introduce hoist_exports (#4651)

README.md

@@ -672,6 +672,9 @@ to be `false` and all symbol names will be omitted.
 
 - `global_defs` (default: `{}`) -- see [conditional compilation](#conditional-compilation)
 
+- `hoist_exports` (default: `true`) -- hoist `export` statements to facilitate
+  various `compress` and `mangle` optimizations.
+
 - `hoist_funs` (default: `false`) -- hoist function declarations
 
 - `hoist_props` (default: `true`) -- hoist properties from constant object and

lib/compress.js

@@ -65,6 +65,7 @@ function Compressor(options, false_by_default) {
         expression      : false,
         functions       : !false_by_default,
         global_defs     : false,
+        hoist_exports   : !false_by_default,
         hoist_funs      : false,
         hoist_props     : !false_by_default,
         hoist_vars      : false,
@@ -183,6 +184,7 @@ merge(Compressor.prototype, {
     },
     compress: function(node) {
         node = node.resolve_defines(this);
+        node.hoist_exports(this);
         if (this.option("expression")) {
             node.process_expression(true);
         }
@@ -260,6 +262,35 @@ merge(Compressor.prototype, {
         return this.TYPE == node.TYPE && this.print_to_string() == node.print_to_string();
     });
 
+    AST_Toplevel.DEFMETHOD("hoist_exports", function(compressor) {
+        if (!compressor.option("hoist_exports")) return;
+        var body = this.body, props = [];
+        for (var i = 0; i < body.length; i++) {
+            var stat = body[i];
+            if (stat instanceof AST_ExportDeclaration) {
+                body[i] = stat = stat.body;
+                if (stat instanceof AST_Definitions) {
+                    stat.definitions.forEach(function(defn) {
+                        defn.name.match_symbol(export_symbol, true);
+                    });
+                } else {
+                    export_symbol(stat.name);
+                }
+            } else if (stat instanceof AST_ExportReferences) {
+                body.splice(i, 1);
+                [].push.apply(props, stat.properties);
+            }
+        }
+        if (props.length) body.push(make_node(AST_ExportReferences, this, { properties: props }));
+
+        function export_symbol(sym) {
+            if (!(sym instanceof AST_SymbolDeclaration)) return;
+            var node = make_node(AST_SymbolExport, sym, sym);
+            node.alias = node.name;
+            props.push(node);
+        }
+    });
+
     AST_Scope.DEFMETHOD("process_expression", function(insert, transform) {
         var self = this;
         var tt = new TreeTransformer(function(node) {
@@ -9871,6 +9902,10 @@ merge(Compressor.prototype, {
         }
     });
 
+    OPT(AST_SymbolExport, function(self) {
+        return self;
+    });
+
     function recursive_ref(compressor, def) {
         var level = 0, node = compressor.self();
         do {

test/compress/exports.js

@@ -142,3 +142,38 @@ mangle_rename: {
         }
     }
 }
+
+hoist_exports: {
+    options = {
+        evaluate: true,
+        hoist_exports: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        const a = 42;
+        export let bbb, { foo: ccc } = a;
+        export function fff(d, { [bbb]: e }) {
+            d(e, fff);
+        }
+        export default a;
+        export default async function g(x, ...{ [ccc]: y }) {
+            (await x)(g, y);
+        }
+    }
+    expect: {
+        let f, { foo: o } = 42;
+        function c(t, { [f]: a }) {
+            t(a, c);
+        }
+        export default 42;
+        export default async function t(a, ...{ [o]: f }) {
+            (await a)(t, f);
+        };
+        export { f as bbb, o as ccc, c as fff };
+    }
+}