Move 'now' into core, implement microsecond truncation.

This is so we don't return a more granular -- and thus subtly different --
timestamp in a TxReport than we put into the store.

core/src/lib.rs

@@ -233,6 +233,28 @@ impl TypedValue {
     }
 }
 
+trait MicrosecondPrecision {
+    /// Truncate the provided `DateTime` to microsecond precision.
+    fn microsecond_precision(self) -> Self;
+}
+
+impl MicrosecondPrecision for DateTime<Utc> {
+    fn microsecond_precision(self) -> DateTime<Utc> {
+        let nanoseconds = self.nanosecond();
+        if nanoseconds % 1000 == 0 {
+            return self;
+        }
+        let microseconds = nanoseconds / 1000;
+        let truncated = microseconds * 1000;
+        self.with_nanosecond(truncated).expect("valid timestamp")
+    }
+}
+
+/// Return the current time as a UTC `DateTime` instance with microsecond precision.
+pub fn now() -> DateTime<Utc> {
+    Utc::now().microsecond_precision()
+}
+
 // We don't do From<i64> or From<Entid> 'cos it's ambiguous.
 
 impl From<bool> for TypedValue {
@@ -245,9 +267,7 @@ impl From<bool> for TypedValue {
 /// `TypedValue::Instant`.
 impl From<DateTime<Utc>> for TypedValue {
     fn from(value: DateTime<Utc>) -> TypedValue {
-        let microseconds = value.nanosecond() / 1000;
-        let truncated = microseconds * 1000;
-        TypedValue::Instant(value.with_nanosecond(truncated).expect("valid timestamp"))
+        TypedValue::Instant(value.microsecond_precision())
     }
 }
 

db/src/lib.rs

@@ -28,11 +28,6 @@ use std::iter::repeat;
 
 use itertools::Itertools;
 
-use mentat_core::{
-    DateTime,
-    Utc,
-};
-
 pub use errors::{Error, ErrorKind, ResultExt, Result};
 
 pub mod db;
@@ -116,8 +111,3 @@ pub fn repeat_values(values_per_tuple: usize, tuples: usize) -> String {
     let values: String = repeat(inner).take(tuples).join(", ");
     values
 }
-
-/// Return the current time as a UTC `DateTime` instance.
-pub fn now() -> DateTime<Utc> {
-    Utc::now()
-}

db/src/tx.rs

@@ -85,6 +85,7 @@ use mentat_core::{
     Schema,
     Utc,
     attribute,
+    now,
 };
 
 use mentat_core::intern_set::InternSet;
@@ -654,7 +655,7 @@ impl<'conn, 'a> Tx<'conn, 'a> {
             }
         }
 
-        tx_instant = self.tx_instant.unwrap_or(::now());
+        tx_instant = self.tx_instant.unwrap_or_else(now);
 
         // Transact [:db/add :db/txInstant NOW :db/tx] if it doesn't exist.
         if self.tx_instant == None {