checker: add a hard to reach limit of 1_000_000 iterations for resolving all generics (usually <10 are enough) (#18477)

vlib/v/checker/checker.v

@@ -14,12 +14,14 @@ import v.errors
 import v.pkgconfig
 
 const (
-	int_min                  = int(0x80000000)
-	int_max                  = int(0x7FFFFFFF)
+	int_min                                        = int(0x80000000)
+	int_max                                        = int(0x7FFFFFFF)
 	// prevent stack overflows by restricting too deep recursion:
-	expr_level_cutoff_limit  = 40
-	stmt_level_cutoff_limit  = 40
-	iface_level_cutoff_limit = 100
+	expr_level_cutoff_limit                        = 40
+	stmt_level_cutoff_limit                        = 40
+	iface_level_cutoff_limit                       = 100
+	generic_fn_cutoff_limit_per_fn                 = 10_000 // how many times post_process_generic_fns, can visit the same function before bailing out
+	generic_fn_postprocess_iterations_cutoff_limit = 1000_000 // how many times the compiler will try to resolve all remaining generic functions
 )
 
 pub const (
@@ -323,7 +325,7 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 	// is needed when the generic type is auto inferred from the call argument.
 	// we may have to loop several times, if there were more concrete types found.
 	mut post_process_generic_fns_iterations := 0
-	for {
+	post_process_iterations_loop: for post_process_generic_fns_iterations <= checker.generic_fn_postprocess_iterations_cutoff_limit {
 		$if trace_post_process_generic_fns_loop ? {
 			eprintln('>>>>>>>>> recheck_generic_fns loop iteration: ${post_process_generic_fns_iterations}')
 		}
@@ -334,7 +336,7 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 						file.generic_fns.map(it.name).str())
 				}
 				c.change_current_file(file)
-				c.post_process_generic_fns()
+				c.post_process_generic_fns() or { break post_process_iterations_loop }
 			}
 		}
 		if !c.need_recheck_generic_fns {

vlib/v/checker/fn.v

@@ -2210,16 +2210,23 @@ fn (mut c Checker) set_node_expected_arg_types(mut node ast.CallExpr, func &ast.
 	}
 }
 
-fn (mut c Checker) post_process_generic_fns() {
+fn (mut c Checker) post_process_generic_fns() ! {
+	mut all_generic_fns := map[string]int{}
 	// Loop thru each generic function concrete type.
 	// Check each specific fn instantiation.
 	for i in 0 .. c.file.generic_fns.len {
 		mut node := c.file.generic_fns[i]
 		c.mod = node.mod
 		fkey := node.fkey()
+		all_generic_fns[fkey]++
+		if all_generic_fns[fkey] > generic_fn_cutoff_limit_per_fn {
+			c.error('generic function visited more than ${generic_fn_cutoff_limit_per_fn} times',
+				node.pos)
+			return error('fkey: ${fkey}')
+		}
 		gtypes := c.table.fn_generic_types[fkey]
 		$if trace_post_process_generic_fns ? {
-			eprintln('> post_process_generic_fns ${node.mod} | ${node.name} | fkey: ${fkey} | gtypes: ${gtypes}')
+			eprintln('> post_process_generic_fns ${node.mod} | ${node.name} | fkey: ${fkey} | gtypes: ${gtypes} | c.file.generic_fns.len: ${c.file.generic_fns.len}')
 		}
 		for concrete_types in gtypes {
 			c.table.cur_concrete_types = concrete_types

vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv:6:2: warning: unused variable: `ff1`
+    4 | 
+    5 | fn main() {
+    6 |     ff1 := f1 // <-- missing e.g. `[int]`
+      |     ~~~
+    7 |     // ff1 := f1[int] <-- is a valid usage with generic return types that are not generic structs
+    8 | }
+vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv:1:1: error: generic function visited more than 10000 times
+    1 | fn f1[T](x T, i int) T {
+      | ~~~~~~~~~~~~~~~~~~~~~~
+    2 |     return x
+    3 | }

vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv

@@ -0,0 +1,8 @@
+fn f1[T](x T, i int) T {
+	return x
+}
+
+fn main() {
+	ff1 := f1 // <-- missing e.g. `[int]`
+	// ff1 := f1[int] <-- is a valid usage with generic return types that are not generic structs
+}