Change:
The feature allows calling any expression of a tuple-like constructor in a : cc
Implemented on 2019-06-07 by in which was reviewed by
Is there anything else needs to be done to resolve this issue?
FCP and stabilization pull request.
Do you think you have time to write a report (e.g. in the style of ) and amend the reference after?
I propose that we stabilize . Tracking issue: Version target: 1.40 (2019-11-05 =beta, 2019-12-19 =stable). 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 : Consistency with other . This should also ensure that constructors implement traits and can be coerced to function pointers, if they are introduced. - Tests various syntactic forms, use in both and items, and constructors in both the current and extern crates. The case in should also get a test.
Thanks! Could you embed this report in a PR with the aforementioned test there as well? I'll FCP that PR.

Discussion:
use crate::hir::def_id::DefId; use crate::hir; use crate::ty::TyCtxt; <del> use syntax_pos::symbol::{sym, Symbol}; </del> <ins> use syntax_pos::symbol::Symbol; </ins> use crate::hir::map::blocks::FnLikeNode; use syntax::attr;

self.is_const_fn_raw(def_id) && match self.is_unstable_const_fn(def_id) { Some(feature_name) => { // has a `rustc_const_unstable` attribute, check whether the user enabled the <del> // corresponding feature gate, const_constructor is not a lib feature, so has // to be checked separately. </del> <ins> // corresponding feature gate. </ins> self.features() .declared_lib_features .iter() .any(|&(sym, _)| sym == feature_name) <del> || (feature_name == sym::const_constructor && self.features().const_constructor) </del> }, // functions without const stability are either stable user written // const fn or the user is using feature gates and we thus don't

/// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it pub fn is_unstable_const_fn(self, def_id: DefId) -> Option<Symbol> { <del> if self.is_constructor(def_id) { Some(sym::const_constructor) } else if self.is_const_fn_raw(def_id) { </del> <ins> if self.is_const_fn_raw(def_id) { </ins> self.lookup_stability(def_id)?.const_stability } else { None

(accepted, macros_in_extern, "1.40.0", Some(49476), None), /// Allows future-proofing enums/structs with the `#[non_exhaustive]` attribute (RFC 2008). (accepted, non_exhaustive, "1.40.0", Some(44109), None), <ins> /// Allows calling constructor functions in `const fn`. (accepted, const_constructor, "1.40.0", Some(61456), None), </ins> // ------------------------------------------------------------------------- // feature-group-end: accepted features

/// Allows the user of associated type bounds. (active, associated_type_bounds, "1.34.0", Some(52662), None), <del> /// Allows calling constructor functions in `const fn`. (active, const_constructor, "1.37.0", Some(61456), None), </del> /// Allows `if/while p && let q = r && ...` chains. (active, let_chains, "1.37.0", Some(53667), None),

#![cfg_attr(const_fn, feature(const_fn))] <del> #![feature(const_constructor)] </del> // Ctor(..) is transformed to Ctor { 0: ... } in HAIR lowering, so directly // calling constructors doesn't require them to be const.

<ins> // revisions: min_const_fn const_fn // run-pass #![cfg_attr(const_fn, feature(const_fn))] trait ConstDefault { const DEFAULT: Self; } #[derive(PartialEq)] enum E { V(i32), W(usize), } impl ConstDefault for E { const DEFAULT: Self = Self::V(23); } impl ConstDefault for Option<i32> { const DEFAULT: Self = Self::Some(23); } impl E { const NON_DEFAULT: Self = Self::W(12); const fn local_fn() -> Self { Self::V(23) } } const fn explicit_qpath() -> E { let _x = <Option<usize>>::Some(23); <E>::W(12) } fn main() { assert!(E::DEFAULT == E::local_fn()); assert!(Option::DEFAULT == Some(23)); assert!(E::NON_DEFAULT == explicit_qpath()); } </ins>

<del> error: `std::prelude::v1::Some` is not yet stable as a const fn --> $DIR/feature-gate-const_constructor.rs:9:37 | LL | const EXTERNAL_CONST: Option<i32> = {Some}(1); |                                     ^^^^^^^^^ | = help: add `#![feature(const_constructor)]` to the crate attributes to enable error: `E::V` is not yet stable as a const fn --> $DIR/feature-gate-const_constructor.rs:12:24 | LL | const LOCAL_CONST: E = {E::V}(1); |                        ^^^^^^^^^ | = help: add `#![feature(const_constructor)]` to the crate attributes to enable error: `std::prelude::v1::Some` is not yet stable as a const fn --> $DIR/feature-gate-const_constructor.rs:17:13 | LL |     let _ = {Some}(1); |             ^^^^^^^^^ | = help: add `#![feature(const_constructor)]` to the crate attributes to enable error: `E::V` is not yet stable as a const fn --> $DIR/feature-gate-const_constructor.rs:23:13 | LL |     let _ = {E::V}(1); |             ^^^^^^^^^ | = help: add `#![feature(const_constructor)]` to the crate attributes to enable error: aborting due to 4 previous errors </del>

<del> // revisions: min_const_fn const_fn #![cfg_attr(const_fn, feature(const_fn))] enum E { V(i32), } const EXTERNAL_CONST: Option<i32> = {Some}(1); //[min_const_fn]~^ ERROR is not yet stable as a const fn //[const_fn]~^^ ERROR is not yet stable as a const fn const LOCAL_CONST: E = {E::V}(1); //[min_const_fn]~^ ERROR is not yet stable as a const fn //[const_fn]~^^ ERROR is not yet stable as a const fn const fn external_fn() { let _ = {Some}(1); //[min_const_fn]~^ ERROR is not yet stable as a const fn //[const_fn]~^^ ERROR is not yet stable as a const fn } const fn local_fn() { let _ = {E::V}(1); //[min_const_fn]~^ ERROR is not yet stable as a const fn //[const_fn]~^^ ERROR is not yet stable as a const fn } fn main() {} </del>