Summary

Remove reflection from the compiler
Remove libdebug
Remove the Poly format trait as well as the :? format specifier

Motivation

In ancient Rust, one of the primary methods of printing a value was via the %? format specifier. This would use reflection at runtime to determine how to print a type. Metadata generated by the compiler (a TyDesc) would be generated to guide the runtime in how to print a type. One of the great parts about reflection was that it was quite easy to print any type. No extra burden was required from the programmer to print something.

There are, however, a number of cons to this approach:

Generating extra metadata for many many types by the compiler can lead to noticeable increases in compile time and binary size.
This form of formatting is inherently not speedy. Widespread usage of %? led to misleading benchmarks about formatting in Rust.
Depending on how metadata is handled, this scheme makes it very difficult to allow recompiling a library without recompiling downstream dependants.

Over time, usage off the ? formatting has fallen out of fashion for the following reasons:

The deriving-based infrastructure was improved greatly and has started seeing much more widespread use, especially for traits like Clone.
The formatting language implementation and syntax has changed. The most common formatter is now {} (an implementation of Show), and it is quite common to see an implementation of Show on nearly all types (frequently via deriving). This form of customizable-per-typformatting largely provides the gap that the original formatting language did not provide, which was limited to only primitives and %?.
Compiler built-ins, such as ~[T] and ~str have been removed from the language, and runtime reflection on Vec<T> and String are far less useful (they just print pointers, not contents).

As a result, the :? formatting specifier is quite rarely used today, and when it is used it’s largely for historical purposes and the output is not of very high quality any more.

The drawbacks and today’s current state of affairs motivate this RFC to recommend removing this infrastructure entirely. It’s possible to add it back in the future with a more modern design reflecting today’s design principles of Rust and the many language changes since the infrastructure was created.

Detailed design

Remove all reflection infrastructure from the compiler. I am not personally super familiar with what exists, but at least these concrete actions will be taken.
- Remove the visit_glue function from TyDesc.
- Remove any form of visit_glue generation.
- (maybe?) Remove the name field of TyDesc.
Remove core::intrinsics::TyVisitor
Remove core::intrinsics::visit_tydesc
Remove libdebug
Remove std::fmt::Poly
Remove the :? format specifier in the formatting language syntax.

Drawbacks

The current infrastructure for reflection, although outdated, represents a significant investment of work in the past which could be a shame to lose. While present in the git history, this infrastructure has been updated over time, and it will no longer receive this attention.

Additionally, given an arbitrary type T, it would now be impossible to print it in literally any situation. Type parameters will now require some bound, such as Show, to allow printing a type.

These two drawbacks are currently not seen as large enough to outweigh the gains from reducing the surface area of the std::fmt API and reduction in maintenance load on the compiler.

Alternatives

The primary alternative to outright removing this infrastructure is to preserve it, but flag it all as #[experimental] or feature-gated. The compiler could require the fmt_poly feature gate to be enabled to enable formatting via :? in a crate. This would mean that any backwards-incompatible changes could continue to be made, and any arbitrary type T could still be printed.

Unresolved questions