internal/rosa/azalea: pass through source ident

For source handle special case.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/azalea/evaluate.go

@@ -142,10 +142,13 @@ var (
 	IdentInputs = unique.Make(Ident("inputs"))
 	// IdentRuntime has the same semantics as [IdentInputs].
 	IdentRuntime = unique.Make(Ident("runtime"))
+	// IdentSource is a special argument in a package declaration where an
+	// assignment of a [Val] with a single [Ident] passes through the evaluator.
+	IdentSource = unique.Make(Ident("source"))
 
-	// ErrInvalidInputs is panicked for an [IdentInputs] argument to [PF]
-	// sharing its value or set for R.
-	ErrInvalidInputs = errors.New("inputs must not be common or bound to scope")
+	// ErrInvalidSpecial is panicked for a special [PF] argument sharing its
+	// value or set for R.
+	ErrInvalidSpecial = errors.New("special must not be common or bound to scope")
 )
 
 // evaluateAny implements [Evaluate].
@@ -291,7 +294,7 @@ func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 				} {
 					if slices.Contains(names, special) {
 						if len(names) != 1 || len(arg.V) != 1 || arg.R {
-							panic(ErrInvalidInputs)
+							panic(ErrInvalidSpecial)
 						}
 						farg.K = names[0]
 						if err := storeE(&farg.V, arg.V[0]); err != nil {
@@ -301,6 +304,20 @@ func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 						continue args
 					}
 				}
+
+				if slices.Contains(names, IdentSource) {
+					if len(names) != 1 || len(arg.V) != 1 || arg.R {
+						panic(ErrInvalidSpecial)
+					}
+					if _, ok = arg.V[0].(Ident); ok {
+						farg.K = names[0]
+						if err := storeE(&farg.V, arg.V[0]); err != nil {
+							panic(err)
+						}
+						fargs = append(fargs, farg)
+						continue args
+					}
+				}
 			}
 
 			if !evaluateAny(d, s, arg.V, &farg.V) {