 +55,8 @@ macro_rules! setup_tracked_fn {
         // True if we `return_ref` flag was given to the function
         return_ref: $return_ref:tt,
 
+        maybe_update_fn: {$($maybe_update_fn:tt)*},
+
         // Annoyingly macro-rules hygiene does not extend to items defined in the macro.
         // We have the procedural macro generate names for those items that are
         // not used elsewhere in the user's code.
@@ -145,6 +147,14 @@ macro_rules! setup_tracked_fn {
                 }
             }
 
+            /// This method isn't used anywhere. It only exitst to enforce the `Self::Output: Update` constraint
+            /// for types that aren't `'static`.
+            ///
+            /// # Safety
+            /// The same safety rules as for `Update` apply.
+            $($maybe_update_fn)*
+
+
             impl $zalsa::function::Configuration for $Configuration {
                 const DEBUG_NAME: &'static str = stringify!($fn_name);
 

components/salsa-macros/src/tracked_fn.rs

@@ -117,6 +117,18 @@ impl Macro {
 
         let return_ref: bool = self.args.return_ref.is_some();
 
+        let maybe_update_fn = quote_spanned! {output_ty.span()=> {
+            #[allow(clippy::all, unsafe_code)]
+            unsafe fn _maybe_update_fn<'db>(old_pointer: *mut #output_ty, new_value: #output_ty) -> bool {
+                unsafe {
+                    use #zalsa::UpdateFallback;
+                    #zalsa::UpdateDispatch::<#output_ty>::maybe_update(
+                        old_pointer, new_value
+                    )
+                }
+            }
+        }};
+
         Ok(crate::debug::dump_tokens(
             fn_name,
             quote![salsa::plumbing::setup_tracked_fn! {
@@ -137,6 +149,7 @@ impl Macro {
                 needs_interner: #needs_interner,
                 lru: #lru,
                 return_ref: #return_ref,
+                maybe_update_fn: { #maybe_update_fn },
                 unused_names: [
                     #zalsa,
                     #Configuration,

components/salsa-macros/src/update.rs

@@ -1,4 +1,5 @@
 use proc_macro2::{Literal, TokenStream};
+use syn::spanned::Spanned;
 use synstructure::BindStyle;
 
 use crate::hygiene::Hygiene;
@@ -34,7 +35,7 @@ pub(crate) fn update_derive(input: syn::DeriveInput) -> syn::Result<TokenStream>
                 .bindings()
                 .iter()
                 .fold(quote!(), |tokens, binding| quote!(#tokens #binding,));
-            let make_new_value = quote! {
+            let make_new_value = quote_spanned! {variant.ast().ident.span()=>
                 let #new_value = if let #variant_pat = #new_value {
                     (#make_tuple)
                 } else {
@@ -46,20 +47,28 @@ pub(crate) fn update_derive(input: syn::DeriveInput) -> syn::Result<TokenStream>
             // For each field, invoke `maybe_update` recursively to update its value.
             // Or the results together (using `|`, not `||`, to avoid shortcircuiting)
             // to get the final return value.
-            let update_fields = variant.bindings().iter().zip(0..).fold(
+            let update_fields = variant.bindings().iter().enumerate().fold(
                 quote!(false),
-                |tokens, (binding, index)| {
+                |tokens, (index, binding)| {
                     let field_ty = &binding.ast().ty;
                     let field_index = Literal::usize_unsuffixed(index);
 
+                    let field_span = binding
+                        .ast()
+                        .ident
+                        .as_ref()
+                        .map(Spanned::span)
+                        .unwrap_or(binding.ast().span());
+
+                    let update_field = quote_spanned! {field_span=>
+                        salsa::plumbing::UpdateDispatch::<#field_ty>::maybe_update(
+                            #binding,
+                            #new_value.#field_index,
+                        )
+                    };
+
                     quote! {
-                        #tokens |
-                            unsafe {
-                                salsa::plumbing::UpdateDispatch::<#field_ty>::maybe_update(
-                                    #binding,
-                                    #new_value.#field_index,
-                                )
-                            }
+                        #tokens | unsafe { #update_field }
                     }
                 },
             );
@@ -77,6 +86,7 @@ pub(crate) fn update_derive(input: syn::DeriveInput) -> syn::Result<TokenStream>
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let tokens = quote! {
         #[allow(clippy::all)]
+        #[automatically_derived]
         unsafe impl #impl_generics salsa::Update for #ident #ty_generics #where_clause {
             unsafe fn maybe_update(#old_pointer: *mut Self, #new_value: Self) -> bool {
                 use ::salsa::plumbing::UpdateFallback as _;

examples/calc/ir.rs

@@ -65,7 +65,7 @@ pub enum ExpressionData<'db> {
     Call(FunctionId<'db>, Vec<Expression<'db>>),
 }
 
-#[derive(Eq, PartialEq, Copy, Clone, Hash, Debug, salsa::Update)]
+#[derive(Eq, PartialEq, Copy, Clone, Hash, Debug)]
 pub enum Op {
     Add,
     Subtract,

src/function.rs

@@ -9,7 +9,7 @@ use crate::{
     salsa_struct::SalsaStructInDb,
     zalsa::{IngredientIndex, MemoIngredientIndex, Zalsa},
     zalsa_local::QueryOrigin,
-    Cycle, Database, Id, Revision, Update,
+    Cycle, Database, Id, Revision,
 };
 
 use self::delete::DeletedEntries;
@@ -43,7 +43,7 @@ pub trait Configuration: Any {
     type Input<'db>: Send + Sync;
 
     /// The value computed by the function.
-    type Output<'db>: fmt::Debug + Send + Sync + Update;
+    type Output<'db>: fmt::Debug + Send + Sync;
 
     /// Determines whether this function can recover from being a participant in a cycle
     /// (and, if so, how).

src/update.rs

@@ -1,6 +1,7 @@
 use std::{
     collections::{BTreeMap, BTreeSet, HashMap, HashSet},
     hash::{BuildHasher, Hash},
+    marker::PhantomData,
     path::PathBuf,
     sync::Arc,
 };
@@ -188,6 +189,29 @@ where
     }
 }
 
+unsafe impl<A> Update for smallvec::SmallVec<A>
+where
+    A: smallvec::Array,
+    A::Item: Update,
+{
+    unsafe fn maybe_update(old_pointer: *mut Self, new_vec: Self) -> bool {
+        let old_vec: &mut smallvec::SmallVec<A> = unsafe { &mut *old_pointer };
+
+        if old_vec.len() != new_vec.len() {
+            old_vec.clear();
+            old_vec.extend(new_vec);
+            return true;
+        }
+
+        let mut changed = false;
+        for (old_element, new_element) in old_vec.iter_mut().zip(new_vec) {
+            changed |= A::Item::maybe_update(old_element, new_element);
+        }
+
+        changed
+    }
+}
+
 macro_rules! maybe_update_set {
     ($old_pointer: expr, $new_set: expr) => {{
         let old_pointer = $old_pointer;
@@ -291,6 +315,26 @@ where
     }
 }
 
+unsafe impl<T> Update for Box<[T]>
+where
+    T: Update,
+{
+    unsafe fn maybe_update(old_pointer: *mut Self, new_box: Self) -> bool {
+        let old_box: &mut Box<[T]> = unsafe { &mut *old_pointer };
+
+        if old_box.len() == new_box.len() {
+            let mut changed = false;
+            for (old_element, new_element) in old_box.iter_mut().zip(new_box) {
+                changed |= T::maybe_update(old_element, new_element);
+            }
+            changed
+        } else {
+            *old_box = new_box;
+            true
+        }
+    }
+}
+
 unsafe impl<T> Update for Arc<T>
 where
     T: Update,
@@ -398,6 +442,9 @@ fallback_impl! {
     PathBuf,
 }
 
+#[cfg(feature = "compact_str")]
+fallback_impl! { compact_str::CompactString, }
+
 macro_rules! tuple_impl {
     ($($t:ident),*; $($u:ident),*) => {
         unsafe impl<$($t),*> Update for ($($t,)*)
@@ -451,3 +498,9 @@ where
         }
     }
 }
+
+unsafe impl<T> Update for PhantomData<T> {
+    unsafe fn maybe_update(_old_pointer: *mut Self, _new_value: Self) -> bool {
+        false
+    }
+}

tests/compile-fail/tracked_fn_return_ref.rs

@@ -1,5 +1,4 @@
 use salsa::Database as Db;
-use salsa::Update;
 
 #[salsa::input]
 struct MyInput {

tests/compile-fail/tracked_fn_return_ref.stderr

@@ -1,42 +1,20 @@
-warning: unused import: `salsa::Update`
- --> tests/compile-fail/tracked_fn_return_ref.rs:2:5
-  |
-2 | use salsa::Update;
-  |     ^^^^^^^^^^^^^
-  |
-  = note: `#[warn(unused_imports)]` on by default
-
-error[E0277]: the trait bound `&'db str: Update` is not satisfied
-  --> tests/compile-fail/tracked_fn_return_ref.rs:16:67
-   |
-16 | fn tracked_fn_return_ref<'db>(db: &'db dyn Db, input: MyInput) -> &'db str {
-   |                                                                   ^^^^^^^^ the trait `Update` is not implemented for `&'db str`
+error: lifetime may not live long enough
+  --> tests/compile-fail/tracked_fn_return_ref.rs:14:1
    |
-   = help: the trait `Update` is implemented for `String`
-note: required by a bound in `salsa::plumbing::function::Configuration::Output`
-  --> src/function.rs
+14 | #[salsa::tracked]
+   | ^^^^^^^^^^^^^^^^^ requires that `'db` must outlive `'static`
+15 | fn tracked_fn_return_ref<'db>(db: &'db dyn Db, input: MyInput) -> &'db str {
+   |                                                                   - lifetime `'db` defined here
    |
-   |     type Output<'db>: fmt::Debug + Send + Sync + Update;
-   |                                                  ^^^^^^ required by this bound in `Configuration::Output`
+   = note: this error originates in the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `ContainsRef<'db>: Update` is not satisfied
-  --> tests/compile-fail/tracked_fn_return_ref.rs:24:6
-   |
-24 | ) -> ContainsRef<'db> {
-   |      ^^^^^^^^^^^^^^^^ the trait `Update` is not implemented for `ContainsRef<'db>`
+error: lifetime may not live long enough
+  --> tests/compile-fail/tracked_fn_return_ref.rs:19:1
    |
-   = help: the following other types implement trait `Update`:
-             ()
-             (A, B)
-             (A, B, C)
-             (A, B, C, D)
-             (A, B, C, D, E)
-             (A, B, C, D, E, F)
-             (A, B, C, D, E, F, G)
-             (A, B, C, D, E, F, G, H)
-           and $N others
-note: required by a bound in `salsa::plumbing::function::Configuration::Output`
-  --> src/function.rs
+19 | #[salsa::tracked]
+   | ^^^^^^^^^^^^^^^^^ requires that `'db` must outlive `'static`
+...
+23 | ) -> ContainsRef<'db> {
+   |      ----------- lifetime `'db` defined here
    |
-   |     type Output<'db>: fmt::Debug + Send + Sync + Update;
-   |                                                  ^^^^^^ required by this bound in `Configuration::Output`
+   = note: this error originates in the attribute macro `salsa::tracked` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/lru.rs

@@ -8,10 +8,10 @@ use std::sync::{
 
 mod common;
 use common::LogDatabase;
-use salsa::{Database as _, Update};
+use salsa::Database as _;
 use test_log::test;
 
-#[derive(Debug, PartialEq, Eq, Update)]
+#[derive(Debug, PartialEq, Eq)]
 struct HotPotato(u32);
 
 thread_local! {

tests/tracked_struct.rs

@@ -0,0 +1,56 @@
+mod common;
+
+use salsa::{Database, Setter};
+
+#[salsa::tracked]
+struct Tracked<'db> {
+    untracked_1: usize,
+
+    untracked_2: usize,
+}
+
+#[salsa::input]
+struct MyInput {
+    field1: usize,
+    field2: usize,
+}
+
+#[salsa::tracked]
+fn intermediate(db: &dyn salsa::Database, input: MyInput) -> Tracked<'_> {
+    Tracked::new(db, input.field1(db), input.field2(db))
+}
+
+#[salsa::tracked]
+fn accumulate(db: &dyn salsa::Database, input: MyInput) -> (usize, usize) {
+    let tracked = intermediate(db, input);
+    let one = read_tracked_1(db, tracked);
+    let two = read_tracked_2(db, tracked);
+
+    (one, two)
+}
+
+#[salsa::tracked]
+fn read_tracked_1<'db>(db: &'db dyn Database, tracked: Tracked<'db>) -> usize {
+    tracked.untracked_1(db)
+}
+
+#[salsa::tracked]
+fn read_tracked_2<'db>(db: &'db dyn Database, tracked: Tracked<'db>) -> usize {
+    tracked.untracked_2(db)
+}
+
+#[test]
+fn execute() {
+    let mut db = salsa::DatabaseImpl::default();
+    let input = MyInput::new(&db, 1, 1);
+
+    assert_eq!(accumulate(&db, input), (1, 1));
+
+    // Should only re-execute `read_tracked_1`.
+    input.set_field1(&mut db).to(2);
+    assert_eq!(accumulate(&db, input), (2, 1));
+
+    // Should only re-execute `read_tracked_2`.
+    input.set_field2(&mut db).to(2);
+    assert_eq!(accumulate(&db, input), (2, 2));
+}

tests/warnings/needless_lifetimes.rs

@@ -1,9 +1,7 @@
-use salsa::Update;
-
 #[salsa::db]
 pub trait Db: salsa::Database {}
 
-#[derive(Debug, PartialEq, Eq, Hash, Update)]
+#[derive(Debug, PartialEq, Eq, Hash)]
 pub struct Item {}
 
 #[salsa::tracked]