refactor: use the default implementation of clone_from in impl const Clone

`Clone::clone_from` gained `#[default_method_body_is_const]` in
[rust-lang/rust#91804][1].

n.b. `vec.rs` is shared by `r3_core` and `r3_kernel` (via a symbolic
link), hence the lack of a scope in the commit message headline.

[1]: rust-lang/rust#91804

Author: yvt

doc/toolchain_limitations.md

@@ -632,30 +632,6 @@ const _: () = assert!(matches!((2..4).next(), Some(2)));
 ```
 
 
-### `[tag:clone_from_default]` `Clone::clone_from` lacks `#[default_method_body_is_const]`
-
-When implementing `const Clone`, you can't use the provided implementation of `Clone::clone_from`.
-
-```rust
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-struct A;
-impl const Clone for A {
-    fn clone(&self) -> Self { A }
-    fn clone_from(&mut self, source: &Self) {}
-}
-```
-
-```rust,compile_fail
-#![feature(const_trait_impl)]
-struct A;
-impl const Clone for A {
-    fn clone(&self) -> Self { A }
-    // error: const trait implementations may not use non-const default functions
-}
-```
-
-
 ### `[tag:iterator_const_default]` `Iterator`'s methods lack `#[default_method_body_is_const]`
 
 Implementing `const Iterator` requires you to implement all of its methods, which is impossible to do correctly.

src/r3_core/src/bind.rs

@@ -235,12 +235,6 @@ impl<System, T> const Clone for Bind<'_, System, T> {
     fn clone(&self) -> Self {
         *self
     }
-
-    // `clone_from` needed due to [ref:clone_from_default]
-    #[inline]
-    fn clone_from(&mut self, source: &Self) {
-        *self = *source;
-    }
 }
 
 /// A [binder][1] that gives `&T` to a bound function.
@@ -296,12 +290,6 @@ impl<System, T> const Clone for BindRef<System, T> {
     fn clone(&self) -> Self {
         *self
     }
-
-    // `clone_from` needed due to [ref:clone_from_default]
-    #[inline]
-    fn clone_from(&mut self, source: &Self) {
-        *self = *source;
-    }
 }
 
 // `BindDefiner` doesn't contain `T`, so this `impl` must use a concrete `T`

src/r3_core/src/utils/alloc.rs

@@ -193,10 +193,6 @@ impl const Clone for ConstAllocator {
             ref_count: self.ref_count,
         }
     }
-
-    fn clone_from(&mut self, source: &Self) {
-        *self = source.clone();
-    }
 }
 
 impl const Drop for ConstAllocator {

src/r3_core/src/utils/freeze.rs

@@ -82,11 +82,6 @@ impl<T: Copy> const Clone for Frozen<T> {
         // Don't use `T as Clone` because it could expose interior mutability.
         *self
     }
-
-    #[inline]
-    fn clone_from(&mut self, source: &Self) {
-        *self = *source;
-    }
 }
 
 impl<T: Copy + ~const fmt::Debug> const fmt::Debug for Frozen<T> {

src/r3_core/src/utils/vec.rs

@@ -35,10 +35,6 @@ impl<T: ~const Clone + ~const Drop> const Clone for ComptimeVec<T> {
         }
         self.map(clone_shim)
     }
-
-    fn clone_from(&mut self, source: &Self) {
-        *self = source.clone();
-    }
 }
 
 impl<T> const Drop for ComptimeVec<T>