Tracking Issue for type_info

[oli-obk]

This is a tracking issue for the project goal rust-lang/rust-project-goals#406
The feature gate for the issue is #![feature(type_info)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the goal
• Discuss general reflection with the lang team
• Discuss Tracking issue for non_static_type_id #41875 and not requiring 'static with the lang team
• Write an RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should we use fine grained (many) intrinsics for individual information or keep the one big intrinsic returning an enum of the information?
• what semver implications does all this have and what guarantees of semver do we retain?
• should we even do this at all? Reflection allows causing monomorphization time errors similar to const { assert!(size_of::<T>(), 5) }, but for much more fine-grained details of a generic parameter T.
• Is something lifetime-aware even possible at all?
• Should we use a non-const-eval based scheme (const generics based or even proc macro based?)

Implementation history

• Reflection MVP #146923
• Make Type::of support unsized types #151019
• Support arrays in type reflection #151031
• Support slices in type reflection #151118
• Support pointers in type reflection #151119
• Support primitives in type info reflection #151123
• feat: Support references in reflection type info #151222
• Support trait objects in type info reflection #151239
• Support ADT types in type info reflection #151142
• Reflection TypeId::trait_info_of #152003
• Reflection TypeKind::FnPtr #152173
• Do not require 'static for obtaining reflection information. #152381