pref: add -check-syntax for just parsing files, without checking them

vlib/v/builder/c.v

@@ -13,6 +13,9 @@ pub fn (mut b Builder) gen_c(v_files []string) string {
 	t1 := time.ticks()
 	parse_time := t1 - t0
 	b.info('PARSE: ${parse_time}ms')
+	if b.pref.only_check_syntax {
+		return ''
+	}
 	//
 	b.generic_struct_insts_to_concrete()
 	b.checker.check_files(b.parsed_files)

vlib/v/builder/cc.v

@@ -76,6 +76,12 @@ fn (mut v Builder) cc() {
 	if v.pref.is_verbose {
 		println('builder.cc() pref.out_name="$v.pref.out_name"')
 	}
+	if v.pref.only_check_syntax {
+		if v.pref.is_verbose {
+			println('builder.cc returning early, since pref.only_check_syntax is true')
+		}
+		return
+	}
 	v.build_thirdparty_obj_files()
 	vexe := pref.vexe_path()
 	vdir := os.dir(vexe)

vlib/v/pref/pref.v

@@ -115,6 +115,7 @@ pub mut:
 	is_parallel         bool
 	error_limit         int
 	is_vweb             bool // skip _ var warning in templates
+	only_check_syntax   bool // when true, just parse the files, then stop, before running checker
 }
 
 pub fn parse_args(args []string) (&Preferences, string) {
@@ -126,6 +127,9 @@ pub fn parse_args(args []string) (&Preferences, string) {
 		arg := args[i]
 		current_args := args[i..]
 		match arg {
+			'-check-syntax' {
+				res.only_check_syntax = true
+			}
 			'-v' {
 				res.is_verbose = true
 			}