fix fallback impl for select_unpredictable intrinsic

library/core/src/intrinsics/mod.rs

@@ -55,7 +55,7 @@
 #![allow(missing_docs)]
 
 use crate::ffi::va_list::{VaArgSafe, VaList};
-use crate::marker::{ConstParamTy, Destruct, DiscriminantKind, PointeeSized, Tuple};
+use crate::marker::{ConstParamTy, DiscriminantKind, PointeeSized, Tuple};
 use crate::{mem, ptr};
 
 mod bounds;
@@ -482,11 +482,14 @@ pub const fn unlikely(b: bool) -> bool {
 #[rustc_nounwind]
 #[miri::intrinsic_fallback_is_spec]
 #[inline]
-pub const fn select_unpredictable<T>(b: bool, true_val: T, false_val: T) -> T
-where
-    T: [const] Destruct,
-{
-    if b { true_val } else { false_val }
+pub const fn select_unpredictable<T>(b: bool, true_val: T, false_val: T) -> T {
+    if b {
+        forget(false_val);
+        true_val
+    } else {
+        forget(true_val);
+        false_val
+    }
 }
 
 /// A guard for unsafe functions that cannot ever be executed if `T` is uninhabited:

src/tools/miri/tests/pass/intrinsics/select-unpredictable-drop.rs

@@ -0,0 +1,19 @@
+//! Check that `select_unpredictable` properly forgets the value it does not select.
+#![feature(core_intrinsics)]
+use std::cell::Cell;
+use std::intrinsics::select_unpredictable;
+
+fn main() {
+    let (true_val, false_val) = (Cell::new(false), Cell::new(false));
+    _ = select_unpredictable(true, TraceDrop(&true_val), TraceDrop(&false_val));
+    assert!(true_val.get());
+    assert!(!false_val.get());
+}
+
+struct TraceDrop<'a>(&'a Cell<bool>);
+
+impl<'a> Drop for TraceDrop<'a> {
+    fn drop(&mut self) {
+        self.0.set(true);
+    }
+}