Pre: Replace PartitionMapping trait with newtype. r=grisha

Generally, I think that Mentat is using too many small traits rather
than wrapping types into newtypes.  Wrapping into newtypes is cheap in
Rust, and it makes it easier to reason about the code.

db/src/db.rs

@@ -14,12 +14,10 @@ use failure::{
     ResultExt,
 };
 
-use std::borrow::Borrow;
 use std::collections::HashMap;
 use std::collections::hash_map::{
     Entry,
 };
-use std::fmt::Display;
 use std::iter::{once, repeat};
 use std::ops::Range;
 use std::path::Path;
@@ -1080,20 +1078,14 @@ SELECT EXISTS
     Ok(())
 }
 
-pub trait PartitionMapping {
-    fn allocate_entid<S: ?Sized + Ord + Display>(&mut self, partition: &S) -> i64 where String: Borrow<S>;
-    fn allocate_entids<S: ?Sized + Ord + Display>(&mut self, partition: &S, n: usize) -> Range<i64> where String: Borrow<S>;
-    fn contains_entid(&self, entid: Entid) -> bool;
-}
-
-impl PartitionMapping for PartitionMap {
+impl PartitionMap {
     /// Allocate a single fresh entid in the given `partition`.
-    fn allocate_entid<S: ?Sized + Ord + Display>(&mut self, partition: &S) -> i64 where String: Borrow<S> {
+    pub(crate) fn allocate_entid(&mut self, partition: &str) -> i64 {
         self.allocate_entids(partition, 1).start
     }
 
     /// Allocate `n` fresh entids in the given `partition`.
-    fn allocate_entids<S: ?Sized + Ord + Display>(&mut self, partition: &S, n: usize) -> Range<i64> where String: Borrow<S> {
+    pub(crate) fn allocate_entids(&mut self, partition: &str, n: usize) -> Range<i64> {
         match self.get_mut(partition) {
             Some(partition) => {
                 let idx = partition.index;
@@ -1105,7 +1097,7 @@ impl PartitionMapping for PartitionMap {
         }
     }
 
-    fn contains_entid(&self, entid: Entid) -> bool {
+    pub(crate) fn contains_entid(&self, entid: Entid) -> bool {
         self.values().any(|partition| partition.contains_entid(entid))
     }
 }
@@ -1114,6 +1106,10 @@ impl PartitionMapping for PartitionMap {
 mod tests {
     extern crate env_logger;
 
+    use std::borrow::{
+        Borrow,
+    };
+
     use super::*;
     use debug::{TestConn,tempids};
     use edn::{

db/src/tx.rs

@@ -60,7 +60,6 @@ use std::iter::{
 use db;
 use db::{
     MentatStoring,
-    PartitionMapping,
 };
 use edn::{
     InternSet,

db/src/types.rs

@@ -10,10 +10,17 @@
 
 #![allow(dead_code)]
 
-use std::collections::HashMap;
 use std::collections::{
     BTreeMap,
     BTreeSet,
+    HashMap,
+};
+use std::iter::{
+    FromIterator,
+};
+use std::ops::{
+    Deref,
+    DerefMut,
 };
 
 extern crate mentat_core;
@@ -66,7 +73,28 @@ impl Partition {
 }
 
 /// Map partition names to `Partition` instances.
-pub type PartitionMap = BTreeMap<String, Partition>;
+#[derive(Clone, Debug, Default, Eq, Hash, Ord, PartialOrd, PartialEq)]
+pub struct PartitionMap(BTreeMap<String, Partition>);
+
+impl Deref for PartitionMap {
+    type Target = BTreeMap<String, Partition>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for PartitionMap {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl FromIterator<(String, Partition)> for PartitionMap {
+    fn from_iter<T: IntoIterator<Item=(String, Partition)>>(iter: T) -> Self {
+        PartitionMap(iter.into_iter().collect())
+    }
+}
 
 /// Represents the metadata required to query from, or apply transactions to, a Mentat store.
 ///