Auto merge of #47300 - remexre:duration-constructors-as-const-fns, r=alexcrichton

Makes the constructors of Duration const fns.

This affects `Duration::new`, `Duration::from_secs`, `Duration::from_millis`, `Duration::from_micros`, and `Duration::from_nanos`.

src/libstd/time/duration.rs

@@ -94,7 +94,7 @@ pub fn new(secs: u64, nanos: u32) -> Duration {
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
-    pub fn from_secs(secs: u64) -> Duration {
+    pub const fn from_secs(secs: u64) -> Duration {
         Duration { secs: secs, nanos: 0 }
     }
 
@@ -112,10 +112,11 @@ pub fn from_secs(secs: u64) -> Duration {
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
-    pub fn from_millis(millis: u64) -> Duration {
-        let secs = millis / MILLIS_PER_SEC;
-        let nanos = ((millis % MILLIS_PER_SEC) as u32) * NANOS_PER_MILLI;
-        Duration { secs: secs, nanos: nanos }
+    pub const fn from_millis(millis: u64) -> Duration {
+        Duration {
+            secs: millis / MILLIS_PER_SEC,
+            nanos: ((millis % MILLIS_PER_SEC) as u32) * NANOS_PER_MILLI,
+        }
     }
 
     /// Creates a new `Duration` from the specified number of microseconds.
@@ -133,10 +134,11 @@ pub fn from_millis(millis: u64) -> Duration {
     /// ```
     #[unstable(feature = "duration_from_micros", issue = "44400")]
     #[inline]
-    pub fn from_micros(micros: u64) -> Duration {
-        let secs = micros / MICROS_PER_SEC;
-        let nanos = ((micros % MICROS_PER_SEC) as u32) * NANOS_PER_MICRO;
-        Duration { secs: secs, nanos: nanos }
+    pub const fn from_micros(micros: u64) -> Duration {
+        Duration {
+            secs: micros / MICROS_PER_SEC,
+            nanos: ((micros % MICROS_PER_SEC) as u32) * NANOS_PER_MICRO,
+        }
     }
 
     /// Creates a new `Duration` from the specified number of nanoseconds.
@@ -154,10 +156,11 @@ pub fn from_micros(micros: u64) -> Duration {
     /// ```
     #[unstable(feature = "duration_extras", issue = "46507")]
     #[inline]
-    pub fn from_nanos(nanos: u64) -> Duration {
-        let secs = nanos / (NANOS_PER_SEC as u64);
-        let nanos = (nanos % (NANOS_PER_SEC as u64)) as u32;
-        Duration { secs: secs, nanos: nanos }
+    pub const fn from_nanos(nanos: u64) -> Duration {
+        Duration {
+            secs: nanos / (NANOS_PER_SEC as u64),
+            nanos: (nanos % (NANOS_PER_SEC as u64)) as u32,
+        }
     }
 
     /// Returns the number of _whole_ seconds contained by this `Duration`.