Tracking issue for calling standard library `const fn`s

[SimonSapin]

The functions below in the standard library are const fn with individual feature gates for calling them in constant expressions. This issue is tracking the stabilization of such calls.

CC #24111, the super-set feature which also includes defining new const fn items.

• std::sync::Once::new
• core::cell::Cell::new
• core::cell::RefCell::new
• core::cell::UnsafeCell::new
• core::sync::AtomicBool::new
• core::sync::AtomicPtr::new
• core::sync::Atomic$IntegerType::new
• core::mem::size_of
• core::mem::align_of
• core::ptr::null
• core::ptr::null_mut
• <$IntegerType>::max_value
• <$IntegerType>::min_value

Items in core:: are also re-exported in std::.

[SimonSapin]

Feature gates are:

• const_once_new
• const_cell_new
• const_refcell_new
• const_unsafe_cell_new
• const_size_of
• const_align_of
• const_ptr_null
• const_ptr_null_mut
• const_min_value
• const_max_value
• const_atomic_bool_new
• const_atomic_ptr_new
• const_atomic_i8_new
• const_atomic_u8_new
• const_atomic_i16_new
• const_atomic_u16_new
• const_atomic_i32_new
• const_atomic_u32_new
• const_atomic_i64_new
• const_atomic_u64_new
• const_atomic_isize_new
• const_atomic_usize_new

[aturon]

Kicking off stabilization discussion for the const-ness of these functions in std.

@rfcbot fcp merge

[rfcbot]

Team member @aturon has proposed to merge this. The next step is review by the rest of the tagged teams:

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[chabapok]

How to about mark std::time::Duration::{from_millis, new, from_secs, from_micros} as const_fn?

[sfackler]

new can panic so it can't be const.

[durka]

@sfackler sure it can, the same way an indexing expression can be const (panic turns into const evaluation error).

[sfackler]

@durka How?

#![feature(const_fn)]

const fn foo(a: i32) {
    if a > 10 {
        panic!();
    }
}

fn main() {}

error[E0019]: constant function contains unimplemented expression type
 --> src/main.rs:4:5
  |
4 | /     if a > 10 {
5 | |         panic!();
6 | |     }
  | |_____^

error: aborting due to previous error

#![feature(const_fn)]

const fn foo(a: i32) {
    panic!();
}

fn main() {}

error[E0015]: calls in constant functions are limited to constant functions, struct and enum constructors
 --> src/main.rs:4:5
  |
4 |     panic!();
  |     ^^^^^^^^^
  |
  = note: this error originates in a macro outside of the current crate

error: aborting due to previous error