Auto merge of #55278 - Centril:constification-1, r=alexcrichton

Minor standard library constification

This PR makes some bits of the standard library into `const fn`s.
I've tried to be as aggressive as I possibly could in the constification.
The list is rather small due to how restrictive `const fn` is at the moment.

r? @oli-obk cc @rust-lang/libs

Stable public APIs affected:
+ [x] `Cell::as_ptr`
+ [x] `UnsafeCell::get`
+ [x] `char::is_ascii`
+ [x] `iter::empty`
+ [x] `ManuallyDrop::{new, into_inner}`
+ [x] `RangeInclusive::{start, end}`
+ [x] `NonNull::as_ptr`
+ [x] `{[T], str}::as_ptr`
+ [x] `Duration::{as_secs, subsec_millis, subsec_micros, subsec_nanos}`
+ [x] `CStr::as_ptr`
+ [x] `Ipv4Addr::is_unspecified`
+ [x] `Ipv6Addr::new`
+ [x] `Ipv6Addr::octets`

Unstable public APIs affected:
+ [x] `Duration::{as_millis, as_micros, as_nanos, as_float_secs}`
+ [x] `Wrapping::{count_ones, count_zeros, trailing_zeros, rotate_left, rotate_right, swap_bytes, reverse_bits, from_be, from_le, to_be, to_le, leading_zeros, is_positive, is_negative, leading_zeros}`
+ [x] `core::convert::identity`

--------------------------

## Removed from list in first pass:

Stable public APIs affected:
+ [ ] `BTree{Map, Set}::{len, is_empty}`
+ [ ] `VecDeque::is_empty`
+ [ ] `String::{is_empty, len}`
+ [ ] `FromUtf8Error::utf8_error`
+ [ ] `Vec<T>::{is_empty, len}`
+ [ ] `Layout::size`
+ [ ] `DecodeUtf16Error::unpaired_surrogate`
+ [ ] `core::fmt::{fill, width, precision, sign_plus, sign_minus, alternate, sign_aware_zero_pad}`
+ [ ] `panic::Location::{file, line, column}`
+ [ ] `{ChunksExact, RChunksExact}::remainder`
+ [ ] `Utf8Error::valid_up_to`
+ [ ] `VacantEntry::key`
+ [ ] `NulError::nul_position`
+ [ ] `IntoStringError::utf8_error`
+ [ ] `IntoInnerError::error`
+ [ ] `io::Chain::get_ref`
+ [ ] `io::Take::{limit, get_ref}`
+ [ ] `SocketAddrV6::{flowinfo, scope_id}`
+ [ ] `PrefixComponent::{kind, as_os_str}`
+ [ ] `Path::{ancestors, display}`
+ [ ] `WaitTimeoutResult::timed_out`
+ [ ] `Receiver::{iter, try_iter}`
+ [ ] `thread::JoinHandle::thread`
+ [ ] `SystemTimeError::duration`

Unstable public APIs affected:
+ [ ] `core::fmt::Arguments::new_v1`
+ [ ] `core::fmt::Arguments::new_v1_formatted`
+ [ ] `Pin::{get_ref, into_ref}`
+ [ ] `Utf8Lossy::chunks`
+ [ ] `LocalWaker::as_waker`
+ [ ] `panic::PanicInfo::{internal_constructor, message, location}`
+ [ ] `panic::Location::{internal_constructor }`

## Removed from list in 2nd pass:

Stable public APIs affected:
+ [ ] `LinkedList::{new, iter, is_empty, len}`
+ [ ] `mem::forget`
+ [ ] `Cursor::{new, get_ref, position}`
+ [ ] `io::{empty, repeat, sink}`
+ [ ] `PoisonError::new`
+ [ ] `thread::Builder::new`
+ [ ] `process::Stdio::{piped, inherit, null}`

Unstable public APIs affected:
+ [ ] `io::Initializer::{zeroing, should_initialize}`

Author: bors

src/libcore/cell.rs

@@ -474,7 +474,7 @@ impl<T: ?Sized> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
-    pub fn as_ptr(&self) -> *mut T {
+    pub const fn as_ptr(&self) -> *mut T {
         self.value.get()
     }
 
@@ -1508,7 +1508,7 @@ impl<T: ?Sized> UnsafeCell<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn get(&self) -> *mut T {
+    pub const fn get(&self) -> *mut T {
         &self.value as *const T as *mut T
     }
 }

src/libcore/char/methods.rs

@@ -903,7 +903,7 @@ impl char {
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
-    pub fn is_ascii(&self) -> bool {
+    pub const fn is_ascii(&self) -> bool {
         *self as u32 <= 0x7F
     }
 

src/libcore/convert.rs

@@ -104,7 +104,6 @@
 /// assert_eq!(vec![1, 3], filtered);
 /// ```
 #[unstable(feature = "convert_id", issue = "53500")]
-#[rustc_const_unstable(feature = "const_convert_id")]
 #[inline]
 pub const fn identity<T>(x: T) -> T { x }
 

src/libcore/iter/sources.rs

@@ -283,7 +283,7 @@ impl<T> Default for Empty<T> {
 /// assert_eq!(None, nope.next());
 /// ```
 #[stable(feature = "iter_empty", since = "1.2.0")]
-pub fn empty<T>() -> Empty<T> {
+pub const fn empty<T>() -> Empty<T> {
     Empty(marker::PhantomData)
 }
 

src/libcore/lib.rs

@@ -82,7 +82,6 @@
 #![feature(const_fn)]
 #![feature(const_int_ops)]
 #![feature(const_fn_union)]
-#![feature(const_manually_drop_new)]
 #![feature(custom_attribute)]
 #![feature(doc_cfg)]
 #![feature(doc_spotlight)]

src/libcore/mem.rs

@@ -942,7 +942,6 @@ impl<T> ManuallyDrop<T> {
     /// ManuallyDrop::new(Box::new(()));
     /// ```
     #[stable(feature = "manually_drop", since = "1.20.0")]
-    #[rustc_const_unstable(feature = "const_manually_drop_new")]
     #[inline]
     pub const fn new(value: T) -> ManuallyDrop<T> {
         ManuallyDrop { value }
@@ -961,7 +960,7 @@ impl<T> ManuallyDrop<T> {
     /// ```
     #[stable(feature = "manually_drop", since = "1.20.0")]
     #[inline]
-    pub fn into_inner(slot: ManuallyDrop<T>) -> T {
+    pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
         slot.value
     }
 

src/libcore/num/flt2dec/estimator.rs

@@ -22,4 +22,3 @@ pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16 {
     // therefore this always underestimates (or is exact), but not much.
     (((nbits + exp as i64) * 1292913986) >> 32) as i16
 }
-

src/libcore/num/flt2dec/mod.rs

@@ -658,4 +658,3 @@ pub fn to_exact_fixed_str<'a, T, F>(mut format_exact: F, v: T,
         }
     }
 }
-

src/libcore/num/wrapping.rs

@@ -387,7 +387,7 @@ assert_eq!(n.count_ones(), 3);
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn count_ones(self) -> u32 {
+                pub const fn count_ones(self) -> u32 {
                     self.0.count_ones()
                 }
             }
@@ -407,7 +407,7 @@ assert_eq!(Wrapping(!0", stringify!($t), ").count_zeros(), 0);
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn count_zeros(self) -> u32 {
+                pub const fn count_zeros(self) -> u32 {
                     self.0.count_zeros()
                 }
             }
@@ -430,7 +430,7 @@ assert_eq!(n.trailing_zeros(), 3);
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn trailing_zeros(self) -> u32 {
+                pub const fn trailing_zeros(self) -> u32 {
                     self.0.trailing_zeros()
                 }
             }
@@ -456,7 +456,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[inline]
             #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-            pub fn rotate_left(self, n: u32) -> Self {
+            pub const fn rotate_left(self, n: u32) -> Self {
                 Wrapping(self.0.rotate_left(n))
             }
 
@@ -481,7 +481,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[inline]
             #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-            pub fn rotate_right(self, n: u32) -> Self {
+            pub const fn rotate_right(self, n: u32) -> Self {
                 Wrapping(self.0.rotate_right(n))
             }
 
@@ -505,7 +505,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[inline]
             #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-            pub fn swap_bytes(self) -> Self {
+            pub const fn swap_bytes(self) -> Self {
                 Wrapping(self.0.swap_bytes())
             }
 
@@ -532,7 +532,7 @@ assert_eq!(n.trailing_zeros(), 3);
             /// ```
             #[unstable(feature = "reverse_bits", issue = "48763")]
             #[inline]
-            pub fn reverse_bits(self) -> Self {
+            pub const fn reverse_bits(self) -> Self {
                 Wrapping(self.0.reverse_bits())
             }
 
@@ -560,7 +560,7 @@ if cfg!(target_endian = \"big\") {
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn from_be(x: Self) -> Self {
+                pub const fn from_be(x: Self) -> Self {
                     Wrapping(<$t>::from_be(x.0))
                 }
             }
@@ -589,7 +589,7 @@ if cfg!(target_endian = \"little\") {
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn from_le(x: Self) -> Self {
+                pub const fn from_le(x: Self) -> Self {
                     Wrapping(<$t>::from_le(x.0))
                 }
             }
@@ -618,7 +618,7 @@ if cfg!(target_endian = \"big\") {
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn to_be(self) -> Self {
+                pub const fn to_be(self) -> Self {
                     Wrapping(self.0.to_be())
                 }
             }
@@ -647,7 +647,7 @@ if cfg!(target_endian = \"little\") {
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn to_le(self) -> Self {
+                pub const fn to_le(self) -> Self {
                     Wrapping(self.0.to_le())
                 }
             }
@@ -707,7 +707,7 @@ assert_eq!(n.leading_zeros(), 3);
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn leading_zeros(self) -> u32 {
+                pub const fn leading_zeros(self) -> u32 {
                     self.0.leading_zeros()
                 }
             }
@@ -784,7 +784,7 @@ assert!(!Wrapping(-10", stringify!($t), ").is_positive());
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn is_positive(self) -> bool {
+                pub const fn is_positive(self) -> bool {
                     self.0.is_positive()
                 }
             }
@@ -806,7 +806,7 @@ assert!(!Wrapping(10", stringify!($t), ").is_negative());
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn is_negative(self) -> bool {
+                pub const fn is_negative(self) -> bool {
                     self.0.is_negative()
                 }
             }
@@ -836,7 +836,7 @@ assert_eq!(n.leading_zeros(), 2);
 ```"),
                 #[inline]
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                pub fn leading_zeros(self) -> u32 {
+                pub const fn leading_zeros(self) -> u32 {
                     self.0.leading_zeros()
                 }
             }

src/libcore/ops/range.rs

@@ -416,7 +416,7 @@ impl<Idx> RangeInclusive<Idx> {
     /// ```
     #[stable(feature = "inclusive_range_methods", since = "1.27.0")]
     #[inline]
-    pub fn start(&self) -> &Idx {
+    pub const fn start(&self) -> &Idx {
         &self.start
     }
 
@@ -440,7 +440,7 @@ impl<Idx> RangeInclusive<Idx> {
     /// ```
     #[stable(feature = "inclusive_range_methods", since = "1.27.0")]
     #[inline]
-    pub fn end(&self) -> &Idx {
+    pub const fn end(&self) -> &Idx {
         &self.end
     }