tamer: asg::graph::object::ident::ObjectIndex::<Ident>::bind_definition: Replace ident span

I noticed this while working on a graph traversal.  The unit test used the
same span for both the reference _and_ the binding, so I didn't notice. -_-

The problem with this, though, is that we do not have a separate span
representing the source location of the identifier reference.  The reason is
that we decided to re-use an existing node rather than creating another one,
which would add another inconvenient layer of indirection (and complexity).

So, I may have to add (optional?) spans to edges.

DEV-13708

tamer/src/asg/air.rs

@@ -557,7 +557,7 @@ impl ParseState for AirAggregate {
 
                 // It is important that we do not mark this expression as
                 //   reachable unless we successfully bind the identifier.
-                match oi_ident.bind_definition(asg, oi) {
+                match oi_ident.bind_definition(asg, id, oi) {
                     Ok(_) => Transition(BuildingExpr(
                         oi_pkg,
                         es.reachable_by(id),

tamer/src/asg/air/test.rs

@@ -984,8 +984,9 @@ fn expr_ref_to_ident() {
     let toks = vec![
         Air::ExprOpen(ExprOp::Sum, S1),
         Air::ExprIdent(id_foo),
-        // Reference to an as-of-yet-undefined id (okay)
-        Air::ExprRef(id_bar),
+        // Reference to an as-of-yet-undefined id (okay),
+        //   with a different span than `id_bar`.
+        Air::ExprRef(SPair("bar".into(), S3)),
         Air::ExprClose(S4),
         //
         // Another expression to reference the first
@@ -1018,13 +1019,18 @@ fn expr_ref_to_ident() {
     let oi_ident_bar =
         foo_rels.pop().and_then(ExprRel::narrow::<Ident>).unwrap();
     let ident_bar = oi_ident_bar.resolve(&asg);
-    assert_eq!(ident_bar.span(), id_bar.span());
 
     // The identifier will have originally been `Missing`,
     //   since it did not exist at the point of reference.
     // But it should now properly identify the other expression.
     assert_matches!(ident_bar, Ident::Transparent(..));
 
+    // The span of the identifier must be updated with the defining
+    //   `ExprIdent`,
+    //     otherwise it'll be the location of the `ExprRef` that originally
+    //     added it as `Missing`.
+    assert_eq!(ident_bar.span(), id_bar.span());
+
     let oi_expr_bar = asg.expect_ident_oi::<Expr>(id_bar);
     assert!(oi_ident_bar.is_bound_to(&asg, oi_expr_bar));
 }

tamer/src/asg/graph/object/ident.rs

@@ -1052,6 +1052,7 @@ impl ObjectIndex<Ident> {
     pub fn bind_definition<O: ObjectKind>(
         self,
         asg: &mut Asg,
+        id: SPair,
         definition: ObjectIndex<O>,
     ) -> Result<ObjectIndex<Ident>, AsgError>
     where
@@ -1088,7 +1089,11 @@ impl ObjectIndex<Ident> {
             }
 
             // We are okay to proceed to add an edge to the `definition`.
-            Missing(id) => Ok(Transparent(id)),
+            // Discard the original span
+            //   (which is the location of the first reference _to_ this
+            //     identifier before it was defined),
+            //   and use the newly provided `id` and its span.
+            Missing(_) => Ok(Transparent(id)),
         })
         .map(|ident_oi| ident_oi.add_edge_to(asg, definition))
     }