Tracking issue for const fn pointers · Issue #63997 · rust-lang/rust (original) (raw)

Sub-tracking issue for #57563.

This tracks const fn types and calling fn types in const fn.

From the RFC (https://github.com/oli-obk/rfcs/blob/const_generic_const_fn_bounds/text/0000-const-generic-const-fn-bounds.md#const-function-pointers):

const function pointers

const fn foo(f: fn() -> i32) -> i32 { f() }

is illegal before and with this RFC. While we can change the language to allow this feature, two
questions make themselves known:

1. fn pointers in constants
   const F: fn() -> i32 = ...;
   is already legal in Rust today, even though the F doesn't need to be a const function.
2. Opt out bounds might seem unintuitive?
   const fn foo(f: ?const fn() -> i32) -> i32 {
   // not allowed to call f here, because we can't guarantee that it points to a const fn
   }
   const fn foo(f: fn() -> i32) -> i32 {
   f()
   }

Alternatively one can prefix function pointers to const functions with const:

const fn foo(f: const fn() -> i32) -> i32 { f() } const fn bar(f: fn() -> i32) -> i32 { f() // ERROR }

This opens up the curious situation of const function pointers in non-const functions:

fn foo(f: const fn() -> i32) -> i32 { f() }

Which is useless except for ensuring some sense of "purity" of the function pointer ensuring that
subsequent calls will only modify global state if passed in via arguments.