rust: replace Node::encode_hex with std::fmt::LowerHex

rust: replace Node::encode_hex with std::fmt::LowerHex

This avoids allocating intermediate strings.

Differential Revision: https://phab.mercurial-scm.org/D9860

--- a/rust/hg-core/examples/nodemap/main.rs	Fri Jan 29 13:46:19 2021 +0100
+++ b/rust/hg-core/examples/nodemap/main.rs	Mon Jan 25 12:00:23 2021 +0100
@@ -66,7 +66,7 @@
         .collect();
     if queries < 10 {
         let nodes_hex: Vec<String> =
-            nodes.iter().map(|n| n.encode_hex()).collect();
+            nodes.iter().map(|n| format!("{:x}", n)).collect();
         println!("Nodes: {:?}", nodes_hex);
     }
     let mut last: Option<Revision> = None;
@@ -76,11 +76,11 @@
     }
     let elapsed = start.elapsed();
     println!(
-        "Did {} queries in {:?} (mean {:?}), last was {:?} with result {:?}",
+        "Did {} queries in {:?} (mean {:?}), last was {:x} with result {:?}",
         queries,
         elapsed,
         elapsed / (queries as u32),
-        nodes.last().unwrap().encode_hex(),
+        nodes.last().unwrap(),
         last
     );
 }

--- a/rust/hg-core/src/revlog/node.rs	Fri Jan 29 13:46:19 2021 +0100
+++ b/rust/hg-core/src/revlog/node.rs	Mon Jan 25 12:00:23 2021 +0100
@@ -11,6 +11,7 @@
 use bytes_cast::BytesCast;
 use hex::{self, FromHex, FromHexError};
 use std::convert::TryFrom;
+use std::fmt;
 
 /// The length in bytes of a `Node`
 ///
@@ -80,6 +81,15 @@
     }
 }
 
+impl fmt::LowerHex for Node {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for &byte in &self.data {
+            write!(f, "{:02x}", byte)?
+        }
+        Ok(())
+    }
+}
+
 #[derive(Debug, PartialEq)]
 pub enum NodeError {
     ExactLengthRequired(usize, String),
@@ -124,14 +134,6 @@
             .into())
     }
 
-    /// Convert to hexadecimal string representation
-    ///
-    /// To be used in FFI and I/O only, in order to facilitate future
-    /// changes of hash format.
-    pub fn encode_hex(&self) -> String {
-        hex::encode(self.data)
-    }
-
     /// Provide access to binary data
     ///
     /// This is needed by FFI layers, for instance to return expected
@@ -349,7 +351,7 @@
 
     #[test]
     fn test_node_encode_hex() {
-        assert_eq!(sample_node().encode_hex(), sample_node_hex());
+        assert_eq!(format!("{:x}", sample_node()), sample_node_hex());
     }
 
     #[test]
@@ -391,7 +393,7 @@
                 "testgr".to_string()
             ))
         );
-        let mut long = NULL_NODE.encode_hex();
+        let mut long = format!("{:x}", NULL_NODE);
         long.push('c');
         match NodePrefix::from_hex(&long)
             .expect_err("should be refused as too long")