Use *fallback* trick for tracked-fn Update constraint

[MichaReiser]

Use the Fallback trick similar to what the derive(Update) does to constrain the tracked-fn's return type to Update.

This removes the need to implement Update for types with a 'static lifetime.

I also added a few Update implementations that are needed for Ruff. The not good news is, Ruff now panics with some assertions and I haven't figured out the root cause yet.

Edit: Okay, the panics go away if I back out of the coarse-grained dependency changes... So something goes wrong there, uff.

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[codspeed-hq]

CodSpeed Performance Report

Merging #680 will not alter performance

_{Comparing MichaReiser:micha/tracked-fn-update-fallback (c48a9df) with master (538eaad)}

Summary

✅ 9 untouched benchmarks

[MichaReiser]

The errors are, unfortunately, not very helpful

[Veykril]

Looking at hte actual diagnostic being emitted (it differs slightly due to having more spans to introspect when testing outside of an expansion I think)

error[E0521]: borrowed data escapes outside of function
  --> tests\tracked_fn_no_eq.rs:55:9
   |
52 | fn assert_update_on_return_type<'db>(old_pointer: *mut &'db u32, new_value: &'db u32) -> ! {
   |                                 ---  ----------- `old_pointer` is a reference that is only valid in the function body
   |                                 |
   |                                 lifetime `'db` defined here
...
55 |         salsa::plumbing::UpdateDispatch::<&u32>::maybe_update(old_pointer, new_value);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |         |
   |         `old_pointer` escapes the function body here
   |         argument requires that `'db` must outlive `'static`
   |
   = note: requirement occurs because of a mutable pointer to `&u32`
   = note: mutable pointers are invariant over their type parameter
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

The triggering span seems to be the maybe_update identifier (and/or the full path span), so to improve the diagnostics here we'd need to pass that path / identifier from the proc-macro to the decl macro with an adjusted span

[MichaReiser]

There are two things that are "bad" about the diagnostic:

• The error isn't very actionable: I have to know that the solution here is to implement the Update trait
• The span point into some non existing code. Ideally, we'd use the span of the return type (possibly?)

[MichaReiser]

Okay, I improved the span. I'm not very familiar with macros so maybe there's a better way of doing this rather than generating the entire function in the proc macro?

Anyway, the error message now looks like this which seems decent

error[E0599]: the function or associated item `maybe_update` exists for struct `UpdateDispatch<ParsedModule>`, but its trait bounds were not satisfied
   --> crates/ruff_db/src/parsed.rs:24:50
    |
24  | pub fn parsed_module(db: &dyn Db, file: File) -> ParsedModule {
    |                                                  ^^^^^^^^^^^^ function or associated item cannot be called on `UpdateDispatch<ParsedModule>` due to unsatisfied trait bounds
...
45  | pub struct ParsedModule {
    | ----------------------- doesn't satisfy `ParsedModule: PartialEq` or `ParsedModule: Update`
    |
   ::: /Users/micha/astral/salsa/src/update.rs:32:5
    |
32  |     pub struct Dispatch<D>(PhantomData<D>);
    |     ---------------------- doesn't satisfy `_: UpdateFallback<ParsedModule>`
    |
note: if you're trying to build a new `UpdateDispatch<ParsedModule>`, consider using `UpdateDispatch::<D>::new` which returns `UpdateDispatch<_>`
   --> /Users/micha/astral/salsa/src/update.rs:41:9
    |
41  |         pub fn new() -> Self {
    |         ^^^^^^^^^^^^^^^^^^^^
    = note: the following trait bounds were not satisfied:
            `ParsedModule: Update`
            `ParsedModule: PartialEq`
            which is required by `UpdateDispatch<ParsedModule>: UpdateFallback<ParsedModule>`
note: the trait `Update` must be implemented
   --> /Users/micha/astral/salsa/src/update.rs:134:1
    |
134 | pub unsafe trait Update {
    | ^^^^^^^^^^^^^^^^^^^^^^^
help: consider annotating `ParsedModule` with `#[derive(PartialEq)]`
    |
45  + #[derive(PartialEq)]
46  | pub struct ParsedModule {
    |

[MichaReiser]

It at least points users in the right direction -> add Partialeq or implement Update

[Veykril]

I wonder if https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute is useful here? (unsure given we go through the fallback trick)

[MichaReiser]

Yeah, that might work. Let me create an issue for this