Initial commit of DstLayout::for_dst

This will be used by the custom derive of `KnownLayout` to compute the
`DstLayout` for unsized types.

Makes progress on #29

Author: joshlf

src/lib.rs

@@ -427,6 +427,52 @@ impl DstLayout {
         }
     }
 
+    /// Constructs a `DstLayout` which describes a dynamically-sized type with
+    /// the given properties.
+    ///
+    /// # Safety
+    ///
+    /// Unsafe code may assume that the returned `DstLayout` is the correct
+    /// layout for the described type so long as:
+    /// - The type has the alignment `align`
+    /// - The type has at least one field
+    /// - The byte offset of the first byte of the trailing field is equal to
+    ///   `trailing_field_offset`
+    /// - The trailing field's layout is correctly described by
+    ///   `trailing_field_layout`
+    const fn for_dst(
+        align: NonZeroUsize,
+        trailing_field_offset: usize,
+        trailing_field_layout: DstLayout,
+    ) -> Option<DstLayout> {
+        Some(DstLayout {
+            // SAFETY: The caller has promised that this is the correct
+            // alignment.
+            _align: align,
+            _size_info: match trailing_field_layout._size_info {
+                SizeInfo::Sized { _size } => {
+                    let without_padding = match trailing_field_offset.checked_add(_size) {
+                        Some(without_padding) => without_padding,
+                        None => return None,
+                    };
+
+                    let padding = util::core_layout::_padding_needed_for(without_padding, align);
+                    // SAFETY: TODO
+                    SizeInfo::Sized { _size: without_padding + padding }
+                }
+                SizeInfo::SliceDst(TrailingSliceLayout { _offset, _elem_size }) => {
+                    let offset = match trailing_field_offset.checked_add(_offset) {
+                        Some(offset) => offset,
+                        None => return None,
+                    };
+
+                    // SAFETY: TODO
+                    SizeInfo::SliceDst(TrailingSliceLayout { _offset: offset, _elem_size })
+                }
+            },
+        })
+    }
+
     /// Validates that a cast is sound from a layout perspective.
     ///
     /// Validates that the size and alignment requirements of a type with the
@@ -3684,6 +3730,20 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_dst_layout_for_dst() {
+        macro_rules! test {
+            ($align:expr, $trailing_field_offset:expr, $trailing_field_layout:expr, $expect:expr) => {{
+                let align = NonZeroUsize::new($align).unwrap();
+                let layout =
+                    DstLayout::for_dst(align, $trailing_field_offset, $trailing_field_layout);
+                assert_eq!(layout, $expect);
+            }};
+        }
+
+        // TODO: Write tests.
+    }
+
     // This test takes a long time when running under Miri, so we skip it in
     // that case. This is acceptable because this is a logic test that doesn't
     // attempt to expose UB.