lib: fix implementation of makeAs to prevent hiding error locations

The previous implementation made an index call to collect the value,
hiding the location of errors from the error syntax renderer. Fix and
simplify the macro implementation to avoid this.

lib/collections.go

@@ -25,7 +25,6 @@ import (
 	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common"
 	"github.com/google/cel-go/common/ast"
-	"github.com/google/cel-go/common/operators"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
 	"github.com/google/cel-go/common/types/traits"
@@ -1045,21 +1044,25 @@ func mapValues(val ref.Val) ref.Val {
 	return types.NewRefValList(types.DefaultTypeAdapter, values)
 }
 
-func makeAs(eh parser.ExprHelper, target ast.Expr, args []ast.Expr) (ast.Expr, *common.Error) {
+func makeAs(mef cel.MacroExprFactory, target ast.Expr, args []ast.Expr) (ast.Expr, *cel.Error) {
+	init := target
 	ident := args[0]
+	expr := args[1]
 	if ident.Kind() != ast.IdentKind {
 		return nil, &common.Error{Message: "argument is not an identifier"}
 	}
 	label := ident.AsIdent()
 
-	fn := args[1]
-	target = eh.NewList(target) // Fold is a list comprehension, so fake this.
-	accuExpr := eh.NewAccuIdent()
-	init := eh.NewList() // Also for the result.
-	condition := eh.NewLiteral(types.True)
-	step := eh.NewCall(operators.Add, accuExpr, eh.NewList(fn))
-	fold := eh.NewComprehension(target, label, parser.AccumulatorName, init, condition, step, accuExpr)
-	return eh.NewCall(operators.Index, fold, eh.NewLiteral(types.IntZero)), nil
+	const unused = "_"
+	return mef.NewComprehension(
+		mef.NewList(),
+		unused,
+		label,
+		init,
+		mef.NewLiteral(types.False),
+		mef.NewIdent(label),
+		expr,
+	), nil
 }
 
 // pathSepIndex returns the offset to a non-escaped dot path separator and

testdata/macro_error.txt

@@ -0,0 +1,12 @@
+! mito -data state.json src.cel
+! stdout .
+cmp stderr want_err.txt
+
+-- state.json --
+{"url": "", "id": 0}
+-- src.cel --
+get(state.url+state.id).as(r, r.map(k, k))
+-- want_err.txt --
+failed eval: ERROR: <input>:1:14: no such overload
+ | get(state.url+state.id).as(r, r.map(k, k))
+ | .............^