rustc makes claims about Cargo.toml when there is none in async context

[matthiaskrgr]

I tried this code:

#![feature(type_alias_impl_trait)]
#![no_std]

pub trait Tr<'x> {
    type Fut: core::future::Future<Output = ()>;

    fn f() -> Self::Fut;
}

impl<'x> Tr<'x> for () {
    type Fut = impl core::future::Future<Output = ()>;

    fn f() -> Self::Fut {
        async {
            //if false {
            return ();
            //}
            let res: Undef = ();
            res
        }
    }
}

When I check this file with rustc async.rs --crate-type lib, rust errors

  --> async.rs:16:13
   |
14 |         async {
   |         ----- `async` blocks are only allowed in Rust 2018 or later
15 |             //if false {
16 |             return ();
   |             ^^^^^^ expected identifier, found keyword
   |
   = help: set `edition = "2021"` in `Cargo.toml`
   = note: for more on editions, read https://doc.rust-lang.org/edition-guide

Which is a bit confusing.
Does it want me to set edition = 2021 in Cargo.toml (as in "please set edition = 2021...")?
Why does it complain about a Cargo.toml when there isn't one (as I just checked the file directly with rustc x.rs).

For some other examples, the message is even more confusing

   = note: to `.await` a `Future`, switch to Rust 2018 or later
   = help: set `edition = "2021"` in `Cargo.toml`

because it is not clear if rustc want to say "2021 is set" or "2021 should be set"

[ehuss]

It is giving instructions on how to resolve the issue. It reads fine to me, though maybe a bit terse. Perhaps a few more words could make it clearer.

As for the hint being given when not using cargo, these suggestions should be gated like this to only be shown when using cargo.

[mati865]

IMO rustc should tell the user to add --edition x command line argument when running without Cargo.

[ChayimFriedman2]

Does rustc have a way to know it's being run by Cargo?

[ehuss]

The link in the comment above shows how to do it.