fix compiler error in 'std.fmt.Parser.until'

lib/std/fmt.zig

@@ -11,6 +11,7 @@ const unicode = std.unicode;
 const meta = std.meta;
 const lossyCast = math.lossyCast;
 const expectFmt = std.testing.expectFmt;
+const testing = std.testing;
 
 pub const default_max_depth = 3;
 
@@ -315,7 +316,6 @@ pub const Specifier = union(enum) {
 /// Allows to implement formatters compatible with std.fmt without replicating
 /// the standard library behavior.
 pub const Parser = struct {
-    pos: usize = 0,
     iter: std.unicode.Utf8Iterator,
 
     // Returns a decimal number or null if the current character is not a
@@ -341,13 +341,13 @@ pub const Parser = struct {
     // Returns a substring of the input starting from the current position
     // and ending where `ch` is found or until the end if not found
     pub fn until(self: *@This(), ch: u21) []const u8 {
-        var result: []const u8 = &[_]u8{};
+        const start = self.iter.i;
         while (self.peek(0)) |code_point| {
             if (code_point == ch)
                 break;
-            result = result ++ (self.iter.nextCodepointSlice() orelse &[_]u8{});
+            _ = self.iter.nextCodepoint();
         }
-        return result;
+        return self.iter.bytes[start..self.iter.i];
     }
 
     // Returns one character, if available
@@ -2763,3 +2763,47 @@ test hex {
         try std.testing.expectEqualStrings("[00efcdab78563412]", s);
     }
 }
+
+test "parser until" {
+    { // return substring till ':'
+        var parser: Parser = .{
+            .iter = .{ .bytes = "abc:1234", .i = 0 },
+        };
+        try testing.expectEqualStrings("abc", parser.until(':'));
+    }
+
+    { // return the entire string - `ch` not found
+        var parser: Parser = .{
+            .iter = .{ .bytes = "abc1234", .i = 0 },
+        };
+        try testing.expectEqualStrings("abc1234", parser.until(':'));
+    }
+
+    { // substring is empty - `ch` is the only character
+        var parser: Parser = .{
+            .iter = .{ .bytes = ":", .i = 0 },
+        };
+        try testing.expectEqualStrings("", parser.until(':'));
+    }
+
+    { // empty string and `ch` not found
+        var parser: Parser = .{
+            .iter = .{ .bytes = "", .i = 0 },
+        };
+        try testing.expectEqualStrings("", parser.until(':'));
+    }
+
+    { // substring starts at index 2 and goes upto `ch`
+        var parser: Parser = .{
+            .iter = .{ .bytes = "abc:1234", .i = 2 },
+        };
+        try testing.expectEqualStrings("c", parser.until(':'));
+    }
+
+    { // substring starts at index 4 and goes upto the end - `ch` not found
+        var parser: Parser = .{
+            .iter = .{ .bytes = "abc1234", .i = 4 },
+        };
+        try testing.expectEqualStrings("234", parser.until(':'));
+    }
+}