core: Fix restoration of loop target in try statement

old_loop_target is only assigned if ctx.loop_target is overwritten,
meaning that if ctx.loop_target is never overwritten, ctx.loop_target
will always be overwritten to None.

We fix this by only restoring from old_loop_target if we previously
assigned to old_loop_target.

nac3core/src/codegen/stmt.rs

@@ -789,8 +789,7 @@ pub fn gen_try<'ctx, 'a, G: CodeGenerator>(
         ctx.outer_catch_clauses = old_clauses;
         ctx.unwind_target = old_unwind;
         ctx.return_target = old_return;
-        ctx.loop_target = old_loop_target;
-        old_loop_target = None;
+        ctx.loop_target = old_loop_target.or(ctx.loop_target).take();
 
         let old_unwind = if !finalbody.is_empty() {
             let final_landingpad = ctx.ctx.append_basic_block(current_fun, "try.catch.final");
@@ -911,7 +910,7 @@ pub fn gen_try<'ctx, 'a, G: CodeGenerator>(
         }
 
         ctx.unwind_target = old_unwind;
-        ctx.loop_target = old_loop_target;
+        ctx.loop_target = old_loop_target.or(ctx.loop_target).take();
         ctx.return_target = old_return;
 
         ctx.builder.position_at_end(landingpad);

nac3standalone/demo/src/loop_try_break.py

@@ -0,0 +1,33 @@
+# Break within try statement within a loop
+# Taken from https://book.pythontips.com/en/latest/for_-_else.html
+
+@extern
+def output_int32(x: int32, newline: bool=True):
+	...
+
+@extern
+def output_float64(x: float, newline: bool=True):
+	...
+
+@extern
+def output_str(x: str, newline: bool=True):
+	...
+
+def run() -> int32:
+	for n in range(2, 10):
+		for x in range(2, n):
+			try:
+				if n % x == 0:
+					output_int32(n, newline=False)
+					output_str(" equals ", newline=False)
+					output_int32(x, newline=False)
+					output_str(" * ", newline=False)
+					output_float64(n / x)
+			except:  # Assume this is intended to catch x == 0
+				break
+		else:
+			# loop fell through without finding a factor
+			output_int32(n, newline=False)
+			output_str(" is a prime number")
+
+	return 0