Do not track side-effect-free array methods as side effects

src/ast/nodes/shared/ArrayPrototype.ts

@@ -16,46 +16,46 @@ const NEW_ARRAY_PROPERTIES: ObjectProperty[] = [
 	{ key: 'length', kind: 'init', property: UNKNOWN_LITERAL_NUMBER }
 ];
 
-const METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_BOOLEAN: [ExpressionEntity] = [
+const METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_BOOLEAN: [ExpressionEntity] = [
 	new Method({
 		callsArgs: [0],
-		mutatesSelfAsArray: true,
+		mutatesSelfAsArray: 'deopt-only',
 		returns: null,
 		returnsPrimitive: UNKNOWN_LITERAL_BOOLEAN
 	})
 ];
 
-const METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_NUMBER: [ExpressionEntity] = [
+const METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_NUMBER: [ExpressionEntity] = [
 	new Method({
 		callsArgs: [0],
-		mutatesSelfAsArray: true,
+		mutatesSelfAsArray: 'deopt-only',
 		returns: null,
 		returnsPrimitive: UNKNOWN_LITERAL_NUMBER
 	})
 ];
 
-export const METHOD_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
+const METHOD_MUTATES_SELF_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
 	new Method({
 		callsArgs: null,
-		mutatesSelfAsArray: false,
+		mutatesSelfAsArray: true,
 		returns: () => new ObjectEntity(NEW_ARRAY_PROPERTIES, ARRAY_PROTOTYPE),
 		returnsPrimitive: null
 	})
 ];
 
-const METHOD_MUTATES_SELF_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
+const METHOD_DEOPTS_SELF_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
 	new Method({
 		callsArgs: null,
-		mutatesSelfAsArray: true,
+		mutatesSelfAsArray: 'deopt-only',
 		returns: () => new ObjectEntity(NEW_ARRAY_PROPERTIES, ARRAY_PROTOTYPE),
 		returnsPrimitive: null
 	})
 ];
 
-const METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
+const METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_NEW_ARRAY: [ExpressionEntity] = [
 	new Method({
 		callsArgs: [0],
-		mutatesSelfAsArray: true,
+		mutatesSelfAsArray: 'deopt-only',
 		returns: () => new ObjectEntity(NEW_ARRAY_PROPERTIES, ARRAY_PROTOTYPE),
 		returnsPrimitive: null
 	})
@@ -79,10 +79,19 @@ const METHOD_MUTATES_SELF_RETURNS_UNKNOWN: [ExpressionEntity] = [
 	})
 ];
 
-const METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_UNKNOWN: [ExpressionEntity] = [
+const METHOD_DEOPTS_SELF_RETURNS_UNKNOWN: [ExpressionEntity] = [
+	new Method({
+		callsArgs: null,
+		mutatesSelfAsArray: 'deopt-only',
+		returns: null,
+		returnsPrimitive: UNKNOWN_EXPRESSION
+	})
+];
+
+const METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_UNKNOWN: [ExpressionEntity] = [
 	new Method({
 		callsArgs: [0],
-		mutatesSelfAsArray: true,
+		mutatesSelfAsArray: 'deopt-only',
 		returns: null,
 		returnsPrimitive: UNKNOWN_EXPRESSION
 	})
@@ -110,34 +119,34 @@ export const ARRAY_PROTOTYPE = new ObjectEntity(
 	({
 		__proto__: null,
 		// We assume that accessors have effects as we do not track the accessed value afterwards
-		at: METHOD_MUTATES_SELF_RETURNS_UNKNOWN,
-		concat: METHOD_RETURNS_NEW_ARRAY,
+		at: METHOD_DEOPTS_SELF_RETURNS_UNKNOWN,
+		concat: METHOD_DEOPTS_SELF_RETURNS_NEW_ARRAY,
 		copyWithin: METHOD_MUTATES_SELF_RETURNS_SELF,
-		entries: METHOD_MUTATES_SELF_RETURNS_NEW_ARRAY,
-		every: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_BOOLEAN,
+		entries: METHOD_DEOPTS_SELF_RETURNS_NEW_ARRAY,
+		every: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_BOOLEAN,
 		fill: METHOD_MUTATES_SELF_RETURNS_SELF,
-		filter: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_NEW_ARRAY,
-		find: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_UNKNOWN,
-		findIndex: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_NUMBER,
-		forEach: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_UNKNOWN,
+		filter: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_NEW_ARRAY,
+		find: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_UNKNOWN,
+		findIndex: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_NUMBER,
+		forEach: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_UNKNOWN,
 		includes: METHOD_RETURNS_BOOLEAN,
 		indexOf: METHOD_RETURNS_NUMBER,
 		join: METHOD_RETURNS_STRING,
 		keys: METHOD_RETURNS_UNKNOWN,
 		lastIndexOf: METHOD_RETURNS_NUMBER,
-		map: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_NEW_ARRAY,
+		map: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_NEW_ARRAY,
 		pop: METHOD_MUTATES_SELF_RETURNS_UNKNOWN,
 		push: METHOD_MUTATES_SELF_RETURNS_NUMBER,
-		reduce: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_UNKNOWN,
-		reduceRight: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_UNKNOWN,
+		reduce: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_UNKNOWN,
+		reduceRight: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_UNKNOWN,
 		reverse: METHOD_MUTATES_SELF_RETURNS_SELF,
 		shift: METHOD_MUTATES_SELF_RETURNS_UNKNOWN,
-		slice: METHOD_MUTATES_SELF_RETURNS_NEW_ARRAY,
-		some: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_BOOLEAN,
+		slice: METHOD_DEOPTS_SELF_RETURNS_NEW_ARRAY,
+		some: METHOD_CALLS_ARG_DEOPTS_SELF_RETURNS_BOOLEAN,
 		sort: METHOD_CALLS_ARG_MUTATES_SELF_RETURNS_SELF,
 		splice: METHOD_MUTATES_SELF_RETURNS_NEW_ARRAY,
 		unshift: METHOD_MUTATES_SELF_RETURNS_NUMBER,
-		values: METHOD_MUTATES_SELF_RETURNS_UNKNOWN
+		values: METHOD_DEOPTS_SELF_RETURNS_UNKNOWN
 	} as unknown) as PropertyMap,
 	OBJECT_PROTOTYPE
 );

src/ast/nodes/shared/MethodTypes.ts

@@ -13,7 +13,7 @@ import { ExpressionNode } from './Node';
 
 type MethodDescription = {
 	callsArgs: number[] | null;
-	mutatesSelfAsArray: boolean;
+	mutatesSelfAsArray: boolean | 'deopt-only';
 } & (
 	| {
 			returns: 'self' | (() => ExpressionEntity);
@@ -70,7 +70,7 @@ export class Method extends ExpressionEntity {
 	): boolean {
 		if (
 			path.length > 0 ||
-			(this.description.mutatesSelfAsArray &&
+			(this.description.mutatesSelfAsArray === true &&
 				callOptions.thisParam?.hasEffectsWhenAssignedAtPath(UNKNOWN_INTEGER_PATH, context))
 		) {
 			return true;

test/form/samples/builtin-prototypes/side-effect-free-array-expression/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'Tree-shakes side-effect-free array functions if only their return value is unused'
+};

test/form/samples/builtin-prototypes/side-effect-free-array-expression/_expected.js

@@ -0,0 +1,41 @@
+const foo1 = [1, 2, 3];
+console.log(foo1[0]);
+
+const foo2 = [1, 2, 3];
+console.log(foo2[0]);
+
+const foo3 = [1, 2, 3];
+console.log(foo3[0]);
+
+const foo4 = [1, 2, 3];
+console.log(foo4[0]);
+
+const foo5 = [1, 2, 3];
+console.log(foo5[0]);
+
+const foo6 = [1, 2, 3];
+console.log(foo6[0]);
+
+const foo7 = [1, 2, 3];
+console.log(foo7[0]);
+
+const foo8 = [1, 2, 3];
+console.log(foo8[0]);
+
+const foo9 = [1, 2, 3];
+console.log(foo9[0]);
+
+const foo10 = [1, 2, 3];
+console.log(foo10[0]);
+
+const foo11 = [1, 2, 3];
+console.log(foo11[0]);
+
+const foo12 = [1, 2, 3];
+console.log(foo12[0]);
+
+const foo13 = [1, 2, 3];
+console.log(foo13[0]);
+
+const foo14 = [1, 2, 3];
+console.log(foo14[0]);

test/form/samples/builtin-prototypes/side-effect-free-array-expression/main.js

@@ -0,0 +1,55 @@
+const foo1 = [1, 2, 3];
+foo1.at(0);
+console.log(foo1[0]);
+
+const foo2 = [1, 2, 3];
+foo2.concat([0]);
+console.log(foo2[0]);
+
+const foo3 = [1, 2, 3];
+foo3.entries();
+console.log(foo3[0]);
+
+const foo4 = [1, 2, 3];
+foo4.every(v => v);
+console.log(foo4[0]);
+
+const foo5 = [1, 2, 3];
+foo5.filter(v => v % 1 === 0);
+console.log(foo5[0]);
+
+const foo6 = [1, 2, 3];
+foo6.find(v => v);
+console.log(foo6[0]);
+
+const foo7 = [1, 2, 3];
+foo7.findIndex(v => v);
+console.log(foo7[0]);
+
+const foo8 = [1, 2, 3];
+foo8.forEach(() => {});
+console.log(foo8[0]);
+
+const foo9 = [1, 2, 3];
+foo9.map(v => v);
+console.log(foo9[0]);
+
+const foo10 = [1, 2, 3];
+foo10.reduce((a, v) => a + v, 0);
+console.log(foo10[0]);
+
+const foo11 = [1, 2, 3];
+foo11.reduceRight((a, v) => a + v, 0);
+console.log(foo11[0]);
+
+const foo12 = [1, 2, 3];
+foo12.slice(1);
+console.log(foo12[0]);
+
+const foo13 = [1, 2, 3];
+foo13.some(v => v);
+console.log(foo13[0]);
+
+const foo14 = [1, 2, 3];
+foo14.values();
+console.log(foo14[0]);