Resolve the prelude import in build_reduced_graph

[LorrensP-2158466]

This pr tries to resolve the prelude import at the build_reduced_graph stage.
Part of batched import resolution in #145108 (cherry picked commit) and maybe needed for #139493.

r? petrochenkov

[petrochenkov]

Could you also add this case to tests?

//@ check-pass

#![feature(custom_inner_attributes)]

#![rustfmt::skip]

fn main() {
    let _ = todo!();
}

[LorrensP-2158466]

I am currently unable to implement any of your suggestions; it is the prelude import that can not be resolved during build_reduced_graph. Which makes me think that this change doesn't actually do anything useful? At least the prelude import in stdlib keeps failing with every change.

[petrochenkov]

@Voultapher
This change should fix your issues with #139493 and custom inner attributes.
Could you check it when it's ready?

[LorrensP-2158466]

Suggestions worked, however, we can't emit an error when the prelude is already set, I just used a panic for ease of use, and it was triggered when compiling stdlib:

prelude already set: [Segment { ident: prelude#0, id: Some(NodeId(479)), has_generic_args: false, has_lifetime_args: false, args_span: no-location (#0) }, Segment { ident: rust_2021#0, id: Some(NodeId(480)), has_generic_args: false, has_lifetime_args: false, args_span: no-location (#0) }]

I also can't avoid to only resolve the prelude path when the prelude is not set. Errors like these get emitted:

error[E0433]: failed to resolve: use of undeclared type `String`
   --> library/std/src/panicking.rs:642:29
    |
642 |                 let mut s = String::new();
    |                             ^^^^^^ use of undeclared type `String`
    |
help: consider importing one of these structs
    |
 12 + use crate::prelude::rust_future::String;
    |
 12 + use alloc::string::String;
    |

error[E0433]: failed to resolve: use of undeclared type `Box`
   --> library/std/src/panicking.rs:656:13
    |
656 |             Box::into_raw(Box::new(contents))
    |             ^^^ use of undeclared type `Box`
    |
help: consider importing one of these structs
    |
 12 + use crate::prelude::rust_future::Box;
    |
 12 + use alloc::boxed::Box;

@rustbot ready

[petrochenkov]

Okay, I see, in std and in some tests we first apply the prelude import injected implicitly, and then the prelude import written explicitly.
Let's keep it as is then.

[Voultapher]

@petrochenkov I've tested #139493 with this PR and I can confirm it does resolve the problem in question, but I'm also seeing the same UI test failures reported by the CI here.

[Voultapher]

Small addendum, I had hopped that fixing the custom_inner_attributes issue would also resolve the change in errors produced in the where-allowed test since it uses custom_inner_attributes and a top level non prelude macro, but alas the change in errors produced still occurs.

[LorrensP-2158466]

Which tests are you talking about?

[LorrensP-2158466]

I also tried to make CI green, those changes are in the second commit.

@rustbot ready