Tracking Issue for more complex const parameter types: `feature(adt_const_params)`

[lcnr]

This is a tracking issue for allowing more const parameter types.
The feature gate for this issue is #![feature(adt_const_params)].

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.

Steps

Most of the work here will be coordinated in the #project-const-generics stream on zulip.

• Implement valtrees
• deal with structural equality
• Figure out which types should be allowed as const parameter types and how
• Investigate ICEs around not-wf const arguments (e.g. ICE: expected int of size 8, but got size 1 in rustc_middle/src/ty/consts/int.rs with feature adt_const_params #131052 ICE: InterpErrorInfo(InterpErrorInfoInner { kind: UndefinedBehavior(BoundsCheckFailed #129095 ICE: unable to construct a valtree for the unevaluated constant #128232)
• Ensure no forwards compatibility issues with min_generic_const_args
• Finalize symbol mangling decision for values of arbitrary types (Value mangling #61486 [RFC2603] Extend <const> to include str and structural constants. rfcs#3161)
• Document feature
• Stabilization report with lang FCP explaining all the differences between rfc and current impl

Unresolved Questions

• Is the v0 mangling setup for adt_const_params forwards compatible with unsized_const_params (Tracking Issue for unsized const parameter types: feature(unsized_const_params) #128028)

cc @rust-lang/project-const-generics