[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