Avoid duplicating impure expressions in object rest destructuring

[erikdesjardins]

Q	A
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	no
Tests Added/Pass?	yes
Fixed Tickets	fixes #4904
License	MIT
Doc PR	no
Dependency Changes	no

There were two main issues here (fixed in the first commit):

1. path.traverse was called instead of path.get("id").traverse in the VariableDeclarator visitor. This means that in const { a } = foo(({ ...x }) => {/* ... */}), the visitor visiting { a } = ... would traverse down to the ...x RestProperty, causing unexpected behaviour.

2. The VariableDeclarator's init expression was duplicated directly, which is unsafe if it may have side effects, e.g. in const { x, ...y } = foo();

Edit: I found a few more (will push fixes to this branch fixed in the second commit)

1. Variables may be reordered incorrectly (repl), e.g.

const { x, ...y } = a,
      z = foo(y);

becomes

const { x } = a,
      z = foo(y);

const y = _objectWithoutProperties(a, ["x"]);

1. The entire VariableDeclaration is removed when all properties are rest properties, killing any other variables (repl), e.g.

const { ...y } = a,
      z = foo(y);

becomes

const y = _objectWithoutProperties(a, []);

[mention-bot]

@erikdesjardins, thanks for your PR! By analyzing the history of the files in this pull request, we identified @hzoo and @christophehurpeau to be potential reviewers.

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[hzoo]

Nice work @erikdesjardins, very clear to me!

[erikdesjardins]

@hzoo Thanks! Just to make sure you've seen it (I'm about to push after tests pass):

Edit: I found a few more (will push fixes to this branch)

[erikdesjardins]

Alright, should be good now.

The second commit changes a lot of test expectations, but it's just replacing

var a = ...;
var b = ...;

with

var a = ...,
    b = ...;

[hzoo]

Ah yeah both an optimization and also prevents those issues you mention

[hzoo]

lgtm, nice work!

[hzoo]

Released in https://github.com/babel/babel/releases/tag/v6.22.0