Rollup merge of rust-lang#90081 - woppopo:const_write_bytes, r=oli-obk

Make `intrinsics::write_bytes` const

This is required to constify `MaybeUninit::zeroed` and `(*mut T)::write_bytes`.

Tracking issue: rust-lang#86302

Author: matthiaskrgr

core/src/intrinsics.rs

@@ -2244,13 +2244,29 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
 /// assert_eq!(*v, 42);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
 #[inline]
-pub unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
+pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
     extern "rust-intrinsic" {
+        #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
         fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
     }
 
-    debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
+    #[cfg(debug_assertions)]
+    fn runtime_check<T>(ptr: *mut T) {
+        debug_assert!(
+            is_aligned_and_not_null(ptr),
+            "attempt to write to unaligned or null pointer"
+        );
+    }
+    #[cfg(debug_assertions)]
+    const fn compiletime_check<T>(_ptr: *mut T) {}
+    #[cfg(debug_assertions)]
+    // SAFETY: runtime debug-assertions are a best-effort basis; it's fine to
+    // not do them during compile time
+    unsafe {
+        const_eval_select((dst,), compiletime_check, runtime_check);
+    }
 
     // SAFETY: the safety contract for `write_bytes` must be upheld by the caller.
     unsafe { write_bytes(dst, val, count) }

core/tests/intrinsics.rs

@@ -35,3 +35,33 @@ fn test_assume_can_be_in_const_contexts() {
     let rs = unsafe { foo(42, 97) };
     assert_eq!(rs, 0);
 }
+
+#[test]
+#[cfg(not(bootstrap))]
+const fn test_write_bytes_in_const_contexts() {
+    use core::intrinsics::write_bytes;
+
+    const TEST: [u32; 3] = {
+        let mut arr = [1u32, 2, 3];
+        unsafe {
+            write_bytes(arr.as_mut_ptr(), 0, 2);
+        }
+        arr
+    };
+
+    assert!(TEST[0] == 0);
+    assert!(TEST[1] == 0);
+    assert!(TEST[2] == 3);
+
+    const TEST2: [u32; 3] = {
+        let mut arr = [1u32, 2, 3];
+        unsafe {
+            write_bytes(arr.as_mut_ptr(), 1, 2);
+        }
+        arr
+    };
+
+    assert!(TEST2[0] == 16843009);
+    assert!(TEST2[1] == 16843009);
+    assert!(TEST2[2] == 3);
+}