node: manually implement Debug

I got too irritated today with the default Debug implementation of
hg::revlog::Node while playing with a new parser. This isn't quite
what I wanted, but it wasn't much code and it at least gives you
output that's easy to visually compare to a node.hex()ed identifier
from the Python side of things.

Sadly, this doesn't influence the output in lldb or the VSCode
debugger extension that uses lldb under the covers, but it at least
means debug prints are a little more useful.

Differential Revision: https://phab.mercurial-scm.org/D12608

rust/hg-core/src/revlog/node.rs

@@ -53,12 +53,21 b' type NodeData = [u8; NODE_BYTES_LENGTH];'
 /// the size or return an error at runtime.
 ///
 /// [`nybbles_len`]: #method.nybbles_len
-#[derive(Copy, Clone, Debug, PartialEq, BytesCast, derive_more::From)]
+#[derive(Copy, Clone, PartialEq, BytesCast, derive_more::From)]
 #[repr(transparent)]
 pub struct Node {
     data: NodeData,
 }
 
+impl fmt::Debug for Node {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let n = format!("{:x?}", self.data);
+        // We're using debug_tuple because it makes the output a little
+        // more compact without losing data.
+        f.debug_tuple("Node").field(&n).finish()
+    }
+}
+
 /// The node value for NULL_REVISION
 pub const NULL_NODE: Node = Node {
     data: [0; NODE_BYTES_LENGTH],