This is a tracking issue for the experimental implementation of RFC "Hierarchy of Sized traits" (rust-lang/rfcs#3729).
The feature gate for the issue is #![feature(sized_hierarchy)].

Approved by the language team for experimentation on Zulip.

This feature is part of the "Scalable Vectors" project goal from 2025h1 and 2025h2. It was discussed with the language team in the 2024/11/13 design meeting and the 2025/02/05 design meeting.

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Steps

1. Implement non-const parts of RFC (Sized Hierarchy: Part I #137944)
   • Introduces the MetaSized and PointeeSized traits and adds an implicit MetaSized supertrait to all traits by default
     • PointeeSized is a "fake" trait which is stripped out during lowering
   • There's value in this half of the changes, as it unblocks extern type, just not scalable vectors
   • ?Sized can be rewritten to MetaSized over an edition boundary and prohibit that syntax if desired
2. Add #[rustc_no_implicit_bounds] (add #![rustc_no_implicit_bounds] #142671)
   • Requested by t-types. Useful for type system tests for debugging or simplification.
3. Investigate relaxing Deref::Target

   • As with any associated type, the default Sized bound of Deref::Target cannot be relaxed backwards compatibly:

     trait Deref {
         type Target: ?Sized;
     }
     
     fn needs_metasized<T: ?Sized>() {}
     fn caller<T: Deref>() -> usize {
         needs_metasized::<<T as Deref>::Target>()
         //~^ error! the trait bound `<T as Deref>::Target: MetaSized` is not satisfied 
     }

     This is a known limitation of the proposal and for most use cases of the new sizedness traits is unlikely to be a major issue

     However, Deref is particularly tricky because in the limited relaxations in the standard library performed so far (because of the use of extern type in a couple places), plenty of code that could have been PointeeSized ended up being MetaSized only because of Deref. It seems unlikely enough that much code is relying solely on Deref::Target's ?Sized bound, so attempt to relax this to find out if it will break any code

   • Currently blocked on hir_analysis: add missing sizedness bounds #142712

   • If this is possible, then revert codegen_llvm: avoid Deref impls w/ extern type #137603

4. Implement const parts of the RFC sizedness
   • Introduces const Sized and const MetaSized and is necessary to unblock scalable vectors
   • To maintain backwards compatibility:
     • T: const Sized is the default bound
     • Explicitly written Sized is interpreted as const Sized
       • There is no way to write a non-const Sized bound
       • Sized could be rewritten to const Sized in the next edition and then Sized would mean non-const Sized as expected
     • ?Sized is interpreted as const MetaSized
     • Default supertrait is const MetaSized
   • All default bounds would be their strictest possible, making existing code incompatible with non-const Sized types, that isn't ideal, but those scalable vectors are the only intended use for non-const Sized which are niche and localised, so is tolerable
   • Currently blocked waiting on the next solver
     • The only practical way to implement this feature is to have the sizedness traits always be const behind-the-scenes but without the user being able to refer to their constness
     • This requires an a sufficient const traits implementation in both the old and next solvers, and the old solver's implementation is not sufficient
5. Investigate and implement edition migration
   • This work involves a complicated edition migration, there are two possibilities:
     1. No migration, keep strictest bounds
        • Simplest and most minimal migration leaves the default bounds as the strictest possible bounds (T: const Sized is default, etc.)
        • This isn't ideal but is the default state after a basic implementation of the traits and constness
        • We could stabilise with this if nothing else is possible
     2. Intelligently migrate to T: Sized as the default
        • T: Sized is the default bound in non-const fns, T: (const) Sized is default bound in const fns, remains the default bounds. There is no default supertrait.
        • Automatically add T: const Sized only when the constness is required to maintain backwards compatibility
          • i.e.
            • fn foo<T> { const { size_of::<T>() } } is migrated
            • fn foo<T> { size_of<T>() } is not migrated
          • Similarly for the const MetaSized default supertrait
        • This would be the smoothest migration for users while having optimal bounds in the next edition that allows non-const Sized types to use most existing code
        • This is likely very hard to implement if at all possible
6. Update RFC following experienced gained with experimentation and discuss with t-lang
7. Wait on stabilisation of prerequisites (i.e. const traits)
8. Adjust documentation (see instructions on rustc-dev-guide)
9. 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
10. Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• What names should be used for the traits?

Implementation history

• Sized Hierarchy: Part I #137944
  • Initial implementation of the non-const parts of the Sized Hierarchy RFC
  • Approved to be merged by t-types in FCP at Sized Hierarchy: Part I #137944 (comment)
  • Pre-requisites:
    • tests: use minicore more #137599
    • type_ir: remove redundant part of comment #137600
    • ssa/mono: deduplicate type_has_metadata #137601
    • feature: fix typo in attribute description #137602
    • codegen_llvm: avoid Deref impls w/ extern type #137603
    • trait_sel: resolve vars in host effects #137604
    • add a "future" edition #137606
    • hir_analysis: skip self type of host effect preds in variances_of #137613
    • re-use Sized fast-path #139577
    • trait_sel: deep reject match_normalize_trait_ref #140978
• add #![rustc_no_implicit_bounds] #142671
  • Follow-up requested by t-types to simplify tests that would benefit from not having any default bounds
• hir_analysis: prohibit dyn PointeeSized #143104
  • Fix for ICE: PointeeSized is removed during lowering #142652
• trait_sel: MetaSized always holds temporarily #144016
  • Temporary fix for where bounds regression in beta+nightly #143992 while a proper fix is implemented
• More robustly deal with relaxed bounds and improve their diagnostics #142693
  • Fix for Sized hierarchy: PointeeSized bounds aren't fully validated during HIR ty lowering #142718
• trait_sel: sizedness + auto trait goals prefer alias candidates  #144064
  • Proper fix for trait_sel: MetaSized always holds temporarily #144016, and pre-requisite to hir_analysis: add missing sizedness bounds #142712
• hir_analysis: add missing sizedness bounds #142712
  • Fixes elaboration of item bounds on associated types to include sizedness
  • Necessary to be able to experiment with relaxing Deref::Target

Known issues

• Regression: overflow evaluating X: MetaSized #143830
  • This is known breakage that was accepted by the types team in Sized Hierarchy: Part I #137944 (comment)
  • It is fixed in the next solver