Auto merge of #100848 - xfix:use-metadata-for-slice-len, r=thomcc

Use ptr::metadata in <[T]>::len implementation

This avoids duplication of ptr::metadata code.

I believe this is acceptable as the previous approach essentially duplicated `ptr::metadata` because back then `rustc_allow_const_fn_unstable` annotation did not exist.

I would like somebody to ping `@rust-lang/wg-const-eval` as the documentation says:

> Always ping `@rust-lang/wg-const-eval` if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.

library/core/src/ptr/metadata.rs

@@ -135,16 +135,16 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
 }
 
 #[repr(C)]
-pub(crate) union PtrRepr<T: ?Sized> {
-    pub(crate) const_ptr: *const T,
-    pub(crate) mut_ptr: *mut T,
-    pub(crate) components: PtrComponents<T>,
+union PtrRepr<T: ?Sized> {
+    const_ptr: *const T,
+    mut_ptr: *mut T,
+    components: PtrComponents<T>,
 }
 
 #[repr(C)]
-pub(crate) struct PtrComponents<T: ?Sized> {
-    pub(crate) data_address: *const (),
-    pub(crate) metadata: <T as Pointee>::Metadata,
+struct PtrComponents<T: ?Sized> {
+    data_address: *const (),
+    metadata: <T as Pointee>::Metadata,
 }
 
 // Manual impl needed to avoid `T: Copy` bound.

library/core/src/ptr/mod.rs

@@ -394,7 +394,6 @@ pub use crate::intrinsics::copy;
 pub use crate::intrinsics::write_bytes;
 
 mod metadata;
-pub(crate) use metadata::PtrRepr;
 #[unstable(feature = "ptr_metadata", issue = "81513")]
 pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};
 

library/core/src/slice/mod.rs

@@ -123,18 +123,11 @@ impl<T> [T] {
     #[lang = "slice_len_fn"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_len", since = "1.39.0")]
+    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     #[inline]
     #[must_use]
-    // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
     pub const fn len(&self) -> usize {
-        // FIXME: Replace with `crate::ptr::metadata(self)` when that is const-stable.
-        // As of this writing this causes a "Const-stable functions can only call other
-        // const-stable functions" error.
-
-        // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
-        // and PtrComponents<T> have the same memory layouts. Only std can make this
-        // guarantee.
-        unsafe { crate::ptr::PtrRepr { const_ptr: self }.components.metadata }
+        ptr::metadata(self)
     }
 
     /// Returns `true` if the slice has a length of 0.