Tracking issue for treating tuple struct and variant constructors as const fn

[matthewjasper]

The const_constructor feature allows calling any expression of a tuple-like constructor in a const fn:

const fn make_options() {
    Some(0); // OK on stable
    (Option::Some)(1); // OK on stable
    let f = Option::Some;
    f(2); // Requires feature gate
}

cc @oli-obk @Centril

[Centril]

Implemented on 2019-06-07 by @matthewjasper in #61209 which was reviewed by @oli-obk

[mexus]

Is there anything else needs to be done to resolve this issue?

[mati865]

FCP and stabilization pull request.

[Centril]

@matthewjasper Do you think you have time to write a report (e.g. in the style of #61682) and amend the reference after?

[matthewjasper]

Stabilization proposal

I propose that we stabilize #![feature(const_constructor)].

Tracking issue: #61456
Version target: 1.40 (2019-11-05 => beta, 2019-12-19 => stable).

What is stabilized

User guide

Tuple struct and tuple variant constructors are now considered to be constant functions. As such a call expression where the callee has a tuple struct or variant constructor "function item" type can be called :

const fn make_options() {
    // These already work because they are special cased:
    Some(0);
    (Option::Some)(1);
    // These also work now:
    let f = Option::Some;
    f(2);
    {Option::Some}(3);
}

Motivation

Consistency with other const fn.

This should also ensure that constructors implement const Fn traits and can be coerced to const fn function pointers, if they are introduced.

Test

https://github.com/rust-lang/rust/blob/0d75ab2293a106eb674ac01860910cfc1580837e/src/test/ui/consts/const_constructor/const-construct-call.rs - Tests various syntactic forms, use in both const fn and const items, and constructors in both the current and extern crates.

The case in #64247 should also get a test.

[Centril]

The case in #64247 should also get a test.

@matthewjasper Thanks! Could you embed this report in a PR with the aforementioned test added there as well? I'll FCP that PR.