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)]