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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Very helpful!

A diagnostic message says "you can add cargo-features = ["edition2021"] to enable this feature", but it doesn't see which section of which file to add it to. For someone who is just trying to build a package and is not a Rust developer, what to do is not at all clear without instructions like the ones here.

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 Do not add cargo-features anywhere — that is an outdated advice.