title: How to execute "Cargo build" command feature wise using features flag?
body: <p>I have a similar kind of structure in my project. I want to build the project feature-wise. <code>example1/main</code> and <code>example1/test</code> are two features of this project.
  I can not change the structure.</p>
  <pre><code>example
  +----example1
  ¦          +-----main
  ¦                ¦   Cargo.toml
  ¦                ¦   
  ¦                +---src
  ¦                ¦ 
  ¦          +-----test
  ¦                ¦   Cargo.toml
  ¦                ¦   
  ¦                +---src
  ¦
  ¦   Cargo.toml
  </code></pre>
  <p>Main manifest ./Cargo.toml:</p>
  <pre><code>[profile.release]
  panic = 'unwind'
  
  [workspace]
  members = [
      'example1/main',
      'example1/test',
  ]
  
  
  </code></pre>
  <p>Workspace manifest ./example1/main/Cargo.toml:</p>
  <pre><code>[package]
  name = 'main'
  version = '2.0.0'
  </code></pre>
  <p>When I am executing <code>cargo build --features main</code> or <code>cargo build --features test</code>, I am getting below error message</p>
  <pre><code>error: --features is not allowed in the root of a virtual workspace
  note: while this was previously accepted, it didn't actually do anything
  </code></pre>
  <p>Can anyone please help me to fix this? Please let me know what configuration I am missing in root cargo.toml file.</p>