Plugins cannot specify the entry option #1270

Here is an example Rollup configuration:

export default {
  plugins: [
    {
      options: function (rollup) {
        rollup.entry = "foo.js";
      }
    }
  ]
};

When using rollup --config I get the following error:

🚨   You must specify an --input (-i) option

If I instead use this config, then it works:

export default {
  entry: "foo.js",
  plugins: [
    {
      options: function (rollup) {
        rollup.entry = "foo.js";
      }
    }
  ]
};

Basically, it seems that Rollup is checking whether the entry option is specified or not, but it does this check before the plugins are run.

I think Rollup should do the check after the plugins run, because a plugin might set the entry in an options hook.