support s[x..y] used in the gcc/clang backtrace expansion in builtin

vlib/builtin/backtraces_nix.c.v

@@ -67,13 +67,10 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 					return false
 				}
 				nr_actual_frames := nr_ptrs - skipframes
-				mut sframes := []string{}
 				//////csymbols := backtrace_symbols(*voidptr(&buffer[skipframes]), nr_actual_frames)
 				csymbols := C.backtrace_symbols(voidptr(&buffer[skipframes]), nr_actual_frames)
 				for i in 0 .. nr_actual_frames {
-					sframes << unsafe { tos2(&u8(csymbols[i])) }
-				}
-				for sframe in sframes {
+					sframe := unsafe { tos2(&u8(csymbols[i])) }
 					executable := sframe.all_before('(')
 					addr := sframe.all_after('[').all_before(']')
 					beforeaddr := sframe.all_before('[')
@@ -92,7 +89,7 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 							output += tos(bp, vstrlen(bp))
 						}
 					}
-					output = output.trim_space() + ':'
+					output = output.trim_chars(' \t\n', .trim_both) + ':'
 					if C.pclose(f) != 0 {
 						eprintln(sframe)
 						continue
@@ -111,7 +108,7 @@ fn print_backtrace_skipping_top_frames_linux(skipframes int) bool {
 					eprint(' | ')
 					eprintln(beforeaddr)
 				}
-				if sframes.len > 0 {
+				if nr_actual_frames > 0 {
 					unsafe { C.free(csymbols) }
 				}
 			}

vlib/builtin/sorted_map.v

@@ -158,6 +158,7 @@ fn (mut n mapnode) split_child(child_index int, mut y mapnode) {
 	n.len++
 }
 
+@[direct_array_access]
 fn (m SortedMap) get(key string, out voidptr) bool {
 	mut node := m.root
 	for {

vlib/builtin/string.v

@@ -1154,7 +1154,8 @@ pub fn (s string) substr_unsafe(start int, _end int) string {
 pub fn (s string) substr_with_check(start int, _end int) !string {
 	end := if _end == max_int { s.len } else { _end } // max_int
 	if start > end || start > s.len || end > s.len || start < 0 || end < 0 {
-		return error('substr(${start}, ${end}) out of bounds (len=${s.len})')
+		return error('substr(' + start.str() + ', ' + end.str() + ') out of bounds (len=' +
+			s.len.str() + ')')
 	}
 	len := end - start
 	if len == s.len {

vlib/builtin/utf8.v

@@ -78,14 +78,15 @@ pub fn utf32_decode_to_buffer(code u32, mut buf &u8) int {
 // it is used in vlib/builtin/string.v,
 // and also in vlib/v/gen/c/cgen.v
 pub fn (_rune string) utf32_code() int {
-	return int(_rune.bytes().utf8_to_utf32() or {
-		// error('more than one utf-8 rune found in this string')
-		rune(0)
-	})
+	if res := _rune.bytes().utf8_to_utf32() {
+		return int(res)
+	}
+	return 0
 }
 
 // convert array of utf8 bytes to single utf32 value
 // will error if more than 4 bytes are submitted
+@[direct_array_access]
 pub fn (_bytes []u8) utf8_to_utf32() !rune {
 	if _bytes.len == 0 {
 		return 0

vlib/v/markused/markused.v

@@ -520,6 +520,10 @@ pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&a
 		}
 	}
 
+	if table.used_features.range_index {
+		walker.fn_by_name(string_idx_str + '.substr')
+	}
+
 	table.used_features.used_fns = walker.used_fns.move()
 	table.used_features.used_consts = walker.used_consts.move()
 	table.used_features.used_globals = walker.used_globals.move()

vlib/v/markused/walker.v

@@ -367,6 +367,10 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 				w.features.used_maps++
 			} else if sym.kind == .array {
 				w.features.used_arrays++
+			} else if sym.kind == .string {
+				if node.index is ast.RangeExpr {
+					w.features.range_index = true
+				}
 			}
 		}
 		ast.InfixExpr {