Clarified the use of var (AV1520) and supplemented AV1707 (names) (#252)

_rules/1520.md

@@ -1,22 +1,33 @@
 ---
 rule_id: 1520
 rule_category: maintainability
-title: Only use `var` when the type is very obvious
+title: Only use `var` when the type is evident
 severity: 1
 ---
-Only use `var` as the result of a LINQ query, or if the type is very obvious from the same statement and using it would improve readability. So don't
+Use `var` for anonymous types (typically resulting from a LINQ query), or if the type is [evident](https://www.jetbrains.com/help/resharper/2021.3/Using_var_Keyword_in_Declarations.html#use-var-when-evident-details).
+Never use `var` for [built-in types](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/built-in-types).
 
-	// what type? int? uint? float?
-	var item = 3;
+	// Projection into anonymous type.
+	var largeOrders =
+		from order in dbContext.Orders
+		where order.Items.Count > 10 && order.TotalAmount > 1000
+		select new { order.Id, order.TotalAmount };
 
-	// Not obvious what base-class or interface to expect.
-	// Also difficult to refactor if you can't search for the class.
-	var myfoo = MyFactoryMethod.Create("arg");
+	// Built-in types.
+	bool isValid = true;
+	string phoneNumber = "(unavailable)";
+	uint pageSize = Math.Max(itemCount, MaxPageSize);
 
-Instead, use `var` like this:
+	// Types are evident.
+	var customer = new Customer();
+	var invoice = Invoice.Create(customer.Id);
+	var user = sessionCache.Resolve<User>("[email protected]");
+	var subscribers = new List<Subscriber>();
+	var summary = shoppingBasket.ToOrderSummary();
 
-	var query = from order in orders where order.Items > 10 and order.TotalValue > 1000;
-	var repository = new RepositoryFactory.Get();
-	var list = new ReadOnlyCollection();
-
-In all of three above examples it is clear what type to expect. For a more detailed rationale about the advantages and disadvantages of using `var`, read Eric Lippert's [Uses and misuses of implicit typing](https://docs.microsoft.com/en-us/archive/blogs/ericlippert/uses-and-misuses-of-implicit-typing).
+	// All other cases.
+	IQueryable<Order> recentOrders = ApplyFilter(order => order.CreatedAt > DateTime.Now.AddDays(-30));
+	LoggerMessage message = Compose(context);
+	ReadOnlySpan<string> key = ExtractKeyFromPair("[email protected]");
+	IDictionary<Category, Product> productsPerCategory =
+		shoppingBasket.Products.ToDictionary(product => product.Category);

_rules/1707.md

@@ -7,3 +7,4 @@ severity: 2
 - Use functional names. For example, `GetLength` is a better name than `GetInt`.
 - Don't use terms like `Enum`, `Class` or `Struct` in a name.
 - Identifiers that refer to a collection type should have plural names.
+- Don't include the type in variable names, except to avoid clashes with other variables.