Meta tracking issue for `const fn`

[Lokathor]

This issue tracks the progress of const fn as well as const evaluation more broadly.

This issue is not for discussion about specific extensions to const fn or const evaluation and only exists to provide links to other places that track the progress of specific issues. If you wish to discuss some subject related to const evaluation or const fn, please find an existing appropriate issue below or create an new issue and comment here with a link to the newly created issue.

If it's impossible to detect what issue the programmer should be pointed to, they can simply be pointed to this issue.

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The const related issues currently on deck are as follows:

• Control flow: Tracking issue for RFC 2342, "Allow if and match in constants" #49146 (1.46)
• Loops: Tracking issue for RFC 2344, "Allow loop in constant evaluation" #52000 (1.46)
• Integer arithmetic: Tracking Issue for more const int functions #53718
• Integration with patterns: Tracking issue for const fn integration with pattern matching #57240
• Floating point arithmetic: Tracking issue for #![feature(const_fn_floating_point_arithmetic)] #57241
• Panics: Tracking issue for RFC 2345, "Allow panicking in constants" (const_panic) #51999 (#![feature(const_err)])
• Comparing raw pointers: Tracking issue for comparing raw pointers in constants #53020
• Dereferencing raw pointers: [tracking issue] dereferencing raw pointers inside constants (const_raw_ptr_deref) #51911
• Raw pointer to usize casts: [tracking issue] raw ptr to usize cast inside constants #51910(#![feature(const_raw_ptr_to_usize_cast)])
• Union field accesses: [tracking issue] union field access inside const fn #51909
• &mut T references and borrows: Tracking issue for &mut T in const contexts (const_mut_refs) #57349 (#![feature(const_mut_refs)])
• FromStr: Tracking issue for FromStr trait usage in const fn #59133
• transmute: Tracking Issue for using std::mem::transmute() in const fn (const_fn_transmute) #53605
• Function pointers (const fn(...) / fn(...)): Tracking issue for const fn pointers #63997
• const unsafe? extern fn: Tracking issue for const extern fn and const unsafe extern fn #64926
• unsizing casts: Tracking issue for unsizing casts in const fns #64992
• platform intrinsics: Request: mark all integer platform intrinsics as const #66061
• async blocks in const fns: Tracking Issue for async {} blocks in const expressions #85368
• mem::discriminant: Tracking Issue for const mem::discriminant #69821(#![feature(const_discriminant)])
• ptr offset methods Tracking Issue for making ptr offset methods "const fn" #71499
• Warnings on long running evaluations: [tracking issue] warnings on long running constant evaluations #49980
• Constant propagation not causing errors: Tracking issue for constant prop not causing add'l errors (RFC #1229) #28238
• const_maybe_uninit_assume_init Tracking Issue for const_maybe_uninit_assume_init #86722

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Open RFCs:

• Calling methods on generic parameters of const fns
  • ?const trait bound opt-out: Tracking issue for RFC 2632 ?const trait bound opt-out #67794(#![feature(const_trait_bound_opt_out)])
  • impl const Trait for Ty: Tracking issue for RFC 2632, impl const Trait for Ty and ~const (tilde const) syntax #67792(#![feature(const_trait_impl)])

Planned RFCs:

• Heap allocation in const fn (this includes everything related to Box)

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Completed:

• const constructors: Tracking issue for treating tuple struct and variant constructors as const fn #61456
• locals, assignments, and destructuring: Tracking issue for RFC 2341, Allow locals and destructuring in const fn #48821
• unsafe operations: Tracking issue for unsafe operations in const fn #55607

[varkor]

The rustc_const_unstable attribute should also contain an issue number, so each const feature can redirect to a specific tracking issue, rather than defaulting to #24111, which is now closed (which might be misleading to users looking for an active issue).

As a rough starting point, you can search for occurrences of "rustc_const_unstable" in the compiler to figure out how the attributes are being handled. This will require changing the diagnostics for rustc_const_unstable to point to specific tracking issues. I can provide more detailed mentoring instructions if someone wants to tackle this.