fix: Add a type-checking fast path for primitive types (#755)

When `Draft<T>` is applied to a large enum type, TypeScript has to do a
lot of unnecessary structural comparisons to confirm that no element of
the enum matches `Function`, `Date`, `RegExp`, etc.  Determining that
they do match `string` or `number`, on the other hand, is trivial.  This
change splits `PrimitiveType` out of `AtomicObject` so that the fast
path can be checked first.

In microsoft/TypeScript#42824, this cut the
check time from ~2.5 seconds to ~0.3 seconds.

compat/pre-3.7/dist/immer.d.ts

@@ -5,6 +5,8 @@ type Tail<T extends any[]> = ((...t: T) => any) extends (
 	? TT
 	: []
 
+type PrimitiveType = number | string | boolean
+
 /** Object types that should never be mapped */
 type AtomicObject =
 	| Function
@@ -13,11 +15,10 @@ type AtomicObject =
 	| Promise<any>
 	| Date
 	| RegExp
-	| Boolean
-	| Number
-	| String
 
-export type Draft<T> = T extends AtomicObject
+export type Draft<T> = T extends PrimitiveType
+	? T
+	: T extends AtomicObject
 	? T
 	: T extends Map<infer K, infer V>
 	? DraftMap<K, V>
@@ -34,7 +35,9 @@ interface DraftMap<K, V> extends Map<Draft<K>, Draft<V>> {}
 interface DraftSet<V> extends Set<Draft<V>> {}
 
 /** Convert a mutable type into a readonly type */
-export type Immutable<T> = T extends AtomicObject
+export type Immutable<T> = T extends PrimitiveType
+	? T
+	: T extends AtomicObject
 	? T
 	: T extends Map<infer K, infer V> // Ideally, but wait for TS 3.7:    ? Omit<ImmutableMap<K, V>, "set" | "delete" | "clear">
 	? ImmutableMap<K, V>

src/types/types-external.ts

@@ -7,15 +7,10 @@ type Tail<T extends any[]> = ((...t: T) => any) extends (
 	? TT
 	: []
 
+type PrimitiveType = number | string | boolean
+
 /** Object types that should never be mapped */
-type AtomicObject =
-	| Function
-	| Promise<any>
-	| Date
-	| RegExp
-	| Boolean
-	| Number
-	| String
+type AtomicObject = Function | Promise<any> | Date | RegExp
 
 /**
  * If the lib "ES2105.collections" is not included in tsconfig.json,
@@ -42,7 +37,9 @@ type WeakReferences = IfAvailable<WeakMap<any, any>> | IfAvailable<WeakSet<any>>
 
 export type WritableDraft<T> = {-readonly [K in keyof T]: Draft<T[K]>}
 
-export type Draft<T> = T extends AtomicObject
+export type Draft<T> = T extends PrimitiveType
+	? T
+	: T extends AtomicObject
 	? T
 	: T extends IfAvailable<ReadonlyMap<infer K, infer V>> // Map extends ReadonlyMap
 	? Map<Draft<K>, Draft<V>>
@@ -55,7 +52,9 @@ export type Draft<T> = T extends AtomicObject
 	: T
 
 /** Convert a mutable type into a readonly type */
-export type Immutable<T> = T extends AtomicObject
+export type Immutable<T> = T extends PrimitiveType
+	? T
+	: T extends AtomicObject
 	? T
 	: T extends IfAvailable<ReadonlyMap<infer K, infer V>> // Map extends ReadonlyMap
 	? ReadonlyMap<Immutable<K>, Immutable<V>>