How to use Rust 2018 or later for `async fn`?

Question

I'm trying to compile a library in Rust and I get an error:

error[E0670]: `async fn` is not permitted in Rust 2015
 --> src/lib.rs:1:1
  |
1 | async fn foo() {
  | ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: set `edition = "2021"` in `Cargo.toml`

This is my Cargo.toml:

[package]
name = "name"
version = "0.1.0"
edition = "2018"

and to compile I use the following command line (documentation here):

rustc --crate-type=lib src/apis/my_file.rs

Does anyone have an idea?

Answer 1

As the name implies Cargo.toml is interpreted by cargo, not rustc if you want to compile with rustc you have to add the edition flag:

rustc --edition=2018 --crate-type=lib src/apis/my_file.rs