[stdlib] add availability annotations and tests

Author: glessard

stdlib/public/core/MutableCollection.swift

@@ -238,6 +238,7 @@ extension MutableCollection {
   ///   the range must be valid indices of the collection.
   ///
   /// - Complexity: O(1)
+  @available(*, unavailable)
   @inlinable
   public subscript(bounds: Range<Index>) -> Slice<Self> {
     get {
@@ -282,7 +283,7 @@ extension MutableCollection {
   }
 }
 
-@available(SwiftStdlib 5.5, *)
+@available(SwiftStdlib 9999, *)
 extension MutableCollection where SubSequence == Slice<Self> {
 
   /// Accesses a contiguous subrange of the collection's elements.
@@ -310,6 +311,7 @@ extension MutableCollection where SubSequence == Slice<Self> {
   ///
   /// - Complexity: O(1)
   @inlinable
+  @_alwaysEmitIntoClient
   public subscript(bounds: Range<Index>) -> Slice<Self> {
     get {
       _failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)

validation-test/stdlib/CollectionDiagnostics.swift

@@ -173,6 +173,33 @@ struct MutableCollectionWithNonDefaultSubSequence: MutableCollection {
   }
 }
 
+struct MutableCollectionWithDefaultSubSequence: MutableCollection {
+  public var startIndex: Int
+  public var endIndex: Int
+
+  public func index(after i: Int) -> Int { i+1 }
+  public subscript(position: Int) -> Int {
+    get { position }
+    set { _ = newValue }
+  }
+}
+
+func subscriptMutableCollectionIgnored() {
+  let cs: MutableCollectionWithNonDefaultSubSequence
+  cs = .init(startIndex: 0, endIndex: 10)
+
+  let badSlice: Slice<MutableCollectionWithNonDefaultSubSequence>
+  badSlice = cs[0..<2] // expected-error {{'subscript(_:)' is unavailable}}
+  cs[3..<5] = badSlice // expected-error {{'subscript(_:)' is unavailable}}
+
+  let ds: MutableCollectionWithDefaultSubSequence
+  ds = .init(startIndex: 0, endIndex: 10)
+
+  let goodSlice: Slice<MutableCollectionWithDefaultSubSequence>
+  goodSlice = ds[0..<2]
+  ds[3..<5] = goodSlice
+}
+
 // FIXME: Remove -verify-ignore-unknown.
 // <unknown>:0: error: unexpected note produced: possibly intended match
 // <unknown>:0: error: unexpected note produced: possibly intended match