checker: disallow function cast outside unsafe (#16030)

vlib/v/checker/checker.v

@@ -2546,6 +2546,12 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		c.error('casting numbers to enums, should be done inside `unsafe{}` blocks', node.pos)
 	}
 
+	if final_to_sym.kind == .function && final_from_sym.kind == .function && !(c.inside_unsafe
+		|| c.file.is_translated) && !c.check_matching_function_symbols(final_from_sym, final_to_sym) {
+		c.error('casting a function value from one function signature, to another function signature, should be done inside `unsafe{}` blocks',
+			node.pos)
+	}
+
 	if to_type == ast.string_type {
 		if from_type in [ast.u8_type, ast.bool_type] {
 			snexpr := node.expr.str()

vlib/v/checker/tests/function_cast_outside_unsafe_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/function_cast_outside_unsafe_err.vv:6:10: error: casting a function value from one function signature, to another function signature, should be done inside `unsafe{}` blocks
+    4 | fn main(){
+    5 |     f := fn(){}
+    6 |     println(FnB(f))  // not FnA()
+      |             ~~~~~~
+    7 | }

vlib/v/checker/tests/function_cast_outside_unsafe_err.vv

@@ -0,0 +1,7 @@
+type FnA = fn()
+type FnB = fn(int)
+
+fn main(){
+	f := fn(){}
+	println(FnB(f))  // not FnA()
+}