[Feature] Add empty separator interface and doc

Sources/OpenSwiftUI/Layout/LayoutFundamentals/Separators/Divider.swift

@@ -0,0 +1,14 @@
+//
+//  Divider.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: TODO
+
+/// A visual element that can be used to separate other content.
+///
+/// When contained in a stack, the divider extends across the minor axis of the
+/// stack, or horizontally when not in a stack.
+public struct Divider: UnaryView, PrimitiveView {
+    public init() {}
+}

Sources/OpenSwiftUI/Layout/LayoutFundamentals/Separators/Spacer.swift

@@ -0,0 +1,124 @@
+//
+//  Spacer.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: TODO
+
+import Foundation
+
+/// A flexible space that expands along the major axis of its containing stack
+/// layout, or on both axes if not contained in a stack.
+///
+/// A spacer creates an adaptive view with no content that expands as much as
+/// it can. For example, when placed within an ``HStack``, a spacer expands
+/// horizontally as much as the stack allows, moving sibling views out of the
+/// way, within the limits of the stack's size.
+/// OpenSwiftUI sizes a stack that doesn't contain a spacer up to the combined
+/// ideal widths of the content of the stack's child views.
+///
+/// The following example provides a simple checklist row to illustrate how you
+/// can use a spacer:
+///
+///     struct ChecklistRow: View {
+///         let name: String
+///
+///         var body: some View {
+///             HStack {
+///                 Image(systemName: "checkmark")
+///                 Text(name)
+///             }
+///             .border(Color.blue)
+///         }
+///     }
+///
+/// ![A figure of a blue rectangular border that marks the boundary of an
+/// HStack, wrapping a checkmark image to the left of the name Megan. The
+/// checkmark and name are centered vertically and separated by system
+/// standard-spacing within the stack.](Spacer-1.png)
+///
+/// Adding a spacer before the image creates an adaptive view with no content
+/// that expands to push the image and text to the right side of the stack.
+/// The stack also now expands to take as much space as the parent view allows,
+/// shown by the blue border that indicates the boundary of the stack:
+///
+///     struct ChecklistRow: View {
+///         let name: String
+///
+///         var body: some View {
+///             HStack {
+///                 Spacer()
+///                 Image(systemName: "checkmark")
+///                 Text(name)
+///             }
+///             .border(Color.blue)
+///         }
+///     }
+///
+/// ![A figure of a blue rectangular border that marks the boundary of an
+/// HStack, wrapping a checkmark image to the left of the name Megan. The
+/// checkmark and name are centered vertically, separated by system-standard
+/// spacing, and pushed to the right side of the stack.](Spacer-2.png)
+///
+/// Moving the spacer between the image and the name pushes those elements to
+/// the left and right sides of the ``HStack``, respectively. Because the stack
+/// contains the spacer, it expands to take as much horizontal space as the
+/// parent view allows; the blue border indicates its size:
+///
+///     struct ChecklistRow: View {
+///         let name: String
+///
+///         var body: some View {
+///             HStack {
+///                 Image(systemName: "checkmark")
+///                 Spacer()
+///                 Text(name)
+///             }
+///             .border(Color.blue)
+///         }
+///     }
+///
+/// ![A figure of a blue rectangular border that marks the boundary of an
+/// HStack, wrapping a checkmark image to the left of the name Megan. The
+/// checkmark and name are centered vertically, with the checkmark on the
+/// left edge of the stack, and the text on the right side of the
+/// stack.](Spacer-3.png)
+///
+/// Adding two spacer views on the outside of the stack leaves the image and
+/// text together, while the stack expands to take as much horizontal space
+/// as the parent view allows:
+///
+///     struct ChecklistRow: View {
+///         let name: String
+///
+///         var body: some View {
+///             HStack {
+///                 Spacer()
+///                 Image(systemName: "checkmark")
+///                 Text(name)
+///                 Spacer()
+///             }
+///             .border(Color.blue)
+///         }
+///     }
+///
+/// ![A figure of a blue rectangular border marks the boundary of an HStack,
+/// wrapping a checkmark image to the left of text spelling the name Megan.
+/// The checkmark and name are centered vertically, separated by
+/// system-standard spacing, and centered horizontally
+/// in the stack.](Spacer-4.png)
+@frozen
+public struct Spacer {
+    /// The minimum length this spacer can be shrunk to, along the axis or axes
+    /// of expansion.
+    ///
+    /// If `nil`, the system default spacing between views is used.
+    public var minLength: CGFloat?
+
+    @inlinable
+    public init(minLength: CGFloat? = nil) {
+        self.minLength = minLength
+    }
+}
+
+extension Spacer: PrimitiveView {}

Sources/OpenSwiftUI/OpenSwiftUI.docc/Layout/LayoutFundamentals/Layout-Fundamentals.md

@@ -15,3 +15,8 @@ custom layout containers, see <doc:Custom-Layout>. For design guidance, see
 [Layout](https://developer.apple.com/design/human-interface-guidelines/foundations/layout) in the Human Interface Guidelines.
 
 ## Topics
+
+### Separators
+
+- ``Spacer``
+- ``Divider``