introduce `awaits` (#4495)

1 files changed, 22 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 0ab65868..d5c22063 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -51,6 +51,7 @@ function Compressor(options, false_by_default) {
         arguments       : !false_by_default,
         arrows          : !false_by_default,
         assignments     : !false_by_default,
+        awaits          : !false_by_default,
         booleans        : !false_by_default,
         collapse_vars   : !false_by_default,
         comparisons     : !false_by_default,
@@ -6651,6 +6652,7 @@ merge(Compressor.prototype, {
         });
         def(AST_AsyncFunction, return_null);
         def(AST_Await, function(compressor) {
+            if (!compressor.option("awaits")) return this;
             var exp = this.expression.drop_side_effect_free(compressor);
             if (exp === this.expression) return this;
             var node = this.clone();
@@ -6811,13 +6813,20 @@ merge(Compressor.prototype, {
             var expressions = trim(this.expressions, compressor, first_in_statement);
             if (!expressions) return null;
             var end = expressions.length - 1;
+            var last = expressions[end];
+            if (compressor.option("awaits") && end > 0 && last instanceof AST_Await && last.expression.is_constant()) {
+                expressions = expressions.slice(0, -1);
+                end--;
+                last.expression = expressions[end];
+                expressions[end] = last;
+            }
             var assign, cond, lhs;
             if (compressor.option("conditionals")
                 && end > 0
                 && (assign = expressions[end - 1]) instanceof AST_Assign
                 && assign.operator == "="
                 && (lhs = assign.left) instanceof AST_SymbolRef
-                && (cond = to_conditional_assignment(compressor, lhs.definition(), assign.right, expressions[end]))) {
+                && (cond = to_conditional_assignment(compressor, lhs.definition(), assign.right, last))) {
                 assign = assign.clone();
                 assign.right = cond;
                 expressions = expressions.slice(0, -2);
@@ -8033,7 +8042,8 @@ merge(Compressor.prototype, {
             }
         }
         var fn = exp instanceof AST_SymbolRef ? exp.fixed_value() : exp;
-        var is_func = fn instanceof AST_Lambda && (!is_async(fn) || compressor.parent() instanceof AST_Await);
+        var is_func = fn instanceof AST_Lambda && (!is_async(fn)
+            || compressor.option("awaits") && compressor.parent() instanceof AST_Await);
         var stat = is_func && fn.first_statement();
         var has_default = false;
         var can_drop = is_func && all(fn.argnames, function(argname, index) {
@@ -8665,6 +8675,7 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_Await, function(self, compressor) {
+        if (!compressor.option("awaits")) return self;
         if (compressor.option("sequences")) {
             var seq = lift_sequence_in_expression(self, compressor);
             if (seq !== self) return seq.optimize(compressor);
@@ -8672,7 +8683,15 @@ merge(Compressor.prototype, {
         if (compressor.option("side_effects")) {
             var exp = self.expression;
             if (exp instanceof AST_Await) return exp;
-            if (exp instanceof AST_UnaryPrefix && exp.expression instanceof AST_Await) return exp;
+            if (exp instanceof AST_UnaryPrefix) {
+                if (exp.expression instanceof AST_Await) return exp;
+                if (exp.operator == "void") return make_node(AST_UnaryPrefix, self, {
+                    operator: "void",
+                    expression: make_node(AST_Await, self, {
+                        expression: exp.expression,
+                    }),
+                });
+            }
         }
         return self;
     });