scanner: implement s.trace/2

vlib/v/scanner/scanner.v

@@ -19,12 +19,13 @@ const (
 
 pub struct Scanner {
 pub mut:
-	file_path         string
-	text              string
-	pos               int
-	line_nr           int
+	file_path         string // '/path/to/file.v'
+	file_base         string // 'file.v'
+	text              string // the whole text of the file
+	pos               int    // current position in the file, first character is s.text[0]
+	line_nr           int    // current line number
 	last_nl_pos       int    // for calculating column
-	is_inside_string  bool
+	is_inside_string  bool   // set to true in a string, *at the start* of an $var or ${expr}
 	is_inter_start    bool   // for hacky string interpolation TODO simplify
 	is_inter_end      bool
 	is_enclosed_inter bool
@@ -109,6 +110,7 @@ pub fn new_vet_scanner_file(file_path string, comments_mode CommentsMode, pref &
 	}
 	mut s := new_vet_scanner(raw_text, comments_mode, pref)
 	s.file_path = file_path
+	s.file_base = os.base(file_path)
 	return s
 }
 
@@ -127,6 +129,7 @@ pub fn new_vet_scanner(text string, comments_mode CommentsMode, pref &pref.Prefe
 		is_fmt: pref.is_fmt
 		comments_mode: comments_mode
 		file_path: 'internal_memory'
+		file_base: 'internal_memory'
 	}
 }
 
@@ -1291,3 +1294,9 @@ pub fn (mut s Scanner) codegen(newtext string) {
 		}
 	}
 }
+
+fn (mut s Scanner) trace(fbase string, message string) {
+	if s.file_base == fbase {
+		println('> s.trace | ${fbase:-10s} | $message')
+	}
+}