Reflect explicit macro import in reference

[Voultapher]

Companion PR for rust-lang/rust#139493

[traviscross]

The [std] crate is added to the root.

Is this sentence needed? I.e., is there any sense in which "the std crate is added to the root" but the core crate is not, or that this is not covered by "Both [core] and [std] are added to the [extern prelude]"?

My reading of the old text is that the sentence was there to support the restrictive clause (that erroneously was preceded by a comma, making the clause seem non-restrictive, i.e. that the part before the comma could stand on its own without the part after it):

The [std] crate is added to the root, along with an implicit [macro_use attribute] pulling in all macros exported from std into the [macro_use prelude].

[traviscross]

Or, maybe this language and the line below

It does three things... Injects the [core] crate into the crate root instead of [std].

are leftovers from Rust 2015.

In Rust 2018 and later, this is a valid library:;

use ::core as c1;
use ::std as c2;

But it's not in Rust 2015. It gives the error

error[E0432]: unresolved import `core`
 --> src/lib.rs:1:5
  |
1 | use ::core as c1;
  |     ^^^^^^^^^^^^ no `core` in the root

unless #![no_std] is used, in which case it accepts that line but not use ::std. Interestingly, I don't see this documented specifically in the 2018 edition guide:

https://doc.rust-lang.org/nightly/edition-guide/rust-2018/path-changes.html

Perhaps this is simply an artifact of the change to make crates in the extern prelude accessible to use declarations. Did that then make it fully unobservable whether core is separately added to the crate root or not, or is there still some way to tell?

cc @ehuss

[traviscross]

Looking, this is all covered in:

• Update no_std to use the attribute template #1913

I'm going to go get that merged.