feature: Add strict mode for accessing non-draftables

In strict mode, accessing a non-draftable property will throw
One can use the unsafe function to perform such operations

Fixes immerjs#686

src/core/immerClass.ts

@@ -37,11 +37,19 @@ export class Immer implements ProducersFns {
 
 	autoFreeze_: boolean = true
 
-	constructor(config?: {useProxies?: boolean; autoFreeze?: boolean}) {
+	strictModeEnabled_: boolean = false
+
+	constructor(config?: {
+		useProxies?: boolean
+		autoFreeze?: boolean
+		strictMode?: boolean
+	}) {
 		if (typeof config?.useProxies === "boolean")
 			this.setUseProxies(config!.useProxies)
 		if (typeof config?.autoFreeze === "boolean")
 			this.setAutoFreeze(config!.autoFreeze)
+		if (typeof config?.strictMode === "boolean")
+			this.setStrictMode(config!.strictMode)
 		this.produce = this.produce.bind(this)
 		this.produceWithPatches = this.produceWithPatches.bind(this)
 	}
@@ -183,6 +191,23 @@ export class Immer implements ProducersFns {
 		this.useProxies_ = value
 	}
 
+	/**
+	 * Pass true to throw errors when attempting to access a non-draftable reference.
+	 *
+	 * By default, strict mode is disabled.
+	 */
+	setStrictMode(value: boolean) {
+		this.strictModeEnabled_ = value
+	}
+
+	unsafe(callback: () => void) {
+		const scope = getCurrentScope()
+
+		scope.unsafeNonDraftabledAllowed_ = true
+		callback()
+		scope.unsafeNonDraftabledAllowed_ = false
+	}
+
 	applyPatches(base: Objectish, patches: Patch[]) {
 		// If a patch replaces the entire state, take that replacement as base
 		// before applying patches

src/core/proxy.ts

@@ -108,7 +108,17 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 			return readPropFromProto(state, source, prop)
 		}
 		const value = source[prop]
-		if (state.finalized_ || !isDraftable(value)) {
+		if (state.finalized_) {
+			return value
+		}
+		if (!isDraftable(value)) {
+			if (
+				state.scope_.immer_.strictModeEnabled_ &&
+				!state.scope_.unsafeNonDraftabledAllowed_
+			) {
+				die(24)
+			}
+
 			return value
 		}
 		// Check for existing draft in modified state.

src/core/scope.ts

@@ -22,6 +22,7 @@ export interface ImmerScope {
 	patchListener_?: PatchListener
 	immer_: Immer
 	unfinalizedDrafts_: number
+	unsafeNonDraftabledAllowed_: boolean
 }
 
 let currentScope: ImmerScope | undefined
@@ -42,7 +43,8 @@ function createScope(
 		// Whenever the modified draft contains a draft from another scope, we
 		// need to prevent auto-freezing so the unowned draft can be finalized.
 		canAutoFreeze_: true,
-		unfinalizedDrafts_: 0
+		unfinalizedDrafts_: 0,
+		unsafeNonDraftabledAllowed_: false
 	}
 }
 

src/immer.ts

@@ -67,6 +67,18 @@ export const setAutoFreeze = immer.setAutoFreeze.bind(immer)
  */
 export const setUseProxies = immer.setUseProxies.bind(immer)
 
+/**
+ * Pass true to throw errors when attempting to access a non-draftable reference.
+ *
+ * By default, strict mode is disabled.
+ */
+export const setStrictMode = immer.setStrictMode.bind(immer)
+
+/**
+ * Allow accessing non-draftable references in strict mode inside the callback.
+ */
+export const unsafe = immer.unsafe.bind(immer)
+
 /**
  * Apply an array of Immer patches to the first argument.
  *

src/utils/errors.ts

@@ -38,7 +38,8 @@ const errors = {
 	},
 	23(thing: string) {
 		return `'original' expects a draft, got: ${thing}`
-	}
+	},
+	24: "Cannot get a non-draftable reference in strict mode. Use the `unsafe` function, add the `immerable` symbol, or disable strict mode"
 } as const
 
 export function die(error: keyof typeof errors, ...args: any[]): never {