Add some more documentation

Author: scottmcm

library/core/src/mem/mod.rs

@@ -1179,13 +1179,39 @@ pub const fn variant_count<T>() -> usize {
  intrinsics::variant_count::<T>()
 }
 
+/// Provides associated constants for various useful properties of types,
+/// to give them a canonical form in our code and make them easier to read.
+///
 /// This is here only to simplify all the ZST checks we need in the library.
 /// It's not on a stabilization track right now.
 #[doc(hidden)]
 #[unstable(feature = "sized_type_properties", issue = "none")]
 pub trait SizedTypeProperties: Sized {
  /// `true` if this type requires no storage.
  /// `false` if its [size](size_of) is greater than zero.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(sized_type_properties)]
+ /// use core::mem::SizedTypeProperties;
+ ///
+ /// fn do_something_with<T>() {
+ /// if T::IS_ZST {
+ /// // ... special approach ...
+ /// } else {
+ /// // ... the normal thing ...
+ /// }
+ /// }
+ ///
+ /// struct MyUnit;
+ /// assert!(MyUnit::IS_ZST);
+ ///
+ /// // For negative checks, consider using UFCS to emphasize the negation
+ /// assert!(!<i32>::IS_ZST);
+ /// // As it can sometimes hide in the type otherwise
+ /// assert!(!String::IS_ZST);
+ /// ```
  #[doc(hidden)]
  #[unstable(feature = "sized_type_properties", issue = "none")]
  const IS_ZST: bool = size_of::<Self>() == 0;