Error if attempting to use marked value as key

When evaluating an HCL expression attempting
to use a marked value as an object key,
return an error rather than falling through
to the cty panic. The error style mimics similar
errors in the area.

hclsyntax/expression.go

@@ -825,6 +825,19 @@ func (e *ObjectConsExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics
 			continue
 		}
 
+		if key.IsMarked() {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity:    hcl.DiagError,
+				Summary:     "Marked value as key",
+				Detail:      "Can't use a marked value as a key.",
+				Subject:     item.ValueExpr.Range().Ptr(),
+				Expression:  item.KeyExpr,
+				EvalContext: ctx,
+			})
+			known = false
+			continue
+		}
+
 		var err error
 		key, err = convert.Convert(key, cty.String)
 		if err != nil {

hclsyntax/expression_test.go

@@ -506,6 +506,19 @@ upper(
 			}),
 			0,
 		},
+		{
+			// Marked values as object keys
+			`{(var.greeting) = "world", "goodbye" = "earth"}`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"var": cty.ObjectVal(map[string]cty.Value{
+						"greeting": cty.StringVal("hello").Mark("marked"),
+					}),
+				},
+			},
+			cty.DynamicVal,
+			1,
+		},
 		{
 			`{"${var.greeting}" = "world"}`,
 			&hcl.EvalContext{