Give the assume intrinsic a fallback body

Author: oli-obk

core/src/intrinsics.rs

@@ -937,20 +937,30 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn unreachable() -> !;
 
-    /// Informs the optimizer that a condition is always true.
-    /// If the condition is false, the behavior is undefined.
-    ///
-    /// No code is generated for this intrinsic, but the optimizer will try
-    /// to preserve it (and its condition) between passes, which may interfere
-    /// with optimization of surrounding code and reduce performance. It should
-    /// not be used if the invariant can be discovered by the optimizer on its
-    /// own, or if it does not enable any significant optimizations.
-    ///
-    /// This intrinsic does not have a stable counterpart.
-    #[rustc_const_stable(feature = "const_assume", since = "1.77.0")]
-    #[rustc_nounwind]
-    pub fn assume(b: bool);
+}
+
+/// Informs the optimizer that a condition is always true.
+/// If the condition is false, the behavior is undefined.
+///
+/// No code is generated for this intrinsic, but the optimizer will try
+/// to preserve it (and its condition) between passes, which may interfere
+/// with optimization of surrounding code and reduce performance. It should
+/// not be used if the invariant can be discovered by the optimizer on its
+/// own, or if it does not enable any significant optimizations.
+///
+/// This intrinsic does not have a stable counterpart.
+#[rustc_const_stable(feature = "const_assume", since = "1.77.0")]
+#[rustc_nounwind]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+pub const unsafe fn assume(b: bool) {
+    if !b {
+        // SAFETY: the caller must guarantee the argument is never `false`
+        unsafe { unreachable() }
+    }
+}
 
+extern "rust-intrinsic" {
     /// Hints to the compiler that branch condition is likely to be true.
     /// Returns the value passed to it.
     ///