This is a tracking issue for rust-lang/const-eval#20
The feature gate for the issue is #![feature(const_heap)].
The general design can be found here

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

• Implement Heap allocations in constants const-eval#20 experimentally
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Do we want to allow the regular Global allocator? This would give us post-monomorpization errors. More details can be found in https://hackmd.io/h2O2vkj3RimrBTfm9hvZWA#Transient-heap-allocations-via-Global
• If we stabilize this, we fundamentally change the nature of const fn: before this change, a const fn() -> T would inevitably have to always return the same value; after this change, that is no longer the case. With ptr_guaranteed_eq, this can even to some extend be observed during CTFE. So in that sense, this feature is much bigger than anything we ever stabilized for CTFE.
• We can already emulate all this via build.rs scripts or proc macros that generate the appropriate constants/statics and references them. Const heap will "just" make generating static/const allocations more convenient.
• Once we permit references to things that (can) contain interior mutability, it would be possible for users to write a const fn foo() -> &'static Cell<i32>, which could then be used to initialize a const FOO: &'static Cell<i32> which is unsound (and only caught by dynamic checks). The tracking issue for interior mutability behind references is Tracking Issue for const_refs_to_cell #80384
• const_heap feature can be used to leak mutable memory into final value of constant #129233
• ICEs when making self-referential values: dealing with cyclic (self-referential) values with const_heap #143869
• Whether or not const_make_global can be called twice on the same allocation.

Implementation history

• add const_allocate intrinsic #79594 adds the base intrinsic for telling the miri-engine to create a virtual heap allocation
• move interpret::MemoryKind::Heap to const eval #79664 makes sure that code outside of the const evaluator has no knowledge about heap allocations
• add const_make_global; err for const_allocate ptrs if didn't call #143595