fixes #6049 add mixin release, acquire to withLock (#6113)

lib/core/locks.nim

@@ -59,6 +59,7 @@ proc signal*(cond: var Cond) {.inline.} =
 template withLock*(a: Lock, body: untyped) =
   ## Acquires the given lock, executes the statements in body and
   ## releases the lock after the statements finish executing.
+  mixin acquire, release
   a.acquire()
   {.locks: [a].}:
     try:

tests/stdlib/tlocks.nim

@@ -0,0 +1,10 @@
+discard """
+  output: '''3'''
+  cmd: "nim $target --threads:on $options $file"
+"""
+
+#bug #6049
+import uselocks
+
+var m = createMyType[int]()
+echo $m.use()

tests/stdlib/uselocks.nim

@@ -0,0 +1,11 @@
+import locks
+
+type MyType* [T] = object
+  lock: Lock
+
+proc createMyType*[T]: MyType[T] =
+  initLock(result.lock)
+
+proc use* (m: var MyType): int =
+  withLock m.lock:
+    result = 3