1//! Extension traits.
2
3use core::fmt::{Alignment, Arguments, Formatter, Result, Write};
4
5mod sealed {
6 pub trait Sealed {}
7
8 impl Sealed for core::fmt::Formatter<'_> {}
9}
10
11/// An extension trait for [`core::fmt::Formatter`].
12pub trait FormatterExt: sealed::Sealed {
13 /// Writes the given arguments to the formatter, padding them with the given width. If `width`
14 /// is incorrect, the resulting output will not be the requested width.
15 fn pad_with_width(&mut self, width: usize, args: Arguments<'_>) -> Result;
16}
17
18impl FormatterExt for Formatter<'_> {
19 fn pad_with_width(&mut self, args_width: usize, args: Arguments<'_>) -> Result {
20 let Some(final_width) = self.width() else {
21 // The caller has not requested a width. Write the arguments as-is.
22 return self.write_fmt(args);
23 };
24 let Some(fill_width @ 1..) = final_width.checked_sub(args_width) else {
25 // No padding will be present. Write the arguments as-is.
26 return self.write_fmt(args);
27 };
28
29 let alignment = self.align().unwrap_or(Alignment::Left);
30 let fill = self.fill();
31
32 let left_fill_width = match alignment {
33 Alignment::Left => 0,
34 Alignment::Right => fill_width,
35 Alignment::Center => fill_width / 2,
36 };
37 let right_fill_width = match alignment {
38 Alignment::Left => fill_width,
39 Alignment::Right => 0,
40 // When the fill is not even on both sides, the extra fill goes on the right.
41 Alignment::Center => (fill_width + 1) / 2,
42 };
43
44 for _ in 0..left_fill_width {
45 self.write_char(fill)?;
46 }
47 self.write_fmt(args)?;
48 for _ in 0..right_fill_width {
49 self.write_char(fill)?;
50 }
51
52 Ok(())
53 }
54}
55