zclsyntax: specific error message for nested attr-only splat

Attribute-only splat expressions cannot have other splats nested inside,
since we're only interested in supporting how these behaved for HIL when
running inside Hashicorp Terraform. More complex cases should be dealt
with using either full splats (bracketed *) or "for" expressions.

zcl/zclsyntax/expression_test.go

@@ -529,6 +529,15 @@ upper(
 			cty.StringVal("Steve"),
 			0,
 		},
+		{
+			`[["hello"], ["goodbye"]].*.*`,
+			nil,
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{cty.StringVal("hello")}),
+				cty.TupleVal([]cty.Value{cty.StringVal("goodbye")}),
+			}),
+			1,
+		},
 
 		{
 			`["hello"][0]`,

zcl/zclsyntax/parser.go

@@ -512,12 +512,21 @@ Traversal:
 					dot := p.Read()
 					if p.Peek().Type != TokenIdent {
 						if !p.recovery {
-							diags = append(diags, &zcl.Diagnostic{
-								Severity: zcl.DiagError,
-								Summary:  "Invalid attribute name",
-								Detail:   "An attribute name is required after a dot.",
-								Subject:  &attrTok.Range,
-							})
+							if p.Peek().Type == TokenStar {
+								diags = append(diags, &zcl.Diagnostic{
+									Severity: zcl.DiagError,
+									Summary:  "Nested splat expression not allowed",
+									Detail:   "A splat expression (*) cannot be used inside another attribute-only splat expression.",
+									Subject:  p.Peek().Range.Ptr(),
+								})
+							} else {
+								diags = append(diags, &zcl.Diagnostic{
+									Severity: zcl.DiagError,
+									Summary:  "Invalid attribute name",
+									Detail:   "An attribute name is required after a dot.",
+									Subject:  &attrTok.Range,
+								})
+							}
 						}
 						p.setRecovery()
 						continue Traversal