Use it in the library, and InstSimplify it away in the easy places

Author: scottmcm

core/src/intrinsics.rs

@@ -2786,6 +2786,7 @@ pub unsafe fn vtable_align(_ptr: *const ()) -> usize {
 /// change the possible layouts of pointers.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 #[cfg(not(bootstrap))]

core/src/ptr/metadata.rs

@@ -2,6 +2,8 @@
 
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
+#[cfg(not(bootstrap))]
+use crate::intrinsics::aggregate_raw_ptr;
 use crate::marker::Freeze;
 
 /// Provides the pointer metadata type of any pointed-to type.
@@ -113,10 +115,17 @@ pub const fn from_raw_parts<T: ?Sized>(
     data_pointer: *const (),
     metadata: <T as Pointee>::Metadata,
 ) -> *const T {
-    // 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 { PtrRepr { components: PtrComponents { data_pointer, metadata } }.const_ptr }
+    #[cfg(bootstrap)]
+    {
+        // 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 { PtrRepr { components: PtrComponents { data_pointer, metadata } }.const_ptr }
+    }
+    #[cfg(not(bootstrap))]
+    {
+        aggregate_raw_ptr(data_pointer, metadata)
+    }
 }
 
 /// Performs the same functionality as [`from_raw_parts`], except that a
@@ -130,10 +139,17 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
     data_pointer: *mut (),
     metadata: <T as Pointee>::Metadata,
 ) -> *mut T {
-    // 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 { PtrRepr { components: PtrComponents { data_pointer, metadata } }.mut_ptr }
+    #[cfg(bootstrap)]
+    {
+        // 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 { PtrRepr { components: PtrComponents { data_pointer, metadata } }.mut_ptr }
+    }
+    #[cfg(not(bootstrap))]
+    {
+        aggregate_raw_ptr(data_pointer, metadata)
+    }
 }
 
 #[repr(C)]

core/tests/ptr.rs

@@ -1163,3 +1163,11 @@ fn test_null_array_as_slice() {
     assert!(ptr.is_null());
     assert_eq!(ptr.len(), 4);
 }
+
+#[test]
+fn test_ptr_from_raw_parts_in_const() {
+    const EMPTY_SLICE_PTR: *const [i32] =
+        std::ptr::slice_from_raw_parts(std::ptr::without_provenance(123), 456);
+    assert_eq!(EMPTY_SLICE_PTR.addr(), 123);
+    assert_eq!(EMPTY_SLICE_PTR.len(), 456);
+}

core/tests/slice.rs

@@ -2678,3 +2678,16 @@ fn test_get_many_mut_duplicate() {
     let mut v = vec![1, 2, 3, 4, 5];
     assert!(v.get_many_mut([1, 3, 3, 4]).is_err());
 }
+
+#[test]
+fn test_slice_from_raw_parts_in_const() {
+    static FANCY: i32 = 4;
+    static FANCY_SLICE: &[i32] = unsafe { std::slice::from_raw_parts(&FANCY, 1) };
+    assert_eq!(FANCY_SLICE.as_ptr(), std::ptr::addr_of!(FANCY));
+    assert_eq!(FANCY_SLICE.len(), 1);
+
+    const EMPTY_SLICE: &[i32] =
+        unsafe { std::slice::from_raw_parts(std::ptr::without_provenance(123456), 0) };
+    assert_eq!(EMPTY_SLICE.as_ptr().addr(), 123456);
+    assert_eq!(EMPTY_SLICE.len(), 0);
+}