Auto merge of #43017 - durka:stabilize-const-invocation, r=eddyb

Individualize feature gates for const fn invocation

This PR changes the meaning of `#![feature(const_fn)]` so it is only required to declare a const fn but not to call one. Based on discussion at #24111. I was hoping we could have an FCP here in order to move that conversation forward.

This sets the stage for future stabilization of the constness of several functions in the standard library (listed below), so could someone please tag the lang team for review.

- `std::cell`
    - `Cell::new`
    - `RefCell::new`
    - `UnsafeCell::new`
- `std::mem`
    - `size_of`
    - `align_of`
- `std::ptr`
    - `null`
    - `null_mut`
- `std::sync`
    - `atomic`
        - `Atomic{Bool,Ptr,Isize,Usize}::new`
    - `once`
        - `Once::new`
- primitives
    - `{integer}::min_value`
    - `{integer}::max_value`

Some other functions are const but they are also unstable or hidden, e.g. `Unique::new` so they don't have to be considered at this time.

After this stabilization, the following `*_INIT` constants in the standard library can be deprecated. I wasn't sure whether to include those deprecations in the current PR.

- `std::sync`
    - `atomic`
        - `ATOMIC_{BOOL,ISIZE,USIZE}_INIT`
    - `once`
        - `ONCE_INIT`

8 files changed, 55 insertions, 2 deletions

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index e0a3b8d52f4..b9c5ff10f87 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -329,6 +329,7 @@ impl<T> Cell<T> {
     /// let c = Cell::new(5);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_cell_new"))]
     #[inline]
     pub const fn new(value: T) -> Cell<T> {
         Cell {
@@ -543,6 +544,7 @@ impl<T> RefCell<T> {
     /// let c = RefCell::new(5);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_refcell_new"))]
     #[inline]
     pub const fn new(value: T) -> RefCell<T> {
         RefCell {
@@ -1188,6 +1190,7 @@ impl<T> UnsafeCell<T> {
     /// let uc = UnsafeCell::new(5);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_unsafe_cell_new"))]
     #[inline]
     pub const fn new(value: T) -> UnsafeCell<T> {
         UnsafeCell { value: value }
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index c270c6ae0db..69612bd2a32 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -85,12 +85,30 @@
 #![feature(prelude_import)]
 #![feature(repr_simd, platform_intrinsics)]
 #![feature(rustc_attrs)]
+#![cfg_attr(not(stage0), feature(rustc_const_unstable))]
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
 
+#![cfg_attr(not(stage0), feature(const_min_value))]
+#![cfg_attr(not(stage0), feature(const_max_value))]
+#![cfg_attr(not(stage0), feature(const_atomic_bool_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_isize_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_usize_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_i8_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_u8_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_i16_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_u16_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_i32_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_u32_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_i64_new))]
+#![cfg_attr(not(stage0), feature(const_atomic_u64_new))]
+#![cfg_attr(not(stage0), feature(const_unsafe_cell_new))]
+#![cfg_attr(not(stage0), feature(const_cell_new))]
+#![cfg_attr(not(stage0), feature(const_nonzero_new))]
+
 #[prelude_import]
 #[allow(unused)]
 use prelude::v1::*;
diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index bae1f4dee14..3e24623dad8 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -189,6 +189,7 @@ pub fn forget<T>(t: T) {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_size_of"))]
 pub const fn size_of<T>() -> usize {
     unsafe { intrinsics::size_of::<T>() }
 }
@@ -280,6 +281,7 @@ pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_align_of"))]
 pub const fn align_of<T>() -> usize {
     unsafe { intrinsics::min_align_of::<T>() }
 }
diff --git a/src/libcore/nonzero.rs b/src/libcore/nonzero.rs
index 3ff1068b937..f075d825f5d 100644
--- a/src/libcore/nonzero.rs
+++ b/src/libcore/nonzero.rs
@@ -68,6 +68,10 @@ pub struct NonZero<T: Zeroable>(T);
 impl<T: Zeroable> NonZero<T> {
     /// Creates an instance of NonZero with the provided value.
     /// You must indeed ensure that the value is actually "non-zero".
+    #[unstable(feature = "nonzero",
+               reason = "needs an RFC to flesh out the design",
+               issue = "27730")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_nonzero_new"))]
     #[inline]
     pub const unsafe fn new_unchecked(inner: T) -> Self {
         NonZero(inner)
diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index c5175287ccf..bf31deae7a6 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -109,6 +109,7 @@ macro_rules! int_impl {
         /// assert_eq!(i8::min_value(), -128);
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_min_value"))]
         #[inline]
         pub const fn min_value() -> Self {
             !0 ^ ((!0 as $UnsignedT) >> 1) as Self
@@ -122,6 +123,7 @@ macro_rules! int_impl {
         /// assert_eq!(i8::max_value(), 127);
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_max_value"))]
         #[inline]
         pub const fn max_value() -> Self {
             !Self::min_value()
@@ -1280,6 +1282,7 @@ macro_rules! uint_impl {
         /// assert_eq!(u8::min_value(), 0);
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_min_value"))]
         #[inline]
         pub const fn min_value() -> Self { 0 }
 
@@ -1291,6 +1294,7 @@ macro_rules! uint_impl {
         /// assert_eq!(u8::max_value(), 255);
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_max_value"))]
         #[inline]
         pub const fn max_value() -> Self { !0 }
 
diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index 2e42e0dfd55..20aa881f4bc 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -76,6 +76,7 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_ptr_null"))]
 pub const fn null<T>() -> *const T { 0 as *const T }
 
 /// Creates a null mutable raw pointer.
@@ -90,6 +91,7 @@ pub const fn null<T>() -> *const T { 0 as *const T }
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_ptr_null_mut"))]
 pub const fn null_mut<T>() -> *mut T { 0 as *mut T }
 
 /// Swaps the values at two mutable locations of the same type, without
@@ -1097,6 +1099,8 @@ impl<T: ?Sized> Unique<T> {
     /// # Safety
     ///
     /// `ptr` must be non-null.
+    #[unstable(feature = "unique", issue = "27730")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_unique_new"))]
     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
         Unique { pointer: NonZero::new_unchecked(ptr), _marker: PhantomData }
     }
@@ -1230,6 +1234,8 @@ impl<T: ?Sized> Shared<T> {
     /// # Safety
     ///
     /// `ptr` must be non-null.
+    #[unstable(feature = "shared", issue = "27730")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_shared_new"))]
     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
         Shared { pointer: NonZero::new_unchecked(ptr), _marker: PhantomData }
     }
diff --git a/src/libcore/sync/atomic.rs b/src/libcore/sync/atomic.rs
index 510e01db0e9..09f3586a8c9 100644
--- a/src/libcore/sync/atomic.rs
+++ b/src/libcore/sync/atomic.rs
@@ -241,6 +241,7 @@ impl AtomicBool {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_atomic_bool_new"))]
     pub const fn new(v: bool) -> AtomicBool {
         AtomicBool { v: UnsafeCell::new(v as u8) }
     }
@@ -649,6 +650,7 @@ impl<T> AtomicPtr<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_atomic_ptr_new"))]
     pub const fn new(p: *mut T) -> AtomicPtr<T> {
         AtomicPtr { p: UnsafeCell::new(p) }
     }
@@ -920,7 +922,7 @@ impl<T> AtomicPtr<T> {
 
 #[cfg(target_has_atomic = "ptr")]
 macro_rules! atomic_int {
-    ($stable:meta,
+    ($stable:meta, $const_unstable:meta,
      $stable_cxchg:meta,
      $stable_debug:meta,
      $stable_access:meta,
@@ -969,6 +971,7 @@ macro_rules! atomic_int {
             /// ```
             #[inline]
             #[$stable]
+            #[cfg_attr(not(stage0), $const_unstable)]
             pub const fn new(v: $int_type) -> Self {
                 $atomic_type {v: UnsafeCell::new(v)}
             }
@@ -1332,6 +1335,7 @@ macro_rules! atomic_int {
 #[cfg(target_has_atomic = "8")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_i8_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1340,6 +1344,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "8")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_u8_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1348,6 +1353,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "16")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_i16_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1356,6 +1362,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "16")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_u16_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1364,6 +1371,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "32")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_i32_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1372,6 +1380,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "32")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_u32_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1380,6 +1389,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "64")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_i64_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1388,6 +1398,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "64")]
 atomic_int! {
     unstable(feature = "integer_atomics", issue = "32976"),
+    rustc_const_unstable(feature = "const_atomic_u64_new"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
     unstable(feature = "integer_atomics", issue = "32976"),
@@ -1396,6 +1407,7 @@ atomic_int! {
 #[cfg(target_has_atomic = "ptr")]
 atomic_int!{
     stable(feature = "rust1", since = "1.0.0"),
+    rustc_const_unstable(feature = "const_atomic_isize_new"),
     stable(feature = "extended_compare_and_swap", since = "1.10.0"),
     stable(feature = "atomic_debug", since = "1.3.0"),
     stable(feature = "atomic_access", since = "1.15.0"),
@@ -1404,6 +1416,7 @@ atomic_int!{
 #[cfg(target_has_atomic = "ptr")]
 atomic_int!{
     stable(feature = "rust1", since = "1.0.0"),
+    rustc_const_unstable(feature = "const_atomic_usize_new"),
     stable(feature = "extended_compare_and_swap", since = "1.10.0"),
     stable(feature = "atomic_debug", since = "1.3.0"),
     stable(feature = "atomic_access", since = "1.15.0"),
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index ab2022b1824..ce4d14b222c 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -11,7 +11,6 @@
 #![deny(warnings)]
 
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(core_float)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]
@@ -42,6 +41,10 @@
 #![feature(try_from)]
 #![feature(unique)]
 
+#![feature(const_atomic_bool_new)]
+#![feature(const_atomic_usize_new)]
+#![feature(const_atomic_isize_new)]
+
 extern crate core;
 extern crate test;
 extern crate rand;