compiler: add `[..2]` & `[2..]` support for slices

vlib/builtin/array.v

@@ -146,6 +146,12 @@ pub fn (s array) right(n int) array {
 	return s.slice(n, s.len)
 }
 
+// used internally for [2..4]
+fn (s array) slice2(start, _end int, end_max bool) array {
+	end := if end_max { s.len } else { _end }
+	return s.slice(start, end)
+}
+
 pub fn (s array) slice(start, _end int) array {
 	mut end := _end
 	if start > end {

vlib/builtin/array_test.v

@@ -118,10 +118,13 @@ fn test_right() {
 	a := [1, 2, 3, 4]
 	b := a.right(1)
 	c := a[1..a.len]
+	d := a[1..]
 	assert b[0] == 2
 	assert b[1] == 3
 	assert c[0] == 2
 	assert c[1] == 3
+	assert d[0] == 2
+	assert d[1] == 3
 }
 
 fn test_right_with_n_bigger_than_array_size() {
@@ -142,10 +145,13 @@ fn test_left() {
 	a := [1, 2, 3]
 	b := a.left(2)
 	c := a[0..2]
+	d := a[..2]
 	assert b[0] == 1
 	assert b[1] == 2
 	assert c[0] == 1
 	assert c[1] == 2
+	assert d[0] == 1
+	assert d[1] == 2
 }
 
 fn test_slice() {

vlib/builtin/string.v

@@ -394,6 +394,12 @@ pub fn (s string) right(n int) string {
 	return s.substr(n, s.len)
 }
 
+// used internally for [2..4]
+fn (s string) substr2(start, _end int, end_max bool) string {
+	end := if end_max { s.len } else { _end }
+	return s.substr(start, end)
+}
+
 // substr
 pub fn (s string) substr(start, end int) string {
 	if start > end || start > s.len || end > s.len || start < 0 || end < 0 {

vlib/builtin/string_test.v

@@ -210,6 +210,10 @@ fn test_runes() {
 	assert s2.substr(1, 4) == 'riv'
 	assert s2[1..4].len == 3
 	assert s2[1..4] == 'riv'
+	assert s2[..4].len == 4
+	assert s2[..4] == 'priv'
+	assert s2[2..].len == 4
+	assert s2[2..] == 'ivet'
 	assert u.substr(1, 4).len == 6
 	assert u.substr(1, 4) == 'рив'
 	assert s2.substr(1, 2) == 'r'

vlib/compiler/gen_c.v

@@ -197,7 +197,7 @@ fn (p mut Parser) index_get(typ string, fn_ph int, cfg IndexConfig) {
 		else {
 			ref := if cfg.is_ptr { '*' } else { '' }
 			if cfg.is_slice {
-				p.gen(' array_slice($ref $index_expr) ')
+				p.gen(' array_slice2($ref $index_expr) ')
 			}	
 			else {
 				p.gen('( *($typ*) array_get($ref $index_expr) )')
@@ -206,7 +206,7 @@ fn (p mut Parser) index_get(typ string, fn_ph int, cfg IndexConfig) {
 	}
 	else if cfg.is_str && !p.builtin_mod {
 		if cfg.is_slice {
-			p.gen('string_substr($index_expr)')
+			p.gen('string_substr2($index_expr)')
 		} else {
 			p.gen('string_at($index_expr)')
 		}

vlib/compiler/parser.v

@@ -2155,14 +2155,21 @@ fn (p mut Parser) index_expr(typ_ string, fn_ph int) string {
 		}
 		// expression inside [ ]
 		if is_arr || is_str {
-			index_pos := p.cgen.cur_line.len
-			T := p.table.find_type(p.expression())
-			// Allows only i8-64 and byte-64 to be used when accessing an array
-			if T.parent != 'int' && T.parent != 'u32' {
-				p.check_types(T.name, 'int')
+			// [2..
+			if p.tok != .dotdot {
+				index_pos := p.cgen.cur_line.len
+				T := p.table.find_type(p.expression())
+				// Allows only i8-64 and byte-64 to be used when accessing an array
+				if T.parent != 'int' && T.parent != 'u32' {
+					p.check_types(T.name, 'int')
+				}
+				if p.cgen.cur_line.right(index_pos).replace(' ', '').int() < 0 {
+					p.error('cannot access negative array index')
+				}
 			}
-			if p.cgen.cur_line.right(index_pos).replace(' ', '').int() < 0 {
-				p.error('cannot access negative array index')
+			// [..
+			else {
+				p.gen('0')
 			}
 			if p.tok == .dotdot {
 				if is_arr {
@@ -2175,7 +2182,15 @@ fn (p mut Parser) index_expr(typ_ string, fn_ph int) string {
 				is_slice = true
 				p.next()
 				p.gen(',')
-				p.check_types(p.expression(), 'int')
+				// ..4]
+				if p.tok != .rsbr {
+					p.check_types(p.expression(), 'int')
+					p.gen(', false')
+				}
+				// ..]
+				else {
+					p.gen('-1, true')
+				}
 			}	
 		}
 		else {