make some functions promotable to reduce fallout

Author: RalfJung

library/core/src/ptr/const_ptr.rs

@@ -43,6 +43,7 @@ impl<T: ?Sized> *const T {
     /// Casts to a pointer of another type.
     #[stable(feature = "ptr_cast", since = "1.38.0")]
     #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[inline]
     pub const fn cast<U>(self) -> *const U {
         self as _

library/core/src/ptr/mod.rs

@@ -196,7 +196,7 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 /// ```
 #[inline(always)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
+#[rustc_promotable] // Operation cannot fail, so it may be promoted.
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
 pub const fn null<T>() -> *const T {
     0 as *const T
@@ -214,7 +214,7 @@ pub const fn null<T>() -> *const T {
 /// ```
 #[inline(always)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
+#[rustc_promotable] // Operation cannot fail, so it may be promoted.
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
 pub const fn null_mut<T>() -> *mut T {
     0 as *mut T

library/core/src/ptr/mut_ptr.rs

@@ -42,6 +42,7 @@ impl<T: ?Sized> *mut T {
     /// Casts to a pointer of another type.
     #[stable(feature = "ptr_cast", since = "1.38.0")]
     #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[inline]
     pub const fn cast<U>(self) -> *mut U {
         self as _

library/core/src/slice/mod.rs

@@ -404,6 +404,7 @@ impl<T> [T] {
     /// [`as_mut_ptr`]: #method.as_mut_ptr
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_as_ptr", since = "1.32.0")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[inline]
     pub const fn as_ptr(&self) -> *const T {
         self as *const [T] as *const T
@@ -432,6 +433,7 @@ impl<T> [T] {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[inline]
     pub const fn as_mut_ptr(&mut self) -> *mut T {
         self as *mut [T] as *mut T

library/std/src/net/ip.rs

@@ -302,6 +302,7 @@ impl Ipv4Addr {
     /// let addr = Ipv4Addr::new(127, 0, 0, 1);
     /// ```
     #[rustc_const_stable(feature = "const_ipv4", since = "1.32.0")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn new(a: u8, b: u8, c: u8, d: u8) -> Ipv4Addr {
         // `s_addr` is stored as BE on all machine and the array is in BE order.
@@ -1045,6 +1046,7 @@ impl Ipv6Addr {
     /// ```
     #[rustc_allow_const_fn_unstable(const_fn_transmute)]
     #[rustc_const_stable(feature = "const_ipv6", since = "1.32.0")]
+    #[rustc_promotable] // Operation cannot fail, so it may be promoted.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn new(a: u16, b: u16, c: u16, d: u16, e: u16, f: u16, g: u16, h: u16) -> Ipv6Addr {
         let addr16 = [