feat: Preserve formatting

[kriskowal]

Closes: #926

Description

This change leverages new Babel support for format preservation.

Security Considerations

None

Scaling Considerations

None

Documentation Considerations

None

Testing Considerations

This change includes a test to perform a narrow spot-check of the verbatim output of a module that should be largely preserved.

Compatibility Considerations

None

Upgrade Considerations

None

• NEWS.md

[kriskowal]

@nicolo-ribaudo Reviewing https://github.com/babel/babel/pull/16708/files#diff-ca2bda59eec9c35846e26c1c6247759c92c26357b80d4dc881fcdaf12df1d7e6R35-R39, it seems like compact: true here should be causing your code to throw, so I suspect this isn’t exercising the new path in my local testing.

[kriskowal]

Found it. Needed to swap out our fork @agoric/babel-generator back to @babel/generator.

[kriskowal]

@nicolo-ribaudo,

I’ll have a push soon, but this looks correct, actually:

Pre-transform:

function TokenType() {}
const beforeExpr = 0;

export function createBinop(name, binop) {
  return new TokenType(name, {
    beforeExpr,
    binop,
  });
}

Post-transform:

function TokenType() {}
const beforeExpr = 0;

       function createBinop(name, binop) {
  return new TokenType(name, {
    beforeExpr,
    binop,
  });
}

I’ll note that there’s a weird but explicable offset on the function declaration because we have erased export!

This was the previous (undesirable) effect:

function TokenType() { }
const beforeExpr=  0;

function        createBinop(name, binop) {
  return new TokenType(name, {
    beforeExpr,
    binop});

 }
})()

And the test I’ve proposed is not yet passing because the generator doesn’t add a newline at the end of the file. That presumably means that it needs to catch up with the final line and column of the file to preserve trailing whitespace, or just add a single newline to the end to make sure the output is a valid UNIX text file.

[kriskowal]

I’m mistaken above about the newlines issue. I’m updating the test fixture and it looks like the test will pass as-is, no changes needed from Babel.

[nicolo-ribaudo]

I'm actually surprised by the spaces here. My expectation was that this would have been printed as compact as possible to increase the chances that the next node could be printed in the right place.

[nicolo-ribaudo]

Oh I see. This code with spaces is generated by you with string concatenation after that Babel runs, and not by Babel itself.

[kriskowal]

Indeed. I also find the spacing arbitrary, emerging from some unnecessary space in the formatting of the code. I’ve pushed a commit to make that compact as expected, regardless of if not being generated by Babel.

[mhofman]

Where is the closing paren for the function expression here?

[nicolo-ribaudo]

Mmm it should be at then end of the file right? It looks like I forgot to flush some tokens somewhere 🤔

[nicolo-ribaudo]

Actually, cloning the PR locally I get this additional line at the end of the file:

})()

Probably @kriskowal accidentally deleted it.

[kriskowal]

Pushed the fix for this. It was present locally when I got the test to pass.