handle std features with a new option in `config.toml`

[onur-ozkan]

Currently there are multiple ways to enable std features during bootstrap, which can be confusing to newcomers on this project.

To simplify this, add a new option under the rustc section in the config.toml, and handle it in the following function:

rust/src/bootstrap/src/lib.rs

Line 646 in 697d953

fn std_features(&self, target: TargetSelection) -> String {

@rustbot label +E-easy +E-mentor +E-help-wanted +T-bootstrap

[shrirambalaji]

@onur-ozkan I'd like to pick this up, haven't contributed to rustc before so might need some mentoring

[onur-ozkan]

@onur-ozkan I'd like to pick this up

You can claim the issue by using rustbot.

haven't contributed to rustc before so might need some mentoring

Feel free to ask me any questions on Zulip, or ping me here under the issue or an open PR.

[shrirambalaji]

@rustbot claim

[onur-ozkan]

Done by #131315.