Self-service

• I'd be willing to implement a fix

Describe the bug

When running the constraints plugin, if the expected field value is an object, it prints [object Object] rather than the JSON stringified (or human readable) form.

To reproduce

const fs = require('fs');

await packageJsonAndInstall({
    bin: {
        key: 'this is visible',
    },
    _bin: {
        key: './bin/entry.js',
    }
});

const constraints = `gen_enforced_field(WorkspaceCwd, 'bin', Value) :- workspace_field(WorkspaceCwd, '_bin', Value).`;
await fs.promises.writeFile('constraints.pro', constraints, 'utf-8');

await expect(yarn(`constraints`)).rejects.toThrow(expect.stringContaining('[object Object]'));

and you'll see:

Environment

System:
OS: macOS 11.4
CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Binaries:
Node: 14.17.3 - /private/var/folders/ft/b9p2qq2n2h31j10zcc8d48qh0000gp/T/xfs-ad65a9aa/node
Yarn: 3.0.0 - /private/var/folders/ft/b9p2qq2n2h31j10zcc8d48qh0000gp/T/xfs-ad65a9aa/yarn

Additional context

The existing value is printed correctly. It's only the expected value having issues.