tamer: asg::air::air: Eliminate todo! for unexpected AirIdent

I had apparently forgotten about this, because I didn't benefit from the
exhaustiveness check; this needs to be eliminated so that this doesn't
happen again, and to provide a proper non-panicking error.

DEV-13162

tamer/src/asg/air.rs

@@ -310,7 +310,9 @@ impl ParseState for AirAggregate {
                 Transition(Toplevel(oi_pkg)).incomplete()
             }
 
-            (st, tok @ AirIdent(..)) => todo!("{st:?}, {tok:?}"),
+            (st @ (Empty | PkgExpr(..) | PkgTpl(..)), AirIdent(tok)) => {
+                Transition(st).err(AsgError::UnexpectedOpaqueIdent(tok.name()))
+            }
         }
     }
 

tamer/src/asg/air/ir.rs

@@ -825,3 +825,18 @@ sum_ir! {
     /// Tokens that may be used to define or apply templates.
     pub sum enum AirBindableTpl = AirTpl | AirBind | AirDoc;
 }
+
+impl AirIdent {
+    /// Name of the identifier described by this token.
+    pub fn name(&self) -> SPair {
+        use AirIdent::*;
+
+        match self {
+            IdentDecl(name, _, _)
+            | IdentExternDecl(name, _, _)
+            | IdentDep(name, _)
+            | IdentFragment(name, _)
+            | IdentRoot(name) => *name,
+        }
+    }
+}

tamer/src/asg/error.rs

@@ -155,6 +155,9 @@ pub enum AsgError {
     ///   depends on first having computed itself,
     ///     which is not possible.
     UnsupportedCycle(Cycle),
+
+    /// An opaque identifier was declared in an invalid context.
+    UnexpectedOpaqueIdent(SPair),
 }
 
 impl Display for AsgError {
@@ -212,6 +215,13 @@ impl Display for AsgError {
             UnsupportedCycle(cycle) => {
                 write!(f, "circular dependency: {cycle}")
             }
+            UnexpectedOpaqueIdent(name) => {
+                write!(
+                    f,
+                    "opaque identifier {} declaration",
+                    TtQuote::wrap(name)
+                )
+            }
         }
     }
 }
@@ -371,6 +381,12 @@ impl Diagnostic for AsgError {
 
                 desc
             }
+            // TODO: This doesn't seem all that helpful.
+            //   What are the circumstances under which this can be hit,
+            //     and what additional information can we provide?
+            UnexpectedOpaqueIdent(name) => vec![name.error(
+                "an opaque identifier declaration was not expected here",
+            )],
         }
     }
 }