Auto merge of rust-lang#74930 - ecstatic-morse:const-size-align-of-val, r=oli-obk

Make `mem::size_of_val` and `mem::align_of_val` unstably const

Implements rust-lang#46571 but does not stabilize it. I wanted this while working on something today.

The only reason not to immediately stabilize are concerns around [custom DSTs](rust-lang#46571 (comment)). That proposal has made zero progress in the last two years and const eval is rich enough to support pretty much any user-defined `len` function as long as nightly features are allowed (`raw_ptr_deref`).

Currently, this raises a `const_err` lint when passed an `extern type`.

r? @oli-obk

cc @rust-lang/wg-const-eval

Author: bors

core/src/intrinsics.rs

@@ -1004,11 +1004,13 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::mem::size_of_val`](../../std/mem/fn.size_of_val.html).
+    #[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
     pub fn size_of_val<T: ?Sized>(_: *const T) -> usize;
     /// The required alignment of the referenced value.
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::mem::align_of_val`](../../std/mem/fn.align_of_val.html).
+    #[rustc_const_unstable(feature = "const_align_of_val", issue = "46571")]
     pub fn min_align_of_val<T: ?Sized>(_: *const T) -> usize;
 
     /// Gets a static string slice containing the name of a type.

core/src/lib.rs

@@ -88,6 +88,8 @@
 #![feature(const_result)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
+#![feature(const_size_of_val)]
+#![feature(const_align_of_val)]
 #![feature(const_type_name)]
 #![feature(const_likely)]
 #![feature(const_unreachable_unchecked)]

core/src/mem/mod.rs

@@ -332,7 +332,8 @@ pub const fn size_of<T>() -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
+#[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
+pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
     intrinsics::size_of_val(val)
 }
 
@@ -466,9 +467,10 @@ pub const fn align_of<T>() -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_align_of_val", issue = "46571")]
 #[allow(deprecated)]
-pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
-    min_align_of_val(val)
+pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
+    intrinsics::min_align_of_val(val)
 }
 
 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.