[wip] Support derive(KnownLayout) on DSTs

Makes progress towards #29.

src/lib.rs

@@ -396,6 +396,12 @@ pub enum _CastType {
 }
 
 impl DstLayout {
+    /// The minimum possible alignment of a type.
+    const MIN_ALIGN: NonZeroUsize = match NonZeroUsize::new(1) {
+        Some(min_align) => min_align,
+        None => unreachable!(),
+    };
+
     /// The maximum theoretic possible alignment of a type.
     ///
     /// For compatibility with future Rust versions, this is defined as the
@@ -419,6 +425,30 @@ impl DstLayout {
         None => unreachable!(),
     };
 
+    /// Constructs a `DstLayout` for a zero-sized type with `repr_align`
+    /// alignment (or 1). If `repr_align` is provided, then it must be a power
+    /// of two.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if the supplied `repr_align` is not a power of two.
+    ///
+    /// # Safety
+    ///
+    /// Unsafe code may assume that the contract of this function is satisfied.
+    #[doc(hidden)]
+    #[inline]
+    pub const fn new_zst(repr_align: Option<NonZeroUsize>) -> DstLayout {
+        let align = match repr_align {
+            Some(align) => align,
+            None => Self::MIN_ALIGN,
+        };
+
+        assert!(align.is_power_of_two());
+
+        DstLayout { _align: align, _size_info: SizeInfo::Sized { _size: 0 } }
+    }
+
     /// Constructs a `DstLayout` which describes `T`.
     ///
     /// # Safety
@@ -490,8 +520,8 @@ impl DstLayout {
     /// We make no guarantees to the behavior of this method if these fragments
     /// cannot appear in a valid Rust type (e.g., the concatenation of the
     /// layouts would lead to a size larger than `isize::MAX`).
-    #[allow(dead_code)]
-    pub(crate) const fn extend(self, field: DstLayout, repr_packed: Option<NonZeroUsize>) -> Self {
+    #[doc(hidden)]
+    pub const fn extend(self, field: DstLayout, repr_packed: Option<NonZeroUsize>) -> Self {
         use util::{core_layout::_padding_needed_for, max, min};
 
         // If `repr_packed` is `None`, there are no alignment constraints, and
@@ -628,8 +658,8 @@ impl DstLayout {
     /// We make no guarantees to the behavior of this method if `self` cannot
     /// appear in a valid Rust type (e.g., because the addition of trailing
     /// padding would lead to a size larger than `isize::MAX`).
-    #[allow(dead_code)]
-    pub(crate) const fn pad_to_align(self) -> Self {
+    #[doc(hidden)]
+    pub const fn pad_to_align(self) -> Self {
         use util::core_layout::_padding_needed_for;
 
         let size_info = match self._size_info {
@@ -6024,7 +6054,7 @@ mod tests {
         assert_impls!(Wrapping<NotZerocopy>: KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
 
         assert_impls!(Unalign<u8>: KnownLayout, FromZeroes, FromBytes, AsBytes, Unaligned);
-        assert_impls!(Unalign<NotZerocopy>: KnownLayout, Unaligned, !FromZeroes, !FromBytes, !AsBytes);
+        assert_impls!(Unalign<NotZerocopy>: Unaligned, !KnownLayout,  !FromZeroes, !FromBytes, !AsBytes);
 
         assert_impls!([u8]: KnownLayout, FromZeroes, FromBytes, AsBytes, Unaligned);
         assert_impls!([NotZerocopy]: !KnownLayout, !FromZeroes, !FromBytes, !AsBytes, !Unaligned);