Architectural groundwork for expansion-time evaluation

[tomassedovic]

Metadata
Point of contact	@oli-obk
Team(s)	compiler, types
Goal document	2026/expansion-time-evaluation

Summary

Lay the architectural groundwork for functions that can be evaluated during macro expansion. This speculative work focuses on "queryifying" the resolver and implementing a restricted trait solver mode, enabling future language features like macro fn and compiler-integrated interop tools.

Tasks and status

---

Note: we have updated the body to match the 2026 goal. Your original text is preserved below.

Details

Metadata
Point of contact	@tmandry
Team(s)	compiler, types
Goal document	2026/expansion-time-evaluation

Summary

Lay the architectural groundwork for functions that can be evaluated during macro expansion. This speculative work focuses on "queryifying" the resolver and implementing a restricted trait solver mode, enabling future language features like macro fn and compiler-integrated interop tools.

Needs funding: This goal needs funding to proceed.

Tasks and status

[tomassedovic]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[oli-obk]

First update:

I did the groundwork refactorings for avoiding node_id_to_def_id splitting into a per-owner table being a perf regression. Not only did that succeed, but it turned out to be a (small) performance improvement to do those refactorings.

The refactorings in question:

• Merge BuildReducedGraphVisitor into DefPathVisitor rust#154945
• Fewer global node_id_to_def_id lookups rust#156173
• Use fine grained component-wise span tracking in use trees rust#153838
• Mark an unreachable match arm as such rust#153659
• Immediately feed visibility on DefId creation rust#155587
• Ensure we don't feed owners from ast lowering if we ever make that query tracked rust#155586
• Remove most uses of def_id_to_node_id rust#156135
• Some cleanups around per parent disambiguators rust#155730
• Remove a bunch of unnecessary explicit lifetimes from the ast validator rust#155619

After those, the split PR was perf neutral

• Split the node_id_to_def_id table into a per-owner table rust#138995

Which was than fairly easy to follow up with

• Eagerly resolve delegations in late resolution rust#157053
• Some cleanups around passing extra lifetime params from the resolver to ast lowering rust#156960
• Some more per owner things rust#157100
• Some more simple per-owner resolver changes rust#157379

which leaves the remaining fields

• partial_res_map
• extra_lifetime_params_map
• lint_buffer (probably easy)
• delegation_infos
• disambiguators (easy, but may be perf-relevant, we'll see)

extra_lifetime_params_map is somewhat blocked on rust-lang/rust#156508 to finish splitting the resolver's owner-relevant maps into per-owner ones, but once crater finishes on that I'm going to start a T-types FCP. In parallel I can still work on the follow-ups, but probably can't land them due to perf reasons, will try. But def will work on the other fields in parallel