clean up empty lines in errors; simplify source line tracking

vlib/compiler/compile_errors.v

@@ -95,11 +95,11 @@ fn (s &Scanner) error_with_col(msg string, col int) {
 	// to find the source file, when the IDE has a different working folder than v itself.
 	eprintln('${fullpath}:${s.line_nr + 1}:${col}: $final_message')
 	
-	if s.should_print_line_on_error && s.file_lines.len > 0 {
-		context_start_line := imax(0,                (s.line_nr - error_context_before + 1 ))
-		context_end_line   := imin(s.file_lines.len, (s.line_nr + error_context_after + 1 ))
+	if s.should_print_line_on_error && s.nlines > 0 {
+		context_start_line := imax(0,          (s.line_nr - error_context_before + 1 ))
+		context_end_line   := imin(s.nlines-1, (s.line_nr + error_context_after  + 1 ))
 		for cline := context_start_line; cline < context_end_line; cline++ {
-			line := '${(cline+1):5d}| ' + s.file_lines[ cline ]
+			line := '${(cline+1):5d}| ' + s.line( cline )
 			coloredline := if cline == s.line_nr && color_on { term.red(line) } else { line }
 			eprintln( coloredline )
 			if cline != s.line_nr { continue }
@@ -225,9 +225,8 @@ fn (s mut Scanner) get_scanner_pos_of_token(t &Token) ScannerPos {
 	// Starting from the start, scan the source lines
 	// till the desired tline is reached, then
 	// s.pos + tcol would be the proper position
-	// of the token. Continue scanning for some more lines of context too.
+	// of the token.
 	s.goto_scanner_position(ScannerPos{})
-	s.file_lines = []string
 
 	mut prevlinepos := 0
 	// NB: TCC BUG workaround: removing the `mut ate:=0 ate++` line
@@ -251,15 +250,13 @@ fn (s mut Scanner) get_scanner_pos_of_token(t &Token) ScannerPos {
 	for {
 		prevlinepos = s.pos
 		if s.pos >= s.text.len { break }		
-		if s.line_nr > tline + 10 { break }
+		if s.line_nr > tline { break }
 		////////////////////////////////////////
 		if tline == s.line_nr {
 			sptoken = s.get_scanner_pos()
 			sptoken.pos += tcol
 		}
 		s.ignore_line() s.eat_single_newline()
-		sline := s.text.substr( prevlinepos, s.pos )//.trim_right('\r\n')
-		s.file_lines << sline
 	}
 	//////////////////////////////////////////////////
 	s.goto_scanner_position(cpos)

vlib/compiler/scanner.v

@@ -39,7 +39,8 @@ mut:
 	should_print_errors_in_color bool
 	should_print_relative_paths_on_error bool
 	quote byte // which quote is used to denote current string: ' or "
-	file_lines   []string // filled *only on error* by rescanning the source till the error (and several lines more)
+	line_ends   []int // the positions of source lines ends   (i.e. \n signs)
+	nlines int  // total number of lines in the source file that were scanned
 }
 
 // new scanner from file.
@@ -632,7 +633,7 @@ fn (s mut Scanner) ident_string() string {
 			s.inc_line_number()
 		}
 		// Don't allow \0
-		if c == `0` && s.pos > 2 && s.text[s.pos - 1] == `\\` {
+		if c == `0` && s.pos > 2 && s.text[s.pos - 1] == slash {
 			s.error('0 character in a string literal')
 		}
 		// Don't allow \x00
@@ -640,14 +641,14 @@ fn (s mut Scanner) ident_string() string {
 			s.error('0 character in a string literal')
 		}
 		// ${var}
-		if c == `{` && prevc == `$` && s.count_symbol_before(s.pos-2, `\\`) % 2 == 0 {
+		if c == `{` && prevc == `$` && s.count_symbol_before(s.pos-2, slash) % 2 == 0 {
 			s.inside_string = true
 			// so that s.pos points to $ at the next step
 			s.pos -= 2
 			break
 		}
 		// $var
-		if (c.is_letter() || c == `_`) && prevc == `$` && s.count_symbol_before(s.pos-2, `\\`) % 2 == 0 {
+		if (c.is_letter() || c == `_`) && prevc == `$` && s.count_symbol_before(s.pos-2, slash) % 2 == 0 {
 			s.inside_string = true
 			s.inter_start = true
 			s.pos -= 2
@@ -748,7 +749,7 @@ fn (s mut Scanner) debug_tokens() {
 
 
 fn (s mut Scanner) ignore_line() {
-	s.eat_to_end_of_line()
+	s.eat_to_end_of_line()	
 	s.inc_line_number()
 }
 
@@ -761,6 +762,21 @@ fn (s mut Scanner) eat_to_end_of_line(){
 fn (s mut Scanner) inc_line_number() {
 	s.last_nl_pos = s.pos
 	s.line_nr++
+	s.line_ends   << s.pos
+	s.nlines++
+}
+
+fn (s Scanner) line(n int) string {
+	mut res := ''
+	if n >= 0 &&
+		n < s.line_ends.len {
+		nline_start := if n == 0 { 0 } else { s.line_ends[ n - 1 ] }
+		nline_end   := s.line_ends[n]
+		if nline_start <= nline_end {
+			res = s.text.substr( nline_start, nline_end )
+		}
+	}	
+	return res.trim_right('\r\n').trim_left('\r\n')
 }
 
 fn is_name_char(c byte) bool {