Add support for /*#__PURE__*/ comments.

src/Module.ts

@@ -1,4 +1,5 @@
 import { IParse, Options as AcornOptions } from 'acorn';
+import walk from 'acorn/dist/walk';
 import * as ESTree from 'estree';
 import { locate } from 'locate-character';
 import MagicString from 'magic-string';
@@ -159,6 +160,38 @@ function handleMissingExport(
 	);
 }
 
+// Find syntax nodes following comments
+function findNodesAfterComments(ast, commentNodes) {
+	const state = {
+		commentIdx: 0,
+		nodes: []
+	};
+
+	(function c(node, state, override) {
+		if (state.commentIdx === commentNodes.length) return;
+		const pos = commentNodes[state.commentIdx].end;
+		if (node.end < pos) return;
+		const type = override || node.type;
+		if (node.start >= pos) {
+			state.commentIdx++;
+			state.nodes.push(node);
+		}
+		walk.base[type](node, state, c);
+	})(ast, state);
+
+	return state.nodes;
+}
+
+function markNodePure(node) {
+	if (node.type === 'ExpressionStatement') {
+		markNodePure(node.expression);
+	} else if (node.type === 'CallExpression') {
+		node.markedPure = true;
+	}
+}
+
+const pureCommentRegex = /^ ?#__PURE__\s*$/;
+
 export default class Module {
 	type: 'Module';
 	private graph: Graph;
@@ -258,6 +291,11 @@ export default class Module {
 
 		timeEnd('generate ast', 3);
 
+		const pureMarkerComments = this.comments.filter(comment => pureCommentRegex.test(comment.text));
+		const nodesAfterPureComments = findNodesAfterComments(this.esTreeAst, pureMarkerComments);
+
+		nodesAfterPureComments.forEach(node => markNodePure(node));
+
 		this.resolvedIds = resolvedIds || Object.create(null);
 
 		// By default, `id` is the filename. Custom resolvers and loaders

src/ast/nodes/CallExpression.ts

@@ -135,6 +135,8 @@ export default class CallExpression extends NodeBase implements DeoptimizableEnt
 		for (const argument of this.arguments) {
 			if (argument.hasEffects(options)) return true;
 		}
+		debugger;
+		if (this.markedPure) return false;
 		return (
 			this.callee.hasEffects(options) ||
 			this.callee.hasEffectsWhenCalledAtPath(

test/function/samples/call-marked-pure/_config.js

@@ -0,0 +1,26 @@
+const assert = require('assert');
+
+module.exports = {
+	description: 'functions marked with pure comment do not have effects',
+	context: {
+		require(id) {
+			if (id === 'socks') {
+				return () => '🧦';
+			}
+		}
+	},
+	code(code) {
+		assert.ok(code.search(/socks\(\)/) === -1);
+	},
+	warnings: [
+		{
+			code: 'UNRESOLVED_IMPORT',
+			importer: 'main.js',
+			message:
+				"'socks' is imported by main.js, but could not be resolved – treating it as an external dependency",
+			source: 'socks',
+			url:
+				'https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency'
+		}
+	]
+};

test/function/samples/call-marked-pure/main.js

@@ -0,0 +1,8 @@
+import socks from 'socks';
+
+/*#__PURE__*/ socks();
+/* #__PURE__*/ socks();
+/*#__PURE__ */ socks();
+/* #__PURE__ */ socks();
+// #__PURE__        
+socks();