Implement intrinsics with fallback bodies

Author: oli-obk

core/src/intrinsics.rs

@@ -2517,79 +2517,79 @@ extern "rust-intrinsic" {
     where
         G: FnOnce<ARG, Output = RET>,
         F: FnOnce<ARG, Output = RET>;
+}
 
-    /// Returns whether the argument's value is statically known at
-    /// compile-time.
-    ///
-    /// This is useful when there is a way of writing the code that will
-    /// be *faster* when some variables have known values, but *slower*
-    /// in the general case: an `if is_val_statically_known(var)` can be used
-    /// to select between these two variants. The `if` will be optimized away
-    /// and only the desired branch remains.
-    ///
-    /// Formally speaking, this function non-deterministically returns `true`
-    /// or `false`, and the caller has to ensure sound behavior for both cases.
-    /// In other words, the following code has *Undefined Behavior*:
-    ///
-    /// ```no_run
-    /// #![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:
-    ///
-    /// ```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
-    /// particular value, ever. However, the compiler will generally make it
-    /// return `true` only if the value of the argument is actually known.
-    ///
-    /// When calling this in a `const fn`, both paths must be semantically
-    /// equivalent, that is, the result of the `true` branch and the `false`
-    /// branch must return the same value and have the same side-effects *no
-    /// matter what*.
-    #[rustc_const_unstable(feature = "is_val_statically_known", issue = "none")]
-    #[rustc_nounwind]
-    pub fn is_val_statically_known<T: Copy>(arg: T) -> bool;
-
-    /// Returns the value of `cfg!(debug_assertions)`, but after monomorphization instead of in
-    /// macro expansion.
-    ///
-    /// This always returns `false` in const eval and Miri. The interpreter provides better
-    /// diagnostics than the checks that this is used to implement. However, this means
-    /// you should only be using this intrinsic to guard requirements that, if violated,
-    /// immediately lead to UB. Otherwise, const-eval and Miri will miss out on those
-    /// checks entirely.
-    ///
-    /// Since this is evaluated after monomorphization, branching on this value can be used to
-    /// implement debug assertions that are included in the precompiled standard library, but can
-    /// be optimized out by builds that monomorphize the standard library code with debug
-    /// assertions disabled. This intrinsic is primarily used by [`assert_unsafe_precondition`].
-    #[rustc_const_unstable(feature = "delayed_debug_assertions", issue = "none")]
-    #[rustc_safe_intrinsic]
-    #[cfg(not(bootstrap))]
-    pub(crate) fn debug_assertions() -> bool;
+/// Returns whether the argument's value is statically known at
+/// compile-time.
+///
+/// This is useful when there is a way of writing the code that will
+/// be *faster* when some variables have known values, but *slower*
+/// in the general case: an `if is_val_statically_known(var)` can be used
+/// to select between these two variants. The `if` will be optimized away
+/// and only the desired branch remains.
+///
+/// Formally speaking, this function non-deterministically returns `true`
+/// or `false`, and the caller has to ensure sound behavior for both cases.
+/// In other words, the following code has *Undefined Behavior*:
+///
+/// ```no_run
+/// #![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:
+///
+/// ```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
+/// particular value, ever. However, the compiler will generally make it
+/// return `true` only if the value of the argument is actually known.
+///
+/// When calling this in a `const fn`, both paths must be semantically
+/// equivalent, that is, the result of the `true` branch and the `false`
+/// branch must return the same value and have the same side-effects *no
+/// matter what*.
+#[rustc_const_unstable(feature = "is_val_statically_known", issue = "none")]
+#[rustc_nounwind]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+pub const unsafe fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
+    false
 }
 
-#[cfg(bootstrap)]
+/// Returns the value of `cfg!(debug_assertions)`, but after monomorphization instead of in
+/// macro expansion.
+///
+/// This always returns `false` in const eval and Miri. The interpreter provides better
+/// diagnostics than the checks that this is used to implement. However, this means
+/// you should only be using this intrinsic to guard requirements that, if violated,
+/// immediately lead to UB. Otherwise, const-eval and Miri will miss out on those
+/// checks entirely.
+///
+/// Since this is evaluated after monomorphization, branching on this value can be used to
+/// implement debug assertions that are included in the precompiled standard library, but can
+/// be optimized out by builds that monomorphize the standard library code with debug
+/// assertions disabled. This intrinsic is primarily used by [`assert_unsafe_precondition`].
 #[rustc_const_unstable(feature = "delayed_debug_assertions", issue = "none")]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
 pub(crate) const fn debug_assertions() -> bool {
     cfg!(debug_assertions)
 }