parser, checker, pref: allow getting notified about unused function params

vlib/v/checker/checker.v

@@ -289,7 +289,13 @@ pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
 				ast.Var {
 					if !obj.is_used && obj.name[0] != `_` {
 						if !c.pref.translated && !c.file.is_translated {
-							c.warn('unused variable: `${obj.name}`', obj.pos)
+							if obj.is_arg {
+								if c.pref.show_unused_params {
+									c.note('unused parameter: `${obj.name}`', obj.pos)
+								}
+							} else {
+								c.warn('unused variable: `${obj.name}`', obj.pos)
+							}
 						}
 					}
 					if obj.is_mut && !obj.is_changed && !c.is_builtin_mod && obj.name != 'it' {

vlib/v/checker/tests/unused_param.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/unused_param.vv:5:33: notice: unused parameter: `unused_param`
+    3 | struct Foo {}
+    4 | 
+    5 | fn (unused_receiver Foo) method(unused_param int) {}
+      |                                 ~~~~~~~~~~~~
+    6 | 
+    7 | fn func(unused_param int) {}
+vlib/v/checker/tests/unused_param.vv:7:9: notice: unused parameter: `unused_param`
+    5 | fn (unused_receiver Foo) method(unused_param int) {}
+    6 | 
+    7 | fn func(unused_param int) {}
+      |         ~~~~~~~~~~~~
+    8 | 
+    9 | fn func2(_ int) {}

vlib/v/checker/tests/unused_param.vv

@@ -0,0 +1,13 @@
+#include <stdio.h>
+
+struct Foo {}
+
+fn (unused_receiver Foo) method(unused_param int) {}
+
+fn func(unused_param int) {}
+
+fn func2(_ int) {}
+
+pub fn pub_func(unused_param int) {}
+
+fn C.puts(unused_c_param &char) int

vlib/v/compiler_errors_test.v

@@ -8,6 +8,7 @@ import benchmark
 
 const skip_files = [
 	'non_existing.vv', // minimize commit diff churn, do not remove
+	'vlib/v/checker/tests/unused_param.vv',
 ]
 
 const skip_on_cstrict = [

vlib/v/parser/fn.v

@@ -442,33 +442,6 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		is_variadic = true
 	}
 	params << params_t
-	if !are_params_type_only {
-		for k, param in params {
-			if p.scope.known_var(param.name) {
-				p.error_with_pos('redefinition of parameter `${param.name}`', param.pos)
-				return ast.FnDecl{
-					scope: unsafe { nil }
-				}
-			}
-			is_stack_obj := !param.typ.has_flag(.shared_f) && (param.is_mut || param.typ.is_ptr())
-			p.scope.register(ast.Var{
-				name:          param.name
-				typ:           param.typ
-				is_mut:        param.is_mut
-				is_auto_deref: param.is_mut
-				is_stack_obj:  is_stack_obj
-				pos:           param.pos
-				is_used:       true
-				is_arg:        true
-				ct_type_var:   if (!is_method || k > 0) && param.typ.has_flag(.generic)
-					&& !param.typ.has_flag(.variadic) {
-					.generic_param
-				} else {
-					.no_comptime
-				}
-			})
-		}
-	}
 	// Return type
 	mut return_type_pos := p.tok.pos()
 	mut return_type := ast.void_type
@@ -515,6 +488,34 @@ run them via `v file.v` instead',
 		p.error_with_pos('cannot declare a static function as a receiver method', name_pos)
 	}
 	// Register
+	if !are_params_type_only {
+		for k, param in params {
+			if p.scope.known_var(param.name) {
+				p.error_with_pos('redefinition of parameter `${param.name}`', param.pos)
+				return ast.FnDecl{
+					scope: unsafe { nil }
+				}
+			}
+			is_stack_obj := !param.typ.has_flag(.shared_f) && (param.is_mut || param.typ.is_ptr())
+			p.scope.register(ast.Var{
+				name:          param.name
+				typ:           param.typ
+				is_mut:        param.is_mut
+				is_auto_deref: param.is_mut
+				is_stack_obj:  is_stack_obj
+				pos:           param.pos
+				is_used:       is_pub || no_body
+					|| (is_method && rec.type_pos == param.type_pos) || p.builtin_mod
+				is_arg:        true
+				ct_type_var:   if (!is_method || k > 0) && param.typ.has_flag(.generic)
+					&& !param.typ.has_flag(.variadic) {
+					.generic_param
+				} else {
+					.no_comptime
+				}
+			})
+		}
+	}
 	if is_method {
 		// Do not allow to modify / add methods to types from other modules
 		// arrays/maps dont belong to a module only their element types do

vlib/v/pref/pref.v

@@ -146,6 +146,7 @@ pub mut:
 	show_c_output          bool   // -show-c-output, print all cc output even if the code was compiled correctly
 	show_callgraph         bool   // -show-callgraph, print the program callgraph, in a Graphviz DOT format to stdout
 	show_depgraph          bool   // -show-depgraph, print the program module dependency graph, in a Graphviz DOT format to stdout
+	show_unused_params     bool   // NOTE: temporary until making it a default.
 	dump_c_flags           string // `-dump-c-flags file.txt` - let V store all C flags, passed to the backend C compiler in `file.txt`, one C flag/value per line.
 	dump_modules           string // `-dump-modules modules.txt` - let V store all V modules, that were used by the compiled program in `modules.txt`, one module per line.
 	dump_files             string // `-dump-files files.txt` - let V store all V or .template file paths, that were used by the compiled program in `files.txt`, one path per line.
@@ -949,6 +950,9 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 				res.parse_line_info(res.line_info)
 				i++
 			}
+			'-check-unused-fn-args' {
+				res.show_unused_params = true
+			}
 			'-use-coroutines' {
 				res.use_coroutines = true
 				$if macos || linux {