json.decode: add check for shared variable (#18237)

vlib/v/checker/fn.v

@@ -635,6 +635,9 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 			}
 		}
 		panic('unreachable')
+	} else if node.args.len > 0 && fn_name == 'json.encode' && node.args[0].typ.has_flag(.shared_f) {
+		c.error('json.encode cannot handle shared data', node.pos)
+		return ast.void_type
 	} else if node.args.len > 0 && fn_name == 'json.decode' {
 		if node.args.len != 2 {
 			c.error("json.decode expects 2 arguments, a type and a string (e.g `json.decode(T, '')`)",

vlib/v/checker/tests/json_decode_shared_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/json_decode_shared_err.vv:6:16: error: json.encode cannot handle shared data
+    4 |     shared data := [1, 2, 3]
+    5 |     rlock data {
+    6 |         println(json.encode(data))
+      |                      ~~~~~~~~~~~~
+    7 |     }
+    8 | }

vlib/v/checker/tests/json_decode_shared_err.vv

@@ -0,0 +1,8 @@
+import json
+
+fn main() {
+	shared data := [1, 2, 3]
+	rlock data {
+		println(json.encode(data))
+	}
+}