Add _assumeNonNegative to OutputSpan's count as well

Author: meg-gupta

stdlib/public/core/Span/OutputSpan.swift

@@ -83,7 +83,7 @@ extension OutputSpan where Element: ~Copyable {
   /// The number of initialized elements in this span.
   @_alwaysEmitIntoClient
   @_semantics("fixed_storage.get_count")
-  public var count: Int { _count }
+  public var count: Int { _assumeNonNegative(_count) }
 
   /// The number of additional elements that can be added to this span.
   @_alwaysEmitIntoClient
@@ -224,7 +224,7 @@ extension OutputSpan where Element: ~Copyable {
   /// The range of initialized positions for this `OutputSpan`.
   @_alwaysEmitIntoClient
   public var indices: Range<Index> {
-    unsafe Range(_uncheckedBounds: (0, _count))
+    unsafe Range(_uncheckedBounds: (0, count))
   }
 
   // SILOptimizer looks for fixed_storage.check_index semantics for bounds check optimizations.

test/SILOptimizer/mutable_span_bounds_check_tests.swift

@@ -237,3 +237,12 @@ public func output_span_init_all_bitwisecopyable<T: P & BitwiseCopyable>(_ os: i
   }
 }
 
+// CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests22outputspan_get_elementySis10OutputSpanVySiG_SitF :
+// CHECK-SIL:   [[REG1:%.*]] = struct_extract %0, #OutputSpan._count
+// CHECK-SIL:   [[REG2:%.*]] = struct_extract [[REG1]], #Int._value
+// CHECK-SIL:   [[REG3:%.*]] = builtin "assumeNonNegative_Int64"([[REG2]]) : $Builtin.Int64
+// CHECK-SIL-LABEL:} // end sil function '$s31mutable_span_bounds_check_tests22outputspan_get_elementySis10OutputSpanVySiG_SitF'
+public func outputspan_get_element(_ v: borrowing OutputSpan<Int>, _ i: Int) -> Int {
+  return v[i]
+}
+