Improve `Debug` impl of `core::time::Duration`

Prior to this, Duration simply derived Debug. Since Duration doesn't
implement `Display`, the only way to inspect its value is to use
`Debug`. Unfortunately, the derived `Debug` impl is far from optimal
for humans. In many cases, Durations are used for some quick'n'dirty
benchmarking (or in general: measuring the time of some code). Correctly
understanding the output of Duration's Debug impl is not easy (e.g.
is "{ secs: 0, nanos: 968360102 }" or "{ secs: 0, nanos 98507324 }"
shorter?).

This commit replaces the derived impl with a manual one. It prints
the duration as seconds (i.e. "3.1803s") if the duration is longer than
a second, otherwise it prints it in either ms, µs or ns (depending on
the duration's length). This already helps readability a lot and it
never omits information that is stored.

This `Debug` impl does *not* respect the following formatting parameters:

- fill/align/padding: difficult to implement, probably not worth it
- alternate # flag: not clear what this should do

1 files changed, 112 insertions, 0 deletions

diff --git a/src/libcore/tests/time.rs b/src/libcore/tests/time.rs
index 042c523f25f..bfb5369cf8f 100644
--- a/src/libcore/tests/time.rs
+++ b/src/libcore/tests/time.rs
@@ -122,3 +122,115 @@ fn checked_div() {
     assert_eq!(Duration::new(1, 0).checked_div(2), Some(Duration::new(0, 500_000_000)));
     assert_eq!(Duration::new(2, 0).checked_div(0), None);
 }
+
+#[test]
+fn debug_formatting_extreme_values() {
+    assert_eq!(
+        format!("{:?}", Duration::new(18_446_744_073_709_551_615, 123_456_789)),
+        "18446744073709551615.123456789s"
+    );
+}
+
+#[test]
+fn debug_formatting_secs() {
+    assert_eq!(format!("{:?}", Duration::new(7, 000_000_000)), "7s");
+    assert_eq!(format!("{:?}", Duration::new(7, 100_000_000)), "7.1s");
+    assert_eq!(format!("{:?}", Duration::new(7, 000_010_000)), "7.00001s");
+    assert_eq!(format!("{:?}", Duration::new(7, 000_000_001)), "7.000000001s");
+    assert_eq!(format!("{:?}", Duration::new(7, 123_456_789)), "7.123456789s");
+
+    assert_eq!(format!("{:?}", Duration::new(88, 000_000_000)), "88s");
+    assert_eq!(format!("{:?}", Duration::new(88, 100_000_000)), "88.1s");
+    assert_eq!(format!("{:?}", Duration::new(88, 000_010_000)), "88.00001s");
+    assert_eq!(format!("{:?}", Duration::new(88, 000_000_001)), "88.000000001s");
+    assert_eq!(format!("{:?}", Duration::new(88, 123_456_789)), "88.123456789s");
+
+    assert_eq!(format!("{:?}", Duration::new(999, 000_000_000)), "999s");
+    assert_eq!(format!("{:?}", Duration::new(999, 100_000_000)), "999.1s");
+    assert_eq!(format!("{:?}", Duration::new(999, 000_010_000)), "999.00001s");
+    assert_eq!(format!("{:?}", Duration::new(999, 000_000_001)), "999.000000001s");
+    assert_eq!(format!("{:?}", Duration::new(999, 123_456_789)), "999.123456789s");
+}
+
+#[test]
+fn debug_formatting_millis() {
+    assert_eq!(format!("{:?}", Duration::new(0, 7_000_000)), "7ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_100_000)), "7.1ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_000_001)), "7.000001ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_123_456)), "7.123456ms");
+
+    assert_eq!(format!("{:?}", Duration::new(0, 88_000_000)), "88ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_100_000)), "88.1ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_000_001)), "88.000001ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_123_456)), "88.123456ms");
+
+    assert_eq!(format!("{:?}", Duration::new(0, 999_000_000)), "999ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_100_000)), "999.1ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_000_001)), "999.000001ms");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_123_456)), "999.123456ms");
+}
+
+#[test]
+fn debug_formatting_micros() {
+    assert_eq!(format!("{:?}", Duration::new(0, 7_000)), "7µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_100)), "7.1µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_001)), "7.001µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 7_123)), "7.123µs");
+
+    assert_eq!(format!("{:?}", Duration::new(0, 88_000)), "88µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_100)), "88.1µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_001)), "88.001µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 88_123)), "88.123µs");
+
+    assert_eq!(format!("{:?}", Duration::new(0, 999_000)), "999µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_100)), "999.1µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_001)), "999.001µs");
+    assert_eq!(format!("{:?}", Duration::new(0, 999_123)), "999.123µs");
+}
+
+#[test]
+fn debug_formatting_nanos() {
+    assert_eq!(format!("{:?}", Duration::new(0, 0)), "0ns");
+    assert_eq!(format!("{:?}", Duration::new(0, 1)), "1ns");
+    assert_eq!(format!("{:?}", Duration::new(0, 88)), "88ns");
+    assert_eq!(format!("{:?}", Duration::new(0, 999)), "999ns");
+}
+
+#[test]
+fn debug_formatting_precision_zero() {
+    assert_eq!(format!("{:.0?}", Duration::new(0, 0)), "0ns");
+    assert_eq!(format!("{:.0?}", Duration::new(0, 123)), "123ns");
+
+    assert_eq!(format!("{:.0?}", Duration::new(0, 1_001)), "1µs");
+    assert_eq!(format!("{:.0?}", Duration::new(0, 1_999)), "1µs");
+
+    assert_eq!(format!("{:.0?}", Duration::new(0, 1_000_001)), "1ms");
+    assert_eq!(format!("{:.0?}", Duration::new(0, 1_999_999)), "1ms");
+
+    assert_eq!(format!("{:.0?}", Duration::new(1, 000_000_001)), "1s");
+    assert_eq!(format!("{:.0?}", Duration::new(1, 999_999_999)), "1s");
+}
+
+#[test]
+fn debug_formatting_precision_two() {
+    // This might seem inconsistent with the other units, but printing
+    // fractional digits for nano seconds would imply more precision than is
+    // actually stored.
+    assert_eq!(format!("{:.2?}", Duration::new(0, 0)), "0ns");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 123)), "123ns");
+
+    assert_eq!(format!("{:.2?}", Duration::new(0, 1_000)), "1.00µs");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 7_001)), "7.00µs");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 7_100)), "7.10µs");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 1_999)), "1.99µs");
+
+    assert_eq!(format!("{:.2?}", Duration::new(0, 1_000_000)), "1.00ms");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 3_001_000)), "3.00ms");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 3_100_000)), "3.10ms");
+    assert_eq!(format!("{:.2?}", Duration::new(0, 1_999_999)), "1.99ms");
+
+    assert_eq!(format!("{:.2?}", Duration::new(1, 000_000_000)), "1.00s");
+    assert_eq!(format!("{:.2?}", Duration::new(4, 001_000_000)), "4.00s");
+    assert_eq!(format!("{:.2?}", Duration::new(2, 100_000_000)), "2.10s");
+    assert_eq!(format!("{:.2?}", Duration::new(8, 999_999_999)), "8.99s");
+}