reflection and comptime

[nikomatsakis]

Metadata
Point of contact	@oli-obk
Team(s)	compiler, lang, libs-api, types
Goal document	2026/reflection-and-comptime

Summary

Finish the implemented reflection scheme based on const fn that can only be called at compile time.
Valdiate it against existing reflection libraries by giving them a nightly feature that obsoletes having derives and makes the derives no-ops.
Obtain T-lang and T-libs-api buy-in for the scheme and write an RFC.
This proposal is solely for producing const eval values, not for putting types back into the type system.
That will be a follow-up once this proposal has a merged MVP.

Tasks and status

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

Details

Metadata
Point of contact	@oli-obk
Team(s)	compiler, lang, libs
Goal document	2025h2/reflection-and-comptime

Summary

Design, implement and experimentally land a reflection scheme based on const fn that can only be called at compile time.
This proposal is solely for producing const eval values, not for putting types back into the type system.
That will be a follow-up once this proposal has a merged MVP.

Tasks and status

• Discussion and moral support (lang )
• Do the work ()

Design language feature to solve problem

• Lang-team experiment (lang, libs )

Implement language feature

• Implementation ()
• Standard reviews (compiler )
• Design meeting (lang )
• Author call for testing blog post

[nikomatsakis]

This issue is intended for status updates only.

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

[oli-obk]

I implemented an initial MVP supporting only tuples and primitives (tho those are just opaque things you can't interact with further), and getting offsets for the tuple fields as well as the size of the tuple: rust-lang/rust#146923

There are two designs of how to expose this from a libs perspective, but after a sync meeting with scottmcm yesterday we came to the conclusion that neither is objectively better at this stage so we're just going to go with the nice end-user UX version for now. For details see the PR description.

Once the MVP lands, I will mentor various interested contributors who will keep adding fields to the Type struct and variants the TypeKind enum.

The next major step is restricting what information you can get from structs outside of the current module or crate. We want to honor visibility, so an initial step would be to just never show private fields, but we want to explore allowing private fields to be shown either just within the current module or via some opt-in marker trait

[nikomatsakis]

Another related PR:

rust-lang/rust#148820

[oli-obk]

Updates

• Add very basic "comptime" fn implementation rust#148820 adds a way to mark functions and intrinsics as only callable during CTFE
• [WIP] Add downcast_trait and downcast_trait_mut rust#144363 has been unblocked and just needs some minor cosmetic work

Blockers

• Reflection MVP rust#146923 (reflection MVP) has not been reviewed yet

[oli-obk]

• Reflection MVP rust#146923 has landed, and we even got the first contribs adding array support to reflection.
  • there are lots more types and type information that we could support, and it's rather easy to add more. Happy to review any work here.
• Add try_as_dyn and try_as_dyn_mut rust#150033 has landed, and I'm working on removing the 'static requirement in Remove 'static requirement on try_as_dyn rust#150161

[oli-obk]

• @BD103 added slices, arrays and raw pointer support
  • Make Type::of support unsized types rust#151019
  • Support arrays in type reflection rust#151031
  • Support slices in type reflection rust#151118
  • Support pointers in type reflection rust#151119
• @SpriteOvO added all of our primitives
  • Support primitives in type info reflection rust#151123
• @clubby789 gave us references
  • feat: Support references in reflection type info rust#151222
• @izagawd made it possible to extract some info from dyn Trait
  • Support trait objects in type info reflection rust#151239

There is ongoing work for Adts and function pointers, both of which will land as MVPs and will need some work to make them respect semver or generally become useful in practice

Removing the 'static bound from try_as_dyn turned out to have many warts, so I'm limiting it to a much smaller subset and will have borrowck emit the 'static requirement if the other rules do not apply (instead of having an unconditional 'static requirement)

[oli-obk]

• I added support for getting reflection information of any type, not just 'static ones
  • Do not require 'static for obtaining reflection information. rust#152381
• @9SonSteroids added a function pointer MVP and trait object support
  • Reflection TypeKind::FnPtr rust#152173
  • Reflection TypeId::trait_info_of rust#152003
• @SpriteOvO added basic struct/enum/union support
  • Support ADT types in type info reflection rust#151142

[oli-obk]

No changes since last time.

I'm writing a document for the lang team meeting on reflection next week