Rollup merge of rust-lang#91918 - fee1-dead:constification0-the-great…

…-constification-begins, r=oli-obk

Constify `bool::then{,_some}`

Note on `~const Drop`: it has no effect when called from runtime functions, when called from const contexts, the trait system ensures that the type can be dropped in const contexts.

library/core/src/bool.rs

@@ -14,8 +14,12 @@ impl bool {
     /// assert_eq!(true.then_some(0), Some(0));
     /// ```
     #[unstable(feature = "bool_to_option", issue = "80967")]
+    #[rustc_const_unstable(feature = "const_bool_to_option", issue = "91917")]
     #[inline]
-    pub fn then_some<T>(self, t: T) -> Option<T> {
+    pub const fn then_some<T>(self, t: T) -> Option<T>
+    where
+        T: ~const Drop,
+    {
         if self { Some(t) } else { None }
     }
 
@@ -29,8 +33,13 @@ impl bool {
     /// assert_eq!(true.then(|| 0), Some(0));
     /// ```
     #[stable(feature = "lazy_bool_to_option", since = "1.50.0")]
+    #[rustc_const_unstable(feature = "const_bool_to_option", issue = "91917")]
     #[inline]
-    pub fn then<T, F: FnOnce() -> T>(self, f: F) -> Option<T> {
+    pub const fn then<T, F>(self, f: F) -> Option<T>
+    where
+        F: ~const FnOnce() -> T,
+        F: ~const Drop,
+    {
         if self { Some(f()) } else { None }
     }
 }

library/core/tests/bool.rs

@@ -88,4 +88,18 @@ fn test_bool_to_option() {
     assert_eq!(true.then_some(0), Some(0));
     assert_eq!(false.then(|| 0), None);
     assert_eq!(true.then(|| 0), Some(0));
+
+    const fn zero() -> i32 {
+        0
+    }
+
+    const A: Option<i32> = false.then_some(0);
+    const B: Option<i32> = true.then_some(0);
+    const C: Option<i32> = false.then(zero);
+    const D: Option<i32> = true.then(zero);
+
+    assert_eq!(A, None);
+    assert_eq!(B, Some(0));
+    assert_eq!(C, None);
+    assert_eq!(D, Some(0));
 }

library/core/tests/lib.rs

@@ -8,6 +8,7 @@
 #![feature(cfg_panic)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_assume)]
+#![feature(const_bool_to_option)]
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
 #![feature(const_maybe_uninit_as_mut_ptr)]