Rust cargo build-bpf - feature `edition2021` is required

gistfile1.txt

cargo build-bpf
Failed to obtain package metadata: Error during execution of `cargo metadata`: error: failed to parse manifest at `/Users/elliot/solana/smart-contract/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 (1.55.0 (32da73ab1 2021-08-23)).
  Consider trying a newer version of Cargo (this may require the nightly release).
  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2021 for more information about the status of this feature.

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The steps to try out the Rust 2021 Edition as follows (more detailed directions can be found here [1]):

Install the most recent nightly: rustup update nightly.
Run cargo +nightly fix --edition.
Edit Cargo.toml and place cargo-features = ["edition2021"] at the top (above [package]), and change the edition field to say edition = "2021".
Run cargo +nightly check to verify it now works in the new edition.

[1] https://doc.rust-lang.org/nightly/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html

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The real fix for this is to upgrade Rust to 1.56 or later. Do not add cargo-features anywhere — that is an outdated advice.