compiler: fix line numbers in unused variable error messages

compiler/fn.v

@@ -195,6 +195,7 @@ fn (p mut Parser) fn_decl() {
 			ref: is_amp
 			ptr: is_mut
 			line_nr: p.scanner.line_nr
+			scanner_pos: p.scanner.get_scanner_pos()
 		}
 		f.args << receiver
 		f.register_var(receiver)
@@ -539,17 +540,11 @@ fn (p mut Parser) check_unused_variables() {
 			break
 		}
 		if !var.is_used && !p.pref.is_repl && !var.is_arg && !p.pref.translated && var.name != '_' {
-			p.scanner.line_nr = var.line_nr - 1
-			if p.pref.is_prod {
-				p.error('`$var.name` declared and not used')
-			} else {
-				p.warn('`$var.name` declared and not used')
-			}
+			p.production_error('`$var.name` declared and not used', var.scanner_pos )
 		}
 		if !var.is_changed && var.is_mut && !p.pref.is_repl &&
 			!p.pref.translated && var.name != '_' {
-			p.scanner.line_nr = var.line_nr - 1
-			p.error('`$var.name` is declared as mutable, but it was never changed')
+			p.error_with_position( '`$var.name` is declared as mutable, but it was never changed', var.scanner_pos )
 		}
 	}
 }
@@ -723,6 +718,7 @@ fn (p mut Parser) fn_args(f mut Fn) {
 				is_arg: true
 				// is_mut: is_mut
 				line_nr: p.scanner.line_nr
+				scanner_pos: p.scanner.get_scanner_pos()        
 			}
 			// f.register_var(v)
 			f.args << v
@@ -766,6 +762,7 @@ fn (p mut Parser) fn_args(f mut Fn) {
 				is_mut: is_mut
 				ptr: is_mut
 				line_nr: p.scanner.line_nr
+				scanner_pos: p.scanner.get_scanner_pos()        
 			}
 			f.register_var(v)
 			f.args << v

compiler/parser.v

@@ -774,6 +774,25 @@ fn (p &Parser) warn(s string) {
 	println('warning: $p.scanner.file_path:${p.scanner.line_nr+1}: $s')
 }
 
+
+fn (p mut Parser) error_with_position(e string, sp ScannerPos) {
+	p.scanner.goto_scanner_position( sp )
+	p.error( e )
+}
+
+fn (p mut Parser) production_error(e string, sp ScannerPos) {
+	if p.pref.is_prod {
+		p.scanner.goto_scanner_position( sp )
+		p.error( e )
+	}else {
+		// on a warning, restore the scanner state after printing the warning:
+		cpos := p.scanner.get_scanner_pos()
+		p.scanner.goto_scanner_position( sp )
+		p.warn(e)
+		p.scanner.goto_scanner_position( cpos )
+	}
+}
+
 fn (p mut Parser) error(s string) {
 	// Dump all vars and types for debugging
 	if p.pref.is_debug {
@@ -1301,6 +1320,7 @@ fn (p mut Parser) var_decl() {
 		p.fspace()
 	}
 	// println('var decl tok=${p.strtok()} ismut=$is_mut')
+	var_scanner_pos := p.scanner.get_scanner_pos()
 	name := p.check_name()
 	p.var_decl_name = name
 	// Don't allow declaring a variable with the same name. Even in a child scope
@@ -1319,6 +1339,8 @@ fn (p mut Parser) var_decl() {
 		typ: typ
 		is_mut: is_mut
 		is_alloc: p.is_alloc || typ.starts_with('array_')
+		scanner_pos: var_scanner_pos
+		line_nr: var_scanner_pos.line_nr
 	})
 	//if p.is_alloc { println('REG VAR IS ALLOC $name') }
 	p.var_decl_name = ''
@@ -3571,8 +3593,8 @@ fn (p mut Parser) go_statement() {
 
 fn (p mut Parser) register_var(v Var) {
 	if v.line_nr == 0 {
-		//v.line_nr = p.scanner.line_nr
-		p.cur_fn.register_var({ v | line_nr: p.scanner.line_nr })
+		spos := p.scanner.get_scanner_pos()
+		p.cur_fn.register_var({ v | scanner_pos: spos, line_nr: spos.line_nr })
 	} else {
 		p.cur_fn.register_var(v)
 	}

compiler/scanner.v

@@ -61,6 +61,24 @@ fn new_scanner(file_path string) &Scanner {
 	return scanner
 }
 
+
+struct ScannerPos {
+mut:
+   pos int
+   line_nr int
+}
+fn (s ScannerPos) str() string {
+	return 'ScannerPos{ ${s.pos:5d} , ${s.line_nr:5d} }'
+}
+fn (s &Scanner) get_scanner_pos() ScannerPos {
+	return ScannerPos{ pos: s.pos line_nr: s.line_nr } 
+}
+fn (s mut Scanner) goto_scanner_position(scp ScannerPos) {
+	s.pos = scp.pos 
+	s.line_nr = scp.line_nr
+}
+
+
 // TODO remove once multiple return values are implemented
 struct ScanRes {
 	tok Token

compiler/table.v

@@ -71,7 +71,6 @@ mut:
 	ref             bool
 	parent_fn       string // Variables can only be defined in functions
 	mod             string // module where this var is stored
-	line_nr         int
 	access_mod      AccessMod
 	is_global       bool // __global (translated from C only)
 	is_used         bool
@@ -79,6 +78,8 @@ mut:
 	scope_level     int
 	is_c            bool // todo remove once `typ` is `Type`, not string
 	moved           bool
+	scanner_pos     ScannerPos // TODO: use only scanner_pos, remove line_nr
+	line_nr         int
 }
 
 struct Type {