Further Implement is_val_statically_known

Author: NCGThompson

core/src/intrinsics.rs

@@ -2525,15 +2525,30 @@ extern "rust-intrinsic" {
     /// or `false`, and the caller has to ensure sound behavior for both cases.
     /// In other words, the following code has *Undefined Behavior*:
     ///
-    /// ```rust
-    /// if !is_val_statically_known(0) { unreachable_unchecked(); }
+    /// ```
+    /// #![feature(is_val_statically_known)]
+    /// #![feature(core_intrinsics)]
+    /// # #![allow(internal_features)]
+    /// use std::hint::unreachable_unchecked;
+    /// use std::intrinsics::is_val_statically_known;
+    ///
+    /// unsafe {
+    ///    if !is_val_statically_known(0) { unreachable_unchecked(); }
+    /// }
     /// ```
     ///
     /// This also means that the following code's behavior is unspecified; it
     /// may panic, or it may not:
     ///
-    /// ```rust,no_run
-    /// assert_eq!(is_val_statically_known(0), black_box(is_val_statically_known(0)))
+    /// ```no_run
+    /// #![feature(is_val_statically_known)]
+    /// #![feature(core_intrinsics)]
+    /// # #![allow(internal_features)]
+    /// use std::intrinsics::is_val_statically_known;
+    ///
+    /// unsafe {
+    ///     assert_eq!(is_val_statically_known(0), is_val_statically_known(0));
+    /// }
     /// ```
     ///
     /// Unsafe code may not rely on `is_val_statically_known` returning any
@@ -2547,15 +2562,14 @@ extern "rust-intrinsic" {
     #[rustc_const_unstable(feature = "is_val_statically_known", issue = "none")]
     #[rustc_nounwind]
     #[cfg(not(bootstrap))]
-    pub fn is_val_statically_known<T>(arg: T) -> bool;
+    pub fn is_val_statically_known<T: Copy>(arg: T) -> bool;
 }
 
 // FIXME: Seems using `unstable` here completely ignores `rustc_allow_const_fn_unstable`
 // and thus compiling stage0 core doesn't work.
-#[rustc_const_stable(feature = "is_val_statically_known", since = "never")]
+#[rustc_const_stable(feature = "is_val_statically_known", since = "0.0.0")]
 #[cfg(bootstrap)]
-pub const unsafe fn is_val_statically_known<T>(t: T) -> bool {
-    mem::forget(t);
+pub const unsafe fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
     false
 }