ast: clarify property name parser (#228)

The property access parser has been getting increasingly gnarly, and
will continue to do so as it evolves to handle accessor ranges. These
changes refactor the parser for simplicity and clarity. As a nice
side-effect, they also improve some error reporting.

ast/interpolation.go

@@ -36,6 +36,7 @@ func parseInterpolate(node syntax.Node, value string) ([]Interpolation, syntax.D
 			value = value[2:]
 		case strings.HasPrefix(value, "${"):
 			rest, access, accessDiags := parsePropertyAccess(node, value[2:])
+
 			diags.Extend(accessDiags...)
 			parts = append(parts, Interpolation{
 				Text:  str.String(),

ast/interpolation_test.go

@@ -21,115 +21,74 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func mkInterp(text string, accessors ...PropertyAccessor) Interpolation {
+	return Interpolation{
+		Text:  text,
+		Value: mkAccess(accessors...),
+	}
+}
+
+func mkAccess(accessors ...PropertyAccessor) *PropertyAccess {
+	if len(accessors) == 0 {
+		return nil
+	}
+	return &PropertyAccess{Accessors: accessors}
+}
+
+func mkPropertyName(name string) *PropertyName {
+	return &PropertyName{Name: name}
+}
+
+func mkPropertySubscript[T string | int](index T) *PropertySubscript {
+	return &PropertySubscript{Index: index}
+}
+
 func TestInvalidInterpolations(t *testing.T) {
 	cases := []struct {
 		text  string
 		parts []Interpolation
 	}{
 		{
-			text: "${foo",
-			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "foo"},
-						},
-					},
-				},
-			},
+			text:  "${foo",
+			parts: []Interpolation{mkInterp("", mkPropertyName("foo"))},
 		},
 		{
 			text: "${foo ",
 			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "foo"},
-						},
-					},
-				},
-				{
-					Text: " ",
-				},
+				mkInterp("", mkPropertyName("foo")),
+				mkInterp(" "),
 			},
 		},
 		{
 			text: `${foo} ${["baz} bar`,
 			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "foo"},
-						},
-					},
-				},
-				{
-					Text: " ",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertySubscript{Index: "baz} bar"},
-						},
-					},
-				},
+				mkInterp("", mkPropertyName("foo")),
+				mkInterp(" ", mkPropertySubscript("baz} bar")),
 			},
 		},
 		{
 			text: `missing ${property[} subscript`,
 			parts: []Interpolation{
-				{
-					Text: "missing ",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "property"},
-						},
-					},
-				},
-				{
-					Text: " subscript",
-				},
+				mkInterp("missing ", mkPropertyName("property"), mkPropertySubscript("")),
+				mkInterp(" subscript"),
 			},
 		},
 		{
 			text: `${[bar].baz}`,
 			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertySubscript{Index: 0},
-							&PropertyName{Name: "baz"},
-						},
-					},
-				},
+				mkInterp("", mkPropertySubscript("bar"), mkPropertyName("baz")),
 			},
 		},
 		{
 			text: `${foo.`,
 			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "foo"},
-						},
-					},
-				},
+				mkInterp("", mkPropertyName("foo"), mkPropertyName("")),
 			},
 		},
 		{
 			text: `${foo[`,
 			parts: []Interpolation{
-				{
-					Text: "",
-					Value: &PropertyAccess{
-						Accessors: []PropertyAccessor{
-							&PropertyName{Name: "foo"},
-						},
-					},
-				},
+				mkInterp("", mkPropertyName("foo"), mkPropertySubscript("")),
 			},
 		},
 	}