omitting edition produces unexpected errors

[sancho20021]

Problem

If there is no edition = "2021" line in Cargo.toml, the project can't be build because of:

error[E0432]: unresolved import `rustfix`
 --> ghostcellify/src/main.rs:1:5
  |
1 | use rustfix::Suggestion;
  |     ^^^^^^^ maybe a missing crate `rustfix`?
  |
  = help: consider adding `extern crate rustfix` to use the `rustfix` crate

error[E0432]: unresolved import `itertools`
 --> ghostcellify/src/main.rs:2:5
  |
2 | use itertools::Itertools;
  |     ^^^^^^^^^ maybe a missing crate `itertools`?
  |
  = help: consider adding `extern crate itertools` to use the `itertools` crate

For more information about this error, try `rustc --explain E0432`.
error: could not compile `ghostcelltf` (bin "ghostcelltf") due to 2 previous errors

This is unexpected. I would like to know why is this happening? What edition is used when edition is omitted?

Steps

1. Project structure and contents:

.
├── Cargo.toml
└── src
    └── main.rs

Cargo.toml:

[package]
name = "ghostcelltf"
version = "0.1.0"
# edition = "2021"

[dependencies]
rustfix = "0.8.2"
itertools = "0.9.0"

src/main.rs:

use rustfix::Suggestion;
use itertools::Itertools;

fn main() {
}

3. How to reproduce?

> cargo build
error[E0432]: unresolved import `itertools`
 --> ghostcellify/src/main.rs:2:5

Note that if explicitly set the edition to 2021, the project builds fine.

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.78.0 (54d8815d0 2024-03-26)
release: 1.78.0
commit-hash: 54d8815d04fa3816edc207bbc4dd36bf18014dbc
commit-date: 2024-03-26
host: x86_64-unknown-linux-gnu
libgit2: 1.7.2 (sys:0.18.2 vendored)
libcurl: 8.6.0-DEV (sys:0.4.72+curl-8.6.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Ubuntu 20.04 (focal) [64-bit]

[weihanglo]

It defaults to 2015, and you need extern crate xxx to make them usable. See also https://doc.rust-lang.org/edition-guide/rust-2018/path-changes.html#no-more-extern-crate

[sancho20021]

Ooo I see, it is said in documentation about 2015 default: https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html#enabling-the-new-edition-to-use-new-features

Thank you, I will close the issue

[sancho20021]

default is 2015 which corresponds to the described behavior. Documentation mentions this