Rollup merge of #147938 - const-clone-slice, r=tgross35

Add const cloning of slices and tests

As discussed in https://github.com/rust-lang/rust/pull/143628#discussion_r2390170336, splitting off slice cloning as a separate PR.
r? @tgross35

library/core/src/slice/mod.rs

@@ -9,6 +9,7 @@
 use crate::clone::TrivialClone;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics::{exact_div, unchecked_sub};
+use crate::marker::Destruct;
 use crate::mem::{self, MaybeUninit, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::ops::{OneSidedRange, OneSidedRangeBound, Range, RangeBounds, RangeInclusive};
@@ -4036,9 +4037,10 @@ impl<T> [T] {
     /// [`split_at_mut`]: slice::split_at_mut
     #[stable(feature = "clone_from_slice", since = "1.7.0")]
     #[track_caller]
-    pub fn clone_from_slice(&mut self, src: &[T])
+    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+    pub const fn clone_from_slice(&mut self, src: &[T])
     where
-        T: Clone,
+        T: [const] Clone + [const] Destruct,
     {
         self.spec_clone_from(src);
     }
@@ -5367,13 +5369,17 @@ const unsafe fn copy_from_slice_impl<T: Clone>(dest: &mut [T], src: &[T]) {
     }
 }
 
-trait CloneFromSpec<T> {
-    fn spec_clone_from(&mut self, src: &[T]);
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+const trait CloneFromSpec<T> {
+    fn spec_clone_from(&mut self, src: &[T])
+    where
+        T: [const] Destruct;
 }
 
-impl<T> CloneFromSpec<T> for [T]
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+impl<T> const CloneFromSpec<T> for [T]
 where
-    T: Clone,
+    T: [const] Clone + [const] Destruct,
 {
     #[track_caller]
     default fn spec_clone_from(&mut self, src: &[T]) {
@@ -5383,15 +5389,19 @@ where
         // But since it can't be relied on we also have an explicit specialization for T: Copy.
         let len = self.len();
         let src = &src[..len];
-        for i in 0..len {
-            self[i].clone_from(&src[i]);
+        // FIXME(const_hack): make this a `for idx in 0..self.len()` loop.
+        let mut idx = 0;
+        while idx < self.len() {
+            self[idx].clone_from(&src[idx]);
+            idx += 1;
         }
     }
 }
 
-impl<T> CloneFromSpec<T> for [T]
+#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
+impl<T> const CloneFromSpec<T> for [T]
 where
-    T: TrivialClone,
+    T: [const] TrivialClone + [const] Destruct,
 {
     #[track_caller]
     fn spec_clone_from(&mut self, src: &[T]) {

library/coretests/tests/clone.rs

@@ -121,3 +121,74 @@ fn cstr_metadata_is_length_with_nul() {
     let bytes: *const [u8] = p as *const [u8];
     assert_eq!(s.to_bytes_with_nul().len(), bytes.len());
 }
+
+#[test]
+fn test_const_clone() {
+    const {
+        let bool: bool = Default::default();
+        let char: char = Default::default();
+        let ascii_char: std::ascii::Char = Default::default();
+        let usize: usize = Default::default();
+        let u8: u8 = Default::default();
+        let u16: u16 = Default::default();
+        let u32: u32 = Default::default();
+        let u64: u64 = Default::default();
+        let u128: u128 = Default::default();
+        let i8: i8 = Default::default();
+        let i16: i16 = Default::default();
+        let i32: i32 = Default::default();
+        let i64: i64 = Default::default();
+        let i128: i128 = Default::default();
+        let f16: f16 = Default::default();
+        let f32: f32 = Default::default();
+        let f64: f64 = Default::default();
+        let f128: f128 = Default::default();
+
+        let bool_clone: bool = bool.clone();
+        let char_clone: char = char.clone();
+        let ascii_char_clone: std::ascii::Char = ascii_char.clone();
+
+        let usize_clone: usize = usize.clone();
+        let u8_clone: u8 = u8.clone();
+        let u16_clone: u16 = u16.clone();
+        let u32_clone: u32 = u32.clone();
+        let u64_clone: u64 = u64.clone();
+        let u128_clone: u128 = u128.clone();
+        let i8_clone: i8 = i8.clone();
+        let i16_clone: i16 = i16.clone();
+        let i32_clone: i32 = i32.clone();
+        let i64_clone: i64 = i64.clone();
+        let i128_clone: i128 = i128.clone();
+        let f16_clone: f16 = f16.clone();
+        let f32_clone: f32 = f32.clone();
+        let f64_clone: f64 = f64.clone();
+        let f128_clone: f128 = f128.clone();
+
+        assert!(bool == bool_clone);
+        assert!(char == char_clone);
+        assert!(ascii_char == ascii_char_clone);
+        assert!(usize == usize_clone);
+        assert!(u8 == u8_clone);
+        assert!(u16 == u16_clone);
+        assert!(u32 == u32_clone);
+        assert!(u64 == u64_clone);
+        assert!(u128 == u128_clone);
+        assert!(i8 == i8_clone);
+        assert!(i16 == i16_clone);
+        assert!(i32 == i32_clone);
+        assert!(i64 == i64_clone);
+        assert!(i128 == i128_clone);
+        assert!(f16 == f16_clone);
+        assert!(f32 == f32_clone);
+        assert!(f64 == f64_clone);
+        assert!(f128 == f128_clone);
+
+        let src: [i32; 4] = [1, 2, 3, 4];
+        let mut dst: [i32; 2] = [0, 0];
+
+        dst.clone_from_slice(&src[2..]);
+
+        assert!(src == [1, 2, 3, 4]);
+        assert!(dst == [3, 4]);
+    }
+}

library/coretests/tests/lib.rs

@@ -19,11 +19,14 @@
 #![feature(clone_to_uninit)]
 #![feature(const_array)]
 #![feature(const_cell_traits)]
+#![feature(const_clone)]
 #![feature(const_cmp)]
 #![feature(const_convert)]
+#![feature(const_default)]
 #![feature(const_destruct)]
 #![feature(const_drop_in_place)]
 #![feature(const_eval_select)]
+#![feature(const_index)]
 #![feature(const_ops)]
 #![feature(const_option_ops)]
 #![feature(const_ref_cell)]