[DEV-7084] TAMER: Correct Ix trait bound for AsgError

The `Debug` bound is inconvenient and requires propagation to any types that
use it.  Further, it's really awkward having `Display` depend on `Debug`; if
we want to render a useful display here, we can write one.

To be clear: IndexType implements Debug.

For now, this is pretty-printed by another part of the code, which we don't
want to implement in `Display` because it requires looking things up from
the graph.

tamer/src/ir/asg/graph.rs

@@ -212,19 +212,13 @@ impl<Ix: IndexType> ObjectRef<Ix> {
     }
 }
 
-impl<Ix> From<NodeIndex<Ix>> for ObjectRef<Ix>
-where
-    Ix: IndexType,
-{
+impl<Ix: IndexType> From<NodeIndex<Ix>> for ObjectRef<Ix> {
     fn from(index: NodeIndex<Ix>) -> Self {
         Self(index)
     }
 }
 
-impl<Ix> From<ObjectRef<Ix>> for NodeIndex<Ix>
-where
-    Ix: IndexType,
-{
+impl<Ix: IndexType> From<ObjectRef<Ix>> for NodeIndex<Ix> {
     fn from(objref: ObjectRef<Ix>) -> Self {
         objref.0
     }
@@ -246,7 +240,7 @@ pub type Node<O> = Option<O>;
 ///   so this stores only owned values.
 /// The caller will know the problem values.
 #[derive(Debug, PartialEq)]
-pub enum AsgError<Ix: Debug> {
+pub enum AsgError<Ix: IndexType> {
     /// An object could not change state in the manner requested.
     ///
     /// See [`Asg::declare`] and [`Asg::set_fragment`] for more
@@ -261,21 +255,19 @@ pub enum AsgError<Ix: Debug> {
     Cycles(Vec<Vec<ObjectRef<Ix>>>),
 }
 
-impl<Ix: Debug> std::fmt::Display for AsgError<Ix> {
+impl<Ix: IndexType> std::fmt::Display for AsgError<Ix> {
     fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
             Self::ObjectTransition(err) => std::fmt::Display::fmt(&err, fmt),
             Self::UnexpectedNode(msg) => {
                 write!(fmt, "unexpected node: {}", msg)
             }
-            Self::Cycles(cycles) => {
-                write!(fmt, "Cyclic dependencies detected: {:?}", cycles)
-            }
+            Self::Cycles(_) => write!(fmt, "cyclic dependencies"),
         }
     }
 }
 
-impl<Ix: Debug> std::error::Error for AsgError<Ix> {
+impl<Ix: IndexType> std::error::Error for AsgError<Ix> {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {
             Self::ObjectTransition(err) => err.source(),
@@ -284,7 +276,7 @@ impl<Ix: Debug> std::error::Error for AsgError<Ix> {
     }
 }
 
-impl<Ix: Debug> From<TransitionError> for AsgError<Ix> {
+impl<Ix: IndexType> From<TransitionError> for AsgError<Ix> {
     fn from(err: TransitionError) -> Self {
         Self::ObjectTransition(err)
     }