zc: add the defer statement

defer expr; and defer { ... } register code that runs when the enclosing
block ends: at the end of the block, and at return, break and continue
that leave it. the tokens are kept at the defer and compiled again at each
exit (block_close, body, kw_return, kw_break, kw_continue), newest first,
before the block's variables are released, so the analyzer sees a deferred
rt.free(p) as the transfer of p where it runs. names mean what they meant
at the defer; values are read when it runs. return, break and continue
inside a deferred statement are errors, reported once. works in unchecked
code and closures (own base per function context).

1 files changed, 10 insertions, 0 deletions

diff --git a/tests/errors/defer_freed_twice.zc b/tests/errors/defer_freed_twice.zc
new file mode 100644
index 0000000..d4fe08d
--- /dev/null
+++ b/tests/errors/defer_freed_twice.zc
@@ -0,0 +1,10 @@
+namespace;
+include rt;
+
+// expect: p: used after it was given away
+fn main() {
+	int* p = rt.new(int);
+	defer rt.free(p);
+	rt.free(p);
+	return 0;
+}