Edition-specific preludes

[m-ou-se]

This changes {std,core}::prelude to export edition-specific preludes under rust_2015, rust_2018 and rust_2021. (As suggested in #51418 (comment).) For now they all just re-export v1::*, but this allows us to add things to the 2021edition prelude soon.

This also changes the compiler to make the automatically injected prelude import dependent on the selected edition.

cc @rust-lang/libs @djc

[rust-highfive]

r? @sfackler

(rust-highfive has picked a reviewer for you, use r? to override)

[m-ou-se]

drive by comment, but v1 now seems to be an extra unnecessary layer of indirection?

Yes, but std::prelude::v1 is public and stable, so we can't remove it. We could deprecate it at some point.

[danielhenrymantilla]

Nice. Could the documentation in of ::std::prelude be adapted too? It uses single-prelude terminology like "the current version of the prelude":

rust/library/std/src/prelude/mod.rs

Line 28 in ee88f46

//! The current version of the prelude (version 1) lives in

[danielhenrymantilla]

(Can always be done later) Could this be done for the other editions as well? If only, I think it will be nice to have such a line appear on the documentation of these preludes.

[m-ou-se]

Could the documentation in of ::std::prelude be adapted too? It uses single-prelude terminology like "the current version of the prelude"

I don't think we should do that yet. These new names for the preludes are unstable, and edition 2021 is also not yet stable. So v1 is still the current/only (stable) version of the prelude.

[bors]

☔ The latest upstream changes (presumably #82281) made this pull request unmergeable. Please resolve the merge conflicts.

[m-ou-se]

r? @nikomatsakis

[nikomatsakis]

The code looks good to me.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 76fd8d7 has been approved by nikomatsakis

[Dylan-DPC-zz]

failed in rollup

@bors r-

[m-ou-se]

@estebank If you have time, I could use some help here. Somehow the E0573 diagnostic suggests to use std::prelude::rust_2021 as an enum, but only when compiling in edition 2015.

use std::result;
impl result {
    fn into_future() -> Err {}
}
fn main() {}

$ rustc +stage1 x.rs --edition=2015
[...]
error[E0573]: expected type, found variant `Err`
 --> x.rs:3:25
  |
3 |     fn into_future() -> Err {}
  |                         ^^^
  |                         |
  |                         not a type
  |                         help: try using the variant's enum: `std::prelude::rust_2021`

$ rustc +stage1 x.rs --edition=2018       
[...]
error[E0573]: expected type, found variant `Err`
 --> x.rs:3:25
  |
3 |     fn into_future() -> Err {}
  |                         ^^^ not a type
  |
help: try using the variant's enum
  |
3 |     fn into_future() -> core::result::Result {}
  |                         ^^^^^^^^^^^^^^^^^^^^
3 |     fn into_future() -> crate::result::Result {}
  |                         ^^^^^^^^^^^^^^^^^^^^^
3 |     fn into_future() -> std::result::Result {}
  |                         ^^^^^^^^^^^^^^^^^^^

[m-ou-se]

This is now blocked on #82942

[estebank]

@m-ou-se the ping is resolved by the open PR, right?

[m-ou-se]

@estebank Yes. I'm still curious why it picks the rust_2021 prelude specifically on edition 2015 though. But I suppose that's not really important.

[m-ou-se]

This should be fine now that #82942 is merged.

@bors r=nikomatsakis

[bors]

📌 Commit 76fd8d7 has been approved by nikomatsakis