Auto merge of rust-lang#97419 - WaffleLapkin:const_from_ptr_range, r=oli-obk

Make `from{,_mut}_ptr_range` const

This PR makes the following APIs `const`:
```rust
// core::slice

pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T];
pub const unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T];
```

Tracking issue: rust-lang#89792.
Feature for `from_ptr_range` as a `const fn`: `slice_from_ptr_range_const`.
Feature for `from_mut_ptr_range` as a `const fn`: `slice_from_mut_ptr_range_const`.

r? `@oli-obk`

Author: bors

library/alloc/src/lib.rs

@@ -130,6 +130,7 @@
 #![feature(ptr_sub_ptr)]
 #![feature(receiver_trait)]
 #![feature(set_ptr_value)]
+#![feature(slice_from_ptr_range)]
 #![feature(slice_group_by)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]

library/alloc/src/slice.rs

@@ -114,6 +114,8 @@ pub use core::slice::EscapeAscii;
 pub use core::slice::SliceIndex;
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub use core::slice::{from_mut, from_ref};
+#[unstable(feature = "slice_from_ptr_range", issue = "89792")]
+pub use core::slice::{from_mut_ptr_range, from_ptr_range};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::slice::{from_raw_parts, from_raw_parts_mut};
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/slice/raw.rs

@@ -213,7 +213,8 @@ pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
 ///
 /// [valid]: ptr#safety
 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
-pub unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
+#[rustc_const_unstable(feature = "const_slice_from_ptr_range", issue = "89792")]
+pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
     // SAFETY: the caller must uphold the safety contract for `from_ptr_range`.
     unsafe { from_raw_parts(range.start, range.end.sub_ptr(range.start)) }
 }
@@ -263,7 +264,8 @@ pub unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
 ///
 /// [valid]: ptr#safety
 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
-pub unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
+#[rustc_const_unstable(feature = "slice_from_mut_ptr_range_const", issue = "89792")]
+pub const unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
     // SAFETY: the caller must uphold the safety contract for `from_mut_ptr_range`.
     unsafe { from_raw_parts_mut(range.start, range.end.sub_ptr(range.start)) }
 }

src/test/ui/const-ptr/allowed_slices.rs

@@ -0,0 +1,106 @@
+// run-pass
+#![feature(
+    const_slice_from_raw_parts,
+    slice_from_ptr_range,
+    const_slice_from_ptr_range,
+    pointer_byte_offsets,
+    const_pointer_byte_offsets
+)]
+use std::{
+    mem::MaybeUninit,
+    ptr,
+    slice::{from_ptr_range, from_raw_parts},
+};
+
+// Dangling is ok, as long as it's either for ZST reads or for no reads
+pub static S0: &[u32] = unsafe { from_raw_parts(dangling(), 0) };
+pub static S1: &[()] = unsafe { from_raw_parts(dangling(), 3) };
+
+// References are always valid of reads of a single element (basically `slice::from_ref`)
+pub static S2: &[u32] = unsafe { from_raw_parts(&D0, 1) };
+pub static S3: &[MaybeUninit<&u32>] = unsafe { from_raw_parts(&D1, 1) };
+
+// Reinterpreting data is fine, as long as layouts match
+pub static S4: &[u8] = unsafe { from_raw_parts((&D0) as *const _ as _, 3) };
+// This is only valid because D1 has uninitialized bytes, if it was an initialized pointer,
+// that would reinterpret pointers as integers which is UB in CTFE.
+pub static S5: &[MaybeUninit<u8>] = unsafe { from_raw_parts((&D1) as *const _ as _, 2) };
+// Even though u32 and [bool; 4] have different layouts, D0 has a value that
+// is valid as [bool; 4], so this is not UB (it's basically a transmute)
+pub static S6: &[bool] = unsafe { from_raw_parts((&D0) as *const _ as _, 4) };
+
+// Structs are considered single allocated objects,
+// as long as you don't reinterpret padding as initialized
+// data everything is ok.
+pub static S7: &[u16] = unsafe {
+    let ptr = (&D2 as *const Struct as *const u16).byte_add(4);
+
+    from_raw_parts(ptr, 3)
+};
+pub static S8: &[MaybeUninit<u16>] = unsafe {
+    let ptr = &D2 as *const Struct as *const MaybeUninit<u16>;
+
+    from_raw_parts(ptr, 6)
+};
+
+pub static R0: &[u32] = unsafe { from_ptr_range(dangling()..dangling()) };
+// from_ptr_range panics on zst
+//pub static R1: &[()] = unsafe { from_ptr_range(dangling(), dangling().byte_add(3)) };
+pub static R2: &[u32] = unsafe {
+    let ptr = &D0 as *const u32;
+    from_ptr_range(ptr..ptr.add(1))
+};
+pub static R3: &[MaybeUninit<&u32>] = unsafe {
+    let ptr = &D1 as *const MaybeUninit<&u32>;
+    from_ptr_range(ptr..ptr.add(1))
+};
+pub static R4: &[u8] = unsafe {
+    let ptr = &D0 as *const u32 as *const u8;
+    from_ptr_range(ptr..ptr.add(3))
+};
+pub static R5: &[MaybeUninit<u8>] = unsafe {
+    let ptr = &D1 as *const MaybeUninit<&u32> as *const MaybeUninit<u8>;
+    from_ptr_range(ptr..ptr.add(2))
+};
+pub static R6: &[bool] = unsafe {
+    let ptr = &D0 as *const u32 as *const bool;
+    from_ptr_range(ptr..ptr.add(4))
+};
+pub static R7: &[u16] = unsafe {
+    let d2 = &D2;
+    let l = &d2.b as *const u32 as *const u16;
+    let r = &d2.d as *const u8 as *const u16;
+
+    from_ptr_range(l..r)
+};
+pub static R8: &[MaybeUninit<u16>] = unsafe {
+    let d2 = &D2;
+    let l = d2 as *const Struct as *const MaybeUninit<u16>;
+    let r = &d2.d as *const u8 as *const MaybeUninit<u16>;
+
+    from_ptr_range(l..r)
+};
+
+// Using valid slice is always valid
+pub static R9: &[u32] = unsafe { from_ptr_range(R0.as_ptr_range()) };
+pub static R10: &[u32] = unsafe { from_ptr_range(R2.as_ptr_range()) };
+
+const D0: u32 = (1 << 16) | 1;
+const D1: MaybeUninit<&u32> = MaybeUninit::uninit();
+const D2: Struct = Struct { a: 1, b: 2, c: 3, d: 4 };
+
+const fn dangling<T>() -> *const T {
+    ptr::NonNull::dangling().as_ptr() as _
+}
+
+#[repr(C)]
+struct Struct {
+    a: u8,
+    // _pad: [MaybeUninit<u8>; 3]
+    b: u32,
+    c: u16,
+    d: u8,
+    // _pad: [MaybeUninit<u8>; 1]
+}
+
+fn main() {}