:; xcrun swift
"crashlog" and "save_crashlog" command installed, use the "--help" option for detailed help
"malloc_info", "ptr_refs", "cstr_refs", "find_variable", and "objc_refs" commands have been installed, use the "--help" options on these commands for detailed help.
Welcome to Apple Swift version 4.2 (swiftlang-1000.0.16.4 clang-1000.0.25.3). Type :help for assistance.
  1> @available(swift, deprecated: 4.2) func f() { print("boo") }
  2> f()
boo
  3>  

:; xcrun swift -deprecated-integrated-repl
***  You are running Swift's integrated REPL,  ***
***  intended for compiler and stdlib          ***
***  development and testing purposes only.    ***
***  The full REPL is built as part of LLDB.   ***
***  Type ':help' for assistance.              ***
(swift) @available(swift, deprecated: 4.2) func f() { print("boo") }
(swift) f()
<REPL Input>:1:1: warning: 'f()' is deprecated
f()
^
boo
(swift)