Recap March 2022 (#439)

* March Recap

Co-authored-by: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>

docs/_posts/2022-03-31-luau-recap-march-2022.md

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+---
+layout: single
+title:  "Luau Recap: March 2022"
+---
+
+Luau is our new language that you can read more about at [https://luau-lang.org](https://luau-lang.org).
+
+[Cross-posted to the [Roblox Developer Forum](https://devforum.roblox.com/t/luau-recap-march-2022/).]
+
+## Singleton types
+
+We added support for singleton types! These allow you to use string or
+boolean literals in types. These types are only inhabited by the
+literal, for example if a variable `x` has type `"foo"`, then `x ==
+"foo"` is guaranteed to be true.
+
+Singleton types are particularly useful when combined with union types,
+for example:
+
+```lua
+type Animals = "Dog" | "Cat" | "Bird"
+```
+
+or:
+
+```lua
+type Falsey = false | nil
+```
+
+In particular, singleton types play well with unions of tables,
+allowing tagged unions (also known as discriminated unions):
+
+```lua
+type Ok<T> = { type: "ok", value: T }
+type Err<E> = { type: "error", error: E }
+type Result<T, E> = Ok<T> | Err<E>
+
+local result: Result<number, string> = ...
+if result.type == "ok" then
+    -- result :: Ok<number>
+    print(result.value)
+else
+    -- result :: Err<string>
+    error(result.error)
+end
+```
+
+The RFC for singleton types is https://github.com/Roblox/luau/blob/master/rfcs/syntax-singleton-types.md
+
+## Width subtyping
+
+A common idiom for programming with tables is to provide a public interface type, but to keep some of the concrete implementation private, for example:
+
+```lua
+type Interface = {
+    name: string,
+}
+
+type Concrete = {
+    name: string,
+    id: number,
+}
+```
+
+Within a module, a developer might use the concrete type, but export functions using the interface type:
+
+```lua
+local x: Concrete = {
+    name = "foo",
+    id = 123,
+}
+
+local function get(): Interface
+    return x
+end
+```
+
+Previously examples like this did not typecheck but now they do!
+
+This language feature is called *width subtyping* (it allows tables to get *wider*, that is to have more properties).
+
+The RFC for width subtyping is https://github.com/Roblox/luau/blob/master/rfcs/sealed-table-subtyping.md
+
+## Typechecking improvements
+
+ * Generic function type inference now works the same for generic types and generic type packs.
+ * We improved some error messages.
+ * There are now fewer crashes (hopefully none!) due to mutating types inside the Luau typechecker.
+ * We fixed a bug that could cause two incompatible copies of the same class to be created.
+ * Luau now copes better with cyclic metatable types (it gives a type error rather than hanging).
+ * Fixed a case where types are not properly bound to all of the subtype when the subtype is a union.
+ * We fixed a bug that confused union and intersection types of table properties.
+ * Functions declared as `function f(x : any)` can now be called as `f()` without a type error.
+
+## API improvements
+
+ * Implement `table.clone` which takes a table and returns a new table that has the same keys/values/metatable. The cloning is shallow - if some keys refer to tables that need to be cloned, that can be done manually by modifying the resulting table.
+
+## Debugger improvements
+
+ * Use the property name as the name of methods in the debugger.
+
+## Performance improvements
+
+ * Optimize table rehashing (~15% faster dictionary table resize on average)
+ * Improve performance of freeing tables (~5% lift on some GC benchmarks)
+ * Improve gathering performance metrics for GC.
+ * Reduce stack memory reallocation.
+