title: Unable to specify `edition2021` in order to use unstable packages in Rust
body: <p>I want to run an example via Cargo but I am facing an error:</p>
  <pre><code>error: failed to parse manifest at `/Users/aviralsrivastava/dev/subxt/Cargo.toml`
  </code></pre>
  <p>The full stacktrace is:</p>
  <pre><code>error: failed to parse manifest at `/Users/aviralsrivastava/dev/subxt/Cargo.toml`
  
  Caused by:
    feature `edition2021` is required
  
    The package requires the Cargo feature called `edition2021`, but that feature is not stabilized in this version of Cargo (1.56.0-nightly (b51439fd8 2021-08-09)).
    Consider adding `cargo-features = [&quot;edition2021&quot;]` to the top of Cargo.toml (above the [package] table) to tell Cargo you are opting in to use this unstable feature.
    See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2021 for more information about the status of this feature.
  </code></pre>
  <p>Based on the suggestion, I go ahead and modify the Cargo.toml:</p>
  <pre><code>  Consider adding `cargo-features = [&quot;edition2021&quot;]` to the top of Cargo.toml (above the [package] table) to tell Cargo you are opting in to use this unstable feature.
  diff --git a/Cargo.toml b/Cargo.toml
  index 26a02c7..186d09b 100644
  --- a/Cargo.toml
  +++ b/Cargo.toml
  @@ -1,6 +1,6 @@
   [workspace]
   members = [&quot;.&quot;, &quot;cli&quot;, &quot;codegen&quot;, &quot;macro&quot;]
  -
  +cargo-features = [&quot;edition2021&quot;]
   [package]
   name = &quot;subxt&quot;
   version = &quot;0.15.0&quot;
  (END)
  </code></pre>
  <p>I still face the same error as if nothing was changed in the toml file.</p>
  <p>How do I resolve the above error in order to use unstable packages?</p>