Breaks magic comments for eslint, Flow, &c.

Prettier 1.14.3 Playground link

Input:

// $FlowFixMe: Missing definitions for bowser?
const _setImmediate = bowser.msie || bowser.msedge ? window.setImmediate.bind(window) : setImmediate;

Output:

// $FlowFixMe: Missing definitions for bowser?
const _setImmediate =
  bowser.msie || bowser.msedge
    ? window.setImmediate.bind(window)
    : setImmediate;

Expected behavior:

Well…I don’t know what the expected behaviour would be, exactly; but what happens is that the magic comment, which applies to the next line, no longer applies to the code it was written for (in this case, the property lookups on bowser), because the line was chopped down. You can get the same effect with various eslint-disable-type directives (eslint-disable-line, eslint-disable-next-line). Result: Flow (or eslint) now fails.

I don’t know what the proper solution is; perhaps this needs discussion. Possibilities include:

• Issue warnings when reformatting AST nodes following known marker comments
• Refuse to change line numbers when reformatting AST nodes following known marker comments
• For some types of comments, it could possibly be automatically fixed: e.g. an eslint-disable-line directive could be replaced by eslint-disable [rule]/eslint-enable [rule] block comments before and after the AST node
• Do nothing but at least put a warning in the docs!

[lydell]

@bardware As mentioned in the docs we recommend using eslint-disable-next-line:

Prettier 1.15.2 Playground link

--parser babylon

Input:

/* eslint-disable-next-line func-names */
(function(){
  return 1;
})()

Output:

/* eslint-disable-next-line func-names */
(function() {
  return 1;
})();

[suchipi]

I think we should start with at least a mention in the docs, and then if we want to do further work beyond that, we can add that later