fix: dont round off floats while formatting (#679)

Co-authored-by: Jacob Pratt <jacob@jhpratt.dev>

Author: dracarys18

tests/formatting.rs

@@ -1,4 +1,5 @@
 use std::io;
+use std::num::NonZeroU8;
 
 use time::format_description::well_known::iso8601::{DateKind, OffsetPrecision, TimePrecision};
 use time::format_description::well_known::{iso8601, Iso8601, Rfc2822, Rfc3339};
@@ -226,6 +227,26 @@ fn iso_8601() -> time::Result<()> {
     Ok(())
 }
 
+#[test]
+fn iso_8601_issue_678() -> time::Result<()> {
+    macro_rules! assert_format_config {
+        ($formatted:literal $(, $($config:tt)+)?) => {
+            assert_eq!(
+                datetime!(2021-01-02 03:04:05.999_999_999 UTC).format(
+                    &Iso8601::<{ iso8601::Config::DEFAULT$($($config)+)?.encode() }>
+                )?,
+                $formatted
+            );
+        };
+    }
+
+    assert_format_config!("2021-01-02T03:04:05.999999999Z", .set_time_precision(TimePrecision::Second { decimal_digits: NonZeroU8::new(9) }));
+    assert_format_config!("2021-01-02T03:04:05.999999Z", .set_time_precision(TimePrecision::Second { decimal_digits: NonZeroU8::new(6) }));
+    assert_format_config!("2021-01-02T03:04:05.999Z", .set_time_precision(TimePrecision::Second { decimal_digits: NonZeroU8::new(3) }));
+
+    Ok(())
+}
+
 #[test]
 fn format_time() -> time::Result<()> {
     let format_output = [

time/src/formatting/mod.rs

@@ -2,6 +2,7 @@
 
 pub(crate) mod formattable;
 mod iso8601;
+
 use core::num::NonZeroU8;
 use std::io;
 
@@ -73,6 +74,10 @@ pub(crate) fn format_float(
 ) -> io::Result<usize> {
     match digits_after_decimal {
         Some(digits_after_decimal) => {
+            // Truncate the decimal points up to the precision
+            let trunc_num = 10_f64.powi(digits_after_decimal.get().cast_signed().extend());
+            let value = f64::trunc(value * trunc_num) / trunc_num;
+
             let digits_after_decimal = digits_after_decimal.get().extend();
             let width = digits_before_decimal.extend::<usize>() + 1 + digits_after_decimal;
             write!(output, "{value:0>width$.digits_after_decimal$}")?;