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Fix broken edge case of async function lowering #390

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merged 6 commits into from
Sep 18, 2020
Merged

Fix broken edge case of async function lowering #390

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2fd4b70
Add async lowering regression test
rtsao Sep 16, 2020
fe48853
Fix bug in async lowering
rtsao Sep 16, 2020
197c8b2
Improve fix to keep arguments optimization when possible
rtsao Sep 17, 2020
d4fe724
Add additional regression test for optimization case
rtsao Sep 17, 2020
ee4a24c
Remove extra whitespace
rtsao Sep 17, 2020
598d53c
Skip param forwarding when obviously cannot throw
rtsao Sep 17, 2020
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2 changes: 1 addition & 1 deletion internal/bundler/snapshots/snapshots_lower.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -51,7 +51,7 @@ export default [
}
},
function() {
return (_0) => __async(this, arguments, function* (bar) {
return (_0, ..._1) => __async(this, [_0, ..._1], function* (bar) {
yield bar;
return [this, arguments];
});
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4 changes: 2 additions & 2 deletions internal/js_parser/js_parser.go
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Expand Up @@ -8987,7 +8987,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
p.pushScopeForVisitPass(js_ast.ScopeFunctionBody, e.Body.Loc)
e.Body.Stmts = p.visitStmtsAndPrependTempRefs(e.Body.Stmts)
p.popScope()
p.lowerFunction(&e.IsAsync, &e.Args, e.Body.Loc, &e.Body.Stmts, &e.PreferExpr, &e.HasRestArg)
p.lowerFunction(&e.IsAsync, &e.Args, e.Body.Loc, &e.Body.Stmts, &e.PreferExpr, &e.HasRestArg, true)
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I suppose adding something like &e.isArrow might be more readable. But I figured omitting such a property on Fn was intentional?

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Yes, that's correct. This change sounds good.

p.popScope()

if p.MangleSyntax && len(e.Body.Stmts) == 1 {
Expand Down Expand Up @@ -9133,7 +9133,7 @@ func (p *parser) visitFn(fn *js_ast.Fn, scopeLoc logger.Loc) {
p.pushScopeForVisitPass(js_ast.ScopeFunctionBody, fn.Body.Loc)
fn.Body.Stmts = p.visitStmtsAndPrependTempRefs(fn.Body.Stmts)
p.popScope()
p.lowerFunction(&fn.IsAsync, &fn.Args, fn.Body.Loc, &fn.Body.Stmts, nil, &fn.HasRestArg)
p.lowerFunction(&fn.IsAsync, &fn.Args, fn.Body.Loc, &fn.Body.Stmts, nil, &fn.HasRestArg, false)
p.popScope()

p.fnOrArrowDataVisit = oldFnOrArrowData
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13 changes: 11 additions & 2 deletions internal/js_parser/js_parser_lower.go
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Expand Up @@ -139,6 +139,7 @@ func (p *parser) lowerFunction(
bodyStmts *[]js_ast.Stmt,
preferExpr *bool,
hasRestArg *bool,
isArrow bool,
) {
// Lower object rest binding patterns in function arguments
if p.UnsupportedFeatures.Has(compat.ObjectRestSpread) {
Expand Down Expand Up @@ -228,7 +229,7 @@ func (p *parser) lowerFunction(
// resulting promise, which needs more complex code to handle
couldThrowErrors := false
for _, arg := range *args {
if _, ok := arg.Binding.Data.(*js_ast.BIdentifier); !ok || arg.Default != nil {
if _, ok := arg.Binding.Data.(*js_ast.BIdentifier); !ok || (arg.Default != nil && couldPotentiallyThrow(arg.Default.Data)) {
couldThrowErrors = true
break
}
Expand Down Expand Up @@ -269,7 +270,7 @@ func (p *parser) lowerFunction(
}

// Forward all arguments from the outer function to the inner function
if p.fnOnlyDataVisit.argumentsRef != nil {
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The problem seems to be argumentsRef is non-nil if the async arrow function is nested inside a regular function. This code path only triggers when the parameters have a default value (and hence couldThrowErrors = true), so both conditions are required for this bug.

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Thanks for the summary.

if !isArrow {
// Normal functions can just use "arguments" to forward everything
forwardedArgs = js_ast.Expr{Loc: bodyLoc, Data: &js_ast.EIdentifier{Ref: *p.fnOnlyDataVisit.argumentsRef}}
} else {
Expand Down Expand Up @@ -2083,3 +2084,11 @@ func (p *parser) maybeLowerSuperPropertyAccessInsideCall(call *js_ast.ECall) {
thisExpr := js_ast.Expr{Loc: call.Target.Loc, Data: &js_ast.EThis{}}
call.Args = append([]js_ast.Expr{thisExpr}, call.Args...)
}

func couldPotentiallyThrow(data js_ast.E) bool {
switch data.(type) {
case *js_ast.ENull, *js_ast.EUndefined, *js_ast.EBoolean, *js_ast.ENumber, *js_ast.EBigInt, *js_ast.EString, *js_ast.EFunction, *js_ast.EArrow:
return false
}
return true
}
22 changes: 22 additions & 0 deletions internal/js_parser/js_parser_test.go
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Expand Up @@ -2655,6 +2655,28 @@ func TestLowerLogicalAssign(t *testing.T) {
expectPrintedTarget(t, 2020, "a()[b()] ||= c", "var _a, _b;\n(_a = a())[_b = b()] || (_a[_b] = c);\n")
}

func TestLowerAsyncFunctions(t *testing.T) {
// Lowered non-arrow functions with argument evaluations should merely use
// "arguments" rather than allocating a new array when forwarding arguments
expectPrintedTarget(t, 2015, "async function foo(a, b = couldThrowErrors()) {console.log(a, b);}", `function foo(_0) {
return __async(this, arguments, function* (a, b = couldThrowErrors()) {
console.log(a, b);
});
}
import {
__async
} from "<runtime>";
`)
// Skip forwarding altogether when parameter evaluation obviously cannot throw
expectPrintedTarget(t, 2015, "async (a, b = 123) => {console.log(a, b);}", `(a, b = 123) => __async(this, null, function* () {
console.log(a, b);
});
import {
__async
} from "<runtime>";
`)
}

func TestLowerClassSideEffectOrder(t *testing.T) {
// The order of computed property side effects must not change
expectPrintedTarget(t, 2015, `class Foo {
Expand Down
14 changes: 14 additions & 0 deletions scripts/end-to-end-tests.js
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Expand Up @@ -1032,6 +1032,20 @@ in.js:24:30: warning: Writing to getter-only property "#getter" will throw
)
`,
}, { async: true }),
test(['in.js', '--outfile=node.js', '--target=es6'],
{
'in.js': `
function nonArrowWrapper() {
return async (x, paramWithDefault = {}) => {
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Some expressions can be statically determined to never throw. Of these, it might be worth handling the subset that are both fairly common and trivial to identify (e.g. false, true, etc.)

if (x !== 123) {
throw 'fail';
}
console.log(paramWithDefault);
};
}
exports.async = () => nonArrowWrapper()(123);
`,
}, { async: true }),
test(['in.js', '--outfile=node.js', '--target=es6'], {
'in.js': `
exports.async = async () => {
Expand Down