var metalsmith = require('metalsmith');
var partial = require('metalsmith-partial');
var templates = require('metalsmith-templates');

metalsmith(__dirname)
  .source('./src')
  .destination('./dest')
  .use(partial({
    directory: './partials', 
    engine: 'eco'
  }))
  .use(templates({
    engine: 'eco',
    inPlace: true
  }))
  .build();

Results in dist/index.html:

<p>The contents</p>

<p>This is a partial</p>