Introduce explicit copy (#15330)

tests/arc/tcopytosink_warning.nim

@@ -0,0 +1,22 @@
+discard """
+  cmd: "nim c --gc:arc $file"
+  nimout: '''tcopytosink_warning.nim(17, 7) Hint: myhint [User]
+tcopytosink_warning.nim(19, 9) Hint: passing 'x' to a sink parameter introduces an implicit copy; if possible, rearrange your program's control flow to prevent it or use 'copy(x)' to hint the compiler it is intentional [Performance]
+'''
+  output: "x"
+"""
+import macros
+
+proc test(v: var seq[string], x: sink string) =
+  v.add x
+
+var v = @["a", "b", "c"]
+var x = "x"
+
+static:
+  hint("myhint")
+test(v, copy(x)) # no warning
+test(v, x)  # produces warning
+
+echo x  # use after sink
+