Update subscript and count for MutableSpan and OutputSpan with @_semantics

Author: meg-gupta

stdlib/public/core/Span/MutableSpan.swift

@@ -236,6 +236,7 @@ extension MutableSpan where Element: ~Copyable {
 extension MutableSpan where Element: ~Copyable {
 
   @_alwaysEmitIntoClient
+  @_semantics("fixed_storage.get_count")
   public var count: Int { _assumeNonNegative(_count) }
 
   @_alwaysEmitIntoClient
@@ -279,7 +280,13 @@ extension MutableSpan where Element: BitwiseCopyable {
 @available(SwiftCompatibilitySpan 5.0, *)
 @_originallyDefinedIn(module: "Swift;CompatibilitySpan", SwiftCompatibilitySpan 6.2)
 extension MutableSpan where Element: ~Copyable {
-
+  // SILOptimizer looks for fixed_storage.check_index semantics for bounds check optimizations.
+  @_semantics("fixed_storage.check_index")
+  @inline(__always)
+  @_alwaysEmitIntoClient
+  internal func _checkIndex(_ position: Index) {
+    _precondition(indices.contains(position), "index out of bounds")
+  }
   /// Accesses the element at the specified position in the `MutableSpan`.
   ///
   /// - Parameter position: The offset of the element to access. `position`
@@ -289,12 +296,12 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   public subscript(_ position: Index) -> Element {
     unsafeAddress {
-      _precondition(indices.contains(position), "index out of bounds")
+      _checkIndex(position)
       return unsafe UnsafePointer(_unsafeAddressOfElement(unchecked: position))
     }
     @lifetime(self: copy self)
     unsafeMutableAddress {
-      _precondition(indices.contains(position), "index out of bounds")
+      _checkIndex(position)
        return unsafe _unsafeAddressOfElement(unchecked: position)
     }
   }

stdlib/public/core/Span/OutputSpan.swift

@@ -193,6 +193,14 @@ extension OutputSpan where Element: ~Copyable {
     unsafe Range(_uncheckedBounds: (0, _count))
   }
 
+  // SILOptimizer looks for fixed_storage.check_index semantics for bounds check optimizations.
+  @_semantics("fixed_storage.check_index")
+  @inline(__always)
+  @_alwaysEmitIntoClient
+  internal func _checkIndex(_ index: Index) {
+    _precondition(indices.contains(index), "Index out of bounds")
+  }
+
   /// Accesses the element at the specified position.
   ///
   /// - Parameter index: A valid index into this span.
@@ -201,12 +209,12 @@ extension OutputSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   public subscript(_ index: Index) -> Element {
     unsafeAddress {
-      _precondition(indices.contains(index), "index out of bounds")
+      _checkIndex(index)
       return unsafe UnsafePointer(_unsafeAddressOfElement(unchecked: index))
     }
     @lifetime(self: copy self)
     unsafeMutableAddress {
-      _precondition(indices.contains(index), "index out of bounds")
+      _checkIndex(index)
       return unsafe _unsafeAddressOfElement(unchecked: index)
     }
   }

stdlib/public/core/Span/Span.swift

@@ -414,6 +414,7 @@ extension Span where Element: ~Copyable {
 @available(SwiftCompatibilitySpan 5.0, *)
 @_originallyDefinedIn(module: "Swift;CompatibilitySpan", SwiftCompatibilitySpan 6.2)
 extension Span where Element: ~Copyable {
+  // SILOptimizer looks for fixed_storage.check_index semantics for bounds check optimizations.
   @_semantics("fixed_storage.check_index")
   @inline(__always)
   @_alwaysEmitIntoClient