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}`

2 files changed, 3 insertions, 4 deletions

diff --git a/src/libstd/ffi/c_str.rs b/src/libstd/ffi/c_str.rs
index 87ffe0f15e4..66718b95408 100644
--- a/src/libstd/ffi/c_str.rs
+++ b/src/libstd/ffi/c_str.rs
@@ -1091,7 +1091,7 @@ impl CStr {
     /// [`CString`]: struct.CString.html
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn as_ptr(&self) -> *const c_char {
+    pub const fn as_ptr(&self) -> *const c_char {
         self.inner.as_ptr()
     }
 
diff --git a/src/libstd/net/ip.rs b/src/libstd/net/ip.rs
index d45a66ef665..2517c45696a 100644
--- a/src/libstd/net/ip.rs
+++ b/src/libstd/net/ip.rs
@@ -424,7 +424,7 @@ impl Ipv4Addr {
     /// assert_eq!(Ipv4Addr::new(45, 22, 13, 197).is_unspecified(), false);
     /// ```
     #[stable(feature = "ip_shared", since = "1.12.0")]
-    pub fn is_unspecified(&self) -> bool {
+    pub const fn is_unspecified(&self) -> bool {
         self.inner.s_addr == 0
     }
 
@@ -862,7 +862,6 @@ impl Ipv6Addr {
     /// let addr = Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_ip")]
     pub const fn new(a: u16, b: u16, c: u16, d: u16, e: u16, f: u16,
                      g: u16, h: u16) -> Ipv6Addr {
         Ipv6Addr {
@@ -1224,7 +1223,7 @@ impl Ipv6Addr {
     ///            [255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
     /// ```
     #[stable(feature = "ipv6_to_octets", since = "1.12.0")]
-    pub fn octets(&self) -> [u8; 16] {
+    pub const fn octets(&self) -> [u8; 16] {
         self.inner.s6_addr
     }
 }