custom-format

```rust use custom_format as cfmt;

use core::fmt;

pub struct DateTime { year: i32, month: u8, month_day: u8, hour: u8, minute: u8, second: u8, nanoseconds: u32, }

macrorules! implcustomformatfordatetime { (match spec { $($spec:literal => $func:expr $(,)?)* }) => { use cfmt::compiletime::{spec, CustomFormat}; $( impl CustomFormat<{ spec($spec) }> for DateTime { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { ($func as fn(&Self, &mut fmt::Formatter) -> fmt::Result)(self, f) } } )* }; }

// Static format specifiers, checked at compile-time implcustomformatfordatetime!(match spec { // Year with pad for at least 4 digits "%Y" => |this, f| write!(f, "{:04}", this.year), // Year % 100 (00..99) "%y" => |this, f| write!(f, "{:02}", (this.year % 100).abs()), // Month of the year, zero-padded (01..12) "%m" => |this, f| write!(f, "{:02}", this.month), // Day of the month, zero-padded (01..31) "%d" => |this, f| write!(f, "{:02}", this.monthday), // Hour of the day, 24-hour clock, zero-padded (00..23) "%H" => |this, f| write!(f, "{:02}", this.hour), // Minute of the hour (00..59) "%M" => |this, f| write!(f, "{:02}", this.minute), // Second of the minute (00..60) "%S" => |this, f| write!(f, "{:02}", this.second), // Date (%m/%d/%y) "%D" => { |this, f| { let month = cfmt::customformatter!("%m", this); let day = cfmt::customformatter!("%d", this); let year = cfmt::customformatter!("%y", this); write!(f, "{}/{}/{}", month, day, year) } } });

// Dynamic format specifiers, checked at runtime impl cfmt::runtime::CustomFormat for DateTime { fn fmt(&self, f: &mut fmt::Formatter, spec: &str) -> fmt::Result { let mut chars = spec.chars(); match (chars.next(), chars.nextback()) { // Nanoseconds with n digits (%nN) (Some('%'), Some('N')) => match chars.asstr().parse() { Ok(n) if n > 0 => { if n <= 9 { write!(f, "{:0width$}", self.nanoseconds / 10u32.pow(9 - n as u32), width = n) } else { write!(f, "{:09}{:0width$}", self.nanoseconds, 0, width = n - 9) } } _ => Err(fmt::Error), }, _ => Err(fmt::Error), } } }

let dt = DateTime { year: 1836, month: 5, month_day: 18, hour: 23, minute: 45, second: 54, nanoseconds: 123456789, };

// Expands to: // // match (&("DateTime"), &dt) { // (arg0, arg1) => ::std::println!( // "The {0:?} is: {1}-{2}-{3} {4}:{5}:{6}.{7}", // arg0, // ::customformat::customformatter!("%Y", arg1), // ::customformat::customformatter!("%m", arg1), // ::customformat::customformatter!("%d", arg1), // ::customformat::customformatter!("%H", arg1), // ::customformat::customformatter!("%M", arg1), // ::customformat::customformatter!("%S", arg1), // ::custom_format::runtime::CustomFormatter::new("%6N", arg1) // ), // } // // Output: The "DateTime" is: 1836-05-18 23:45:54.123456 // // The custom format specifier is interpreted as a compile-time specifier by default, // or as a runtime specifier if it is inside "<>". cfmt::println!( "The {ty:?} is: {dt :%Y}-{dt :%m}-{dt :%d} {dt :%H}:{dt :%M}:{dt :%S}.{dt :<%6N>}", ty = "DateTime", );

// Compile-time error since "%h" is not a valid format specifier // cfmt::println!("{0 :%h}", dt);

// Panic at runtime since "%h" is not a valid format specifier // cfmt::println!("{0 :<%h>}", dt); ```