Auto merge of #65188 - matthewjasper:stabilize-const-constructor, r=Centril

Stabilize `const_constructor`

# 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:

```rust
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);
    <Option<_>>::Some(5);
}
```

### Motivation

Consistency with other `const fn`. Consistency between syntactic path forms.

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

## Tests

* [ui/consts/const_constructor/const-construct-call.rs](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.
* [ui/consts/const_constructor/const_constructor_qpath.rs](https://github.com/rust-lang/rust/blob/1850dfcdabf8258a1f023f26c2c59e96b869dd95/src/test/ui/consts/const_constructor/const_constructor_qpath.rs) - Tests that type qualified paths to enum variants are also considered to be `const fn`.(#64247)

r? @oli-obk

Closes #61456
Closes  #64247

Author: bors

src/librustc/ty/constness.rs

@@ -2,7 +2,7 @@ use crate::ty::query::Providers;
 use crate::hir::def_id::DefId;
 use crate::hir;
 use crate::ty::TyCtxt;
-use syntax_pos::symbol::{sym, Symbol};
+use syntax_pos::symbol::Symbol;
 use crate::hir::map::blocks::FnLikeNode;
 use syntax::attr;
 
@@ -13,14 +13,11 @@ impl<'tcx> TyCtxt<'tcx> {
         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
-                // corresponding feature gate, const_constructor is not a lib feature, so has
-                // to be checked separately.
+                // corresponding feature gate.
                 self.features()
                     .declared_lib_features
                     .iter()
                     .any(|&(sym, _)| sym == feature_name)
-                    || (feature_name == sym::const_constructor
-                        && self.features().const_constructor)
             },
             // functions without const stability are either stable user written
             // const fn or the user is using feature gates and we thus don't
@@ -31,9 +28,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
     /// 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> {
-        if self.is_constructor(def_id) {
-            Some(sym::const_constructor)
-        } else if self.is_const_fn_raw(def_id) {
+        if self.is_const_fn_raw(def_id) {
             self.lookup_stability(def_id)?.const_stability
         } else {
             None

src/libsyntax/feature_gate/accepted.rs

@@ -249,6 +249,8 @@ declare_features! (
     (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),
+    /// Allows calling constructor functions in `const fn`.
+    (accepted, const_constructor, "1.40.0", Some(61456), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

src/libsyntax/feature_gate/active.rs

@@ -488,9 +488,6 @@ declare_features! (
     /// Allows the user of associated type bounds.
     (active, associated_type_bounds, "1.34.0", Some(52662), None),
 
-    /// Allows calling constructor functions in `const fn`.
-    (active, const_constructor, "1.37.0", Some(61456), None),
-
     /// Allows `if/while p && let q = r && ...` chains.
     (active, let_chains, "1.37.0", Some(53667), None),
 

src/test/ui/consts/const_constructor/const-construct-call.rs

@@ -6,8 +6,6 @@
 
 #![cfg_attr(const_fn, feature(const_fn))]
 
-#![feature(const_constructor)]
-
 // Ctor(..) is transformed to Ctor { 0: ... } in HAIR lowering, so directly
 // calling constructors doesn't require them to be const.
 

src/test/ui/consts/const_constructor/const_constructor_qpath.rs

@@ -0,0 +1,40 @@
+// 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());
+}