[Draft] Supertrait item resolution in subtrait impls

[dingxiangfei2009]

r? @ghost

cc @cramertj @petrochenkov

This is pertaining to implementable trait aliases, Deref/Receiver trait migration and supertrait items in subtrait impls.

This is a draft for the resolver and crate metadata changes. Through this patch, the resolver will pick up supertrait items, collected from the trait super-bounds and trait alias in a future extension. This information will be encoded so that the resolver can resolve names in the downstream crates.

I would like to post this draft earlier because there are already significant design changes, which may warrant collecting opinions ahead of time.

Next steps would be

• teach hir-analysis to collect associated items in subtrait items into implied supertrait impls;
• adapt the syntax so that associated items can be paths instead of just identifies today, because associate items on paths is essential for the disambiguation of the design.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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-    fn get_module_supertraits(self, id: DefIndex, sess: &'a Session) -> impl Iterator<Item = DefId> {
+    fn get_module_supertraits(
+        self,
+        id: DefIndex,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = DefId> {
         let supertraits = self.root.tables.module_supertraits.get(self, id);
         supertraits.decode((self, sess)).into_iter()
     }
fmt: checked 6147 files
Build completed unsuccessfully in 0:00:44
  local time: Sun Jul  6 14:09:05 UTC 2025
  network time: Sun, 06 Jul 2025 14:09:06 GMT

[dingxiangfei2009]

This is not the desired end game, yet. This is rather a demonstration that this patch allows name resolution to associate A with that from trait Sup correctly.

[dingxiangfei2009]

cc @tmandry @tomassedovic

The patch is also to unblock arbitrary_self_types.

The next steps are

• (before landing this patch) gate the supertrait behaviour behind #[rustc_supertrait_in_subtrait_impl]
• Teach hir-analysis to generate an impl for supertraits
• Mark Deref with a builtin attribute #[rustc_supertrait_in_subtrait_impl], move Target from Deref to Receiver
  • Test the set up with crater