Rollup merge of rust-lang#147788 - clarfonthey:const-cell, r=oli-obk

const Cell methods

Tracking: rust-lang#147787

r? `@oli-obk`

Author: matthiaskrgr

library/core/src/cell.rs

@@ -252,7 +252,7 @@
 
 use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
-use crate::marker::{PhantomData, Unsize};
+use crate::marker::{Destruct, PhantomData, Unsize};
 use crate::mem::{self, ManuallyDrop};
 use crate::ops::{self, CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
 use crate::panic::const_panic;
@@ -429,7 +429,11 @@ impl<T> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn set(&self, val: T) {
+    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
+    pub const fn set(&self, val: T)
+    where
+        T: [const] Destruct,
+    {
         self.replace(val);
     }
 
@@ -561,7 +565,12 @@ impl<T: Copy> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "cell_update", since = "1.88.0")]
-    pub fn update(&self, f: impl FnOnce(T) -> T) {
+    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
+    pub const fn update(&self, f: impl [const] FnOnce(T) -> T)
+    where
+        // FIXME(const-hack): `Copy` should imply `const Destruct`
+        T: [const] Destruct,
+    {
         let old = self.get();
         self.set(f(old));
     }
@@ -654,7 +663,11 @@ impl<T: Default> Cell<T> {
     /// assert_eq!(c.into_inner(), 0);
     /// ```
     #[stable(feature = "move_cell", since = "1.17.0")]
-    pub fn take(&self) -> T {
+    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
+    pub const fn take(&self) -> T
+    where
+        T: [const] Default,
+    {
         self.replace(Default::default())
     }
 }

library/coretests/tests/lib.rs

@@ -16,6 +16,7 @@
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
+#![feature(const_cell_traits)]
 #![feature(const_cmp)]
 #![feature(const_convert)]
 #![feature(const_destruct)]

library/coretests/tests/manually_drop.rs

@@ -27,3 +27,41 @@ fn smoke() {
     drop(x);
     drop(y);
 }
+
+#[test]
+fn const_drop_in_place() {
+    const COUNTER: usize = {
+        use core::cell::Cell;
+
+        let counter = Cell::new(0);
+
+        // only exists to make `Drop` indirect impl
+        #[allow(dead_code)]
+        struct Test<'a>(Dropped<'a>);
+
+        struct Dropped<'a>(&'a Cell<usize>);
+        impl const Drop for Dropped<'_> {
+            fn drop(&mut self) {
+                self.0.set(self.0.get() + 1);
+            }
+        }
+
+        let mut one = ManuallyDrop::new(Test(Dropped(&counter)));
+        let mut two = ManuallyDrop::new(Test(Dropped(&counter)));
+        let mut three = ManuallyDrop::new(Test(Dropped(&counter)));
+        assert!(counter.get() == 0);
+        unsafe {
+            ManuallyDrop::drop(&mut one);
+        }
+        assert!(counter.get() == 1);
+        unsafe {
+            ManuallyDrop::drop(&mut two);
+        }
+        assert!(counter.get() == 2);
+        unsafe {
+            ManuallyDrop::drop(&mut three);
+        }
+        counter.get()
+    };
+    assert_eq!(COUNTER, 3);
+}

library/coretests/tests/ptr.rs

@@ -1,6 +1,6 @@
 use core::cell::RefCell;
 use core::marker::Freeze;
-use core::mem::{ManuallyDrop, MaybeUninit};
+use core::mem::MaybeUninit;
 use core::num::NonZero;
 use core::ptr;
 use core::ptr::*;
@@ -1045,42 +1045,3 @@ fn test_ptr_default() {
     let default = PtrMutDefaultTest::default();
     assert!(default.ptr.is_null());
 }
-
-#[test]
-fn test_const_drop_in_place() {
-    const COUNTER: usize = {
-        let mut counter = 0;
-        let counter_ptr = &raw mut counter;
-
-        // only exists to make `Drop` indirect impl
-        #[allow(dead_code)]
-        struct Test(Dropped);
-
-        struct Dropped(*mut usize);
-        impl const Drop for Dropped {
-            fn drop(&mut self) {
-                unsafe {
-                    *self.0 += 1;
-                }
-            }
-        }
-
-        let mut one = ManuallyDrop::new(Test(Dropped(counter_ptr)));
-        let mut two = ManuallyDrop::new(Test(Dropped(counter_ptr)));
-        let mut three = ManuallyDrop::new(Test(Dropped(counter_ptr)));
-        assert!(counter == 0);
-        unsafe {
-            ManuallyDrop::drop(&mut one);
-        }
-        assert!(counter == 1);
-        unsafe {
-            ManuallyDrop::drop(&mut two);
-        }
-        assert!(counter == 2);
-        unsafe {
-            ManuallyDrop::drop(&mut three);
-        }
-        counter
-    };
-    assert_eq!(COUNTER, 3);
-}