The const_mut_refs feature gate allows the following in const context:

• &mut arguments
• mutable (re)borrows and raw pointers (&mut and addr_of_mut! expressions)
• dereferencing mutable references and raw pointers (* expressions)

Remaining work

• Anonymous allocations in statics get duplicated for multiple codegen units #79738 static mut FOO: &'static mut i32 = &mut 42; is problematic, because LLVM currently duplicates the inner mutable reference when you do static mut BAR: &'static mut i32 = FOO;. It would probably suffice to give all nested allocations in a static names relative to their original static item. Named allocations are neither deduplicated nor duplicated.
• Currently the interning of the final result of the const relies on types more than it should, given that transmute is available in consts. Fixed in const-eval interning: get rid of type-driven traversal #119044.
• Mutable reference protected by a mutex in static context is considered UB #120450
• Figure out what to do with regression: encountered mutable pointer in final value when "outer scope" rule applies in const/static with interior mutability #121610: that's happening in const-eval interning: accept interior mutable pointers in final value #128543
• Adjust documentation: const_eval: update for const_mut_refs and const_refs_to_cell stabilization reference#1590
• Stabilization PR: Stabilize &mut (and *mut) as well as &Cell (and *const Cell) in const #129195

Open questions

• can we stabilize this without figuring out the const heap story? -> yes, no reason this has to wait for const_heap
• can we stabilize this without cranelift and codegen_gcc support? ICE: hir: index out of bounds: the len is 5 but the index is 5 #122399, ICE: hir: index out of bounds: the len is 5 but the index is 5 rustc_codegen_gcc#473