const select_unpredictable

Author: clarfonthey

library/core/src/hint.rs

@@ -4,6 +4,7 @@
 //!
 //! Hints may be compile time or runtime.
 
+use crate::marker::Destruct;
 use crate::mem::MaybeUninit;
 use crate::{intrinsics, ub_checks};
 
@@ -771,7 +772,11 @@ pub const fn cold_path() {
 /// ```
 #[inline(always)]
 #[stable(feature = "select_unpredictable", since = "1.88.0")]
-pub fn select_unpredictable<T>(condition: bool, true_val: T, false_val: T) -> T {
+#[rustc_const_unstable(feature = "const_select_unpredictable", issue = "145938")]
+pub const fn select_unpredictable<T>(condition: bool, true_val: T, false_val: T) -> T
+where
+    T: [const] Destruct,
+{
     // FIXME(https://github.com/rust-lang/unsafe-code-guidelines/issues/245):
     // Change this to use ManuallyDrop instead.
     let mut true_val = MaybeUninit::new(true_val);

library/core/src/intrinsics/mod.rs

@@ -55,7 +55,7 @@
 #![allow(missing_docs)]
 
 use crate::ffi::va_list::{VaArgSafe, VaListImpl};
-use crate::marker::{ConstParamTy, DiscriminantKind, PointeeSized, Tuple};
+use crate::marker::{ConstParamTy, Destruct, DiscriminantKind, PointeeSized, Tuple};
 use crate::ptr;
 
 mod bounds;
@@ -477,11 +477,15 @@ pub const fn unlikely(b: bool) -> bool {
 /// However unlike the public form, the intrinsic will not drop the value that
 /// is not selected.
 #[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_const_unstable(feature = "const_select_unpredictable", issue = "145938")]
 #[rustc_intrinsic]
 #[rustc_nounwind]
 #[miri::intrinsic_fallback_is_spec]
 #[inline]
-pub fn select_unpredictable<T>(b: bool, true_val: T, false_val: T) -> T {
+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 }
 }
 

library/core/src/lib.rs

@@ -106,6 +106,7 @@
 #![feature(const_cmp)]
 #![feature(const_destruct)]
 #![feature(const_eval_select)]
+#![feature(const_select_unpredictable)]
 #![feature(core_intrinsics)]
 #![feature(coverage_attribute)]
 #![feature(disjoint_bitor)]

library/coretests/tests/hint.rs

@@ -1,25 +1,34 @@
 #[test]
 fn select_unpredictable_drop() {
-    use core::cell::Cell;
-    struct X<'a>(&'a Cell<bool>);
-    impl Drop for X<'_> {
+    struct X(*mut bool);
+    impl const Drop for X {
         fn drop(&mut self) {
-            self.0.set(true);
+            // FIXME(const-hack): Once const cells work, we could remove unsafe code here.
+            unsafe {
+                *self.0 = true;
+            }
         }
     }
 
-    let a_dropped = Cell::new(false);
-    let b_dropped = Cell::new(false);
-    let a = X(&a_dropped);
-    let b = X(&b_dropped);
-    assert!(!a_dropped.get());
-    assert!(!b_dropped.get());
-    let selected = core::hint::select_unpredictable(core::hint::black_box(true), a, b);
-    assert!(!a_dropped.get());
-    assert!(b_dropped.get());
-    drop(selected);
-    assert!(a_dropped.get());
-    assert!(b_dropped.get());
+    const fn do_test() {
+        let mut a_dropped = false;
+        let mut b_dropped = false;
+        let a = X(&raw mut a_dropped);
+        let b = X(&raw mut b_dropped);
+        assert!(!a_dropped);
+        assert!(!b_dropped);
+        let selected = core::hint::select_unpredictable(core::hint::black_box(true), a, b);
+        assert!(!a_dropped);
+        assert!(b_dropped);
+        drop(selected);
+        assert!(a_dropped);
+        assert!(b_dropped);
+    }
+
+    do_test();
+    const {
+        do_test();
+    }
 }
 
 #[test]

library/coretests/tests/lib.rs

@@ -26,6 +26,7 @@
 #![feature(const_option_ops)]
 #![feature(const_ref_cell)]
 #![feature(const_result_trait_fn)]
+#![feature(const_select_unpredictable)]
 #![feature(const_trait_impl)]
 #![feature(control_flow_ok)]
 #![feature(core_float_math)]