Support tracked methods without self

[nikomatsakis]

This is an error today:

#[salsa::tracked]
struct SymTy<'db> {
    ...
}

#[salsa::tracked]
impl<'db> SymTy<'db> {
    #[salsa::tracked]
    pub fn unit(db: &'db dyn Db) -> Self {}
}

...but why? I can still desugar this relatively easily to a regular function calling __unit (see below), so why can't the macro do it?

#[salsa::tracked]
fn __unit(db: &'db dyn Db) -> SymTy<'db> { ... }

[nikomatsakis]

Mentoring instructions

Tracked methods are defined in the proc macro here:

salsa/components/salsa-macros/src/tracked_impl.rs

Lines 33 to 43 in a20b894

	fn try_generate(&self, mut impl_item: syn::ItemImpl) -> syn::Result<TokenStream> {
	let mut member_items = std::mem::take(&mut impl_item.items);
	for member_item in &mut member_items {
	self.modify_member(&impl_item, member_item)?;
	}
	impl_item.items = member_items;
	Ok(crate::debug::dump_tokens(
	format!("impl {:?}", impl_item.self_ty),
	impl_item.into_token_stream(),
	))
	}

It works by modifying tracked methods in place with this method:

salsa/components/salsa-macros/src/tracked_impl.rs

Lines 45 to 53 in a20b894

	#[allow(non_snake_case)]
	fn modify_member(
	&self,
	impl_item: &syn::ItemImpl,
	member_item: &mut syn::ImplItem,
	) -> syn::Result<()> {
	let syn::ImplItem::Fn(fn_item) = member_item else {
	return Ok(());
	};

so that they expand to a call to the setup_method_body! macro-rules macro:

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

Lines 23 to 38 in a20b894

	{
	trait $InnerTrait<$($db_lt)?> {
	fn $inner_fn_name($self, db: $($db_ty)*, $($input_id: $input_ty),*) -> $output_ty;
	}
	impl<$($db_lt)?> $InnerTrait<$($db_lt)?> for $self_ty {
	$inner_fn
	}
	#[$salsa_tracked_attr]
	fn $inner_fn_name<$($db_lt)?>(db: $($db_ty)*, this: $self_ty, $($input_id: $input_ty),*) -> $output_ty {
	<$self_ty as $InnerTrait>::$inner_fn_name(this, db, $($input_id),*)
	}
	$inner_fn_name($db, $self, $($input_id),*)
	}

We need to modify these to be a bit more general.