make everybody_loops preserve item declarations

[QuietMisdreavus]

First half of #52545.

everybody_loops is used by rustdoc to ensure we don't contain erroneous references to platform APIs if one of its uses is pulled in by #[doc(cfg)]. However, you can also implement traits for public types inside of functions. This is used by Diesel (probably others, but they were the example that was reported) to get around a recent macro hygiene fix, which has caused their crate to fail to document. While this won't make the traits show up in documentation (that step comes later), it will at least allow files to be generated.

[rust-highfive]

r? @varkor

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[QuietMisdreavus]

r? @pnkfelix @eddyb

[pnkfelix]

I'll make sure to make a decision about this tomorrow.

[pnkfelix]

Thanks for the fun looking at the design options here via live chat over last few days with @QuietMisdreavus and @eddyb

I have an alternative approach that is factored slightly differently, but until I see a case where this PR breaks, I'm not going to try to push it.

[pnkfelix]

@bors r+

[bors]

📌 Commit 7e77d19 has been approved by pnkfelix

[pnkfelix]

For the record, below (in the details block) is an expanded test case that tries to trip up code here. It might be worth trying to adapt into a real test for rustdoc at some point.

• The goal of the tests I make, in terms of how raw -Z unpretty=everybody_loops is supposed to behave, is that for any test that rustc compiles without error, you should be able to roundtrip the output of -Z unpretty=everybody_loops back into rustc again.
• From what I can see, this PR satisfies that goal for the test below

Details

#![allow(dead_code)]

mod a { fn f() { println!("Hello from a::f"); } }

trait Trait {
    fn m1(&self);
    fn m2(&self) { }
}

struct S1;
struct S2;

mod b {
    fn f() { println!("Hello from b::f"); }
    struct S3;
    impl ::Trait for S3 {
        fn m1(&self) { println!("Hello from S3::m1"); }
        fn m2(&self) { println!("Hello from S3::m2"); }
    }
}

struct S4;

struct S6;

fn main () {
    mod c { fn f() { println!("Hello from c::f"); } }

    const C: [i32; 2] = [1, 2];

    { mod c2 { const C: [i32; 2] = [1, 2]; } };

    { mod c2 { const C: [i32; 2] = [1, 2]; } }

    { mod c2 { const C: [i32; { mod uh_oh {  } 2 }] = [1, 2]; } }

    { mod c2 { const C: [i32; { mod uh_oh { impl ::Trait for ::S6 { fn m1(&self) { } } } 2 }] = [1, 2]; } }

    struct S5;

    mod d {
        trait Trait { fn m1(&self); }
        impl Trait for ::S4 {
            fn m1(&self) { println!("Hello from <S4 as d::Trait>::m1"); }
        }
    }

    impl Trait for S4 {
        fn m1(&self) { println!("Hello from <S4 as Trait>::m1"); }
    }

    impl Trait for S5 {
        fn m1(&self) { println!("Hello from <S4 as Trait>::m1"); }
    }

    impl S1 {
        fn f() { println!("Hello from S1::f"); }
    }

    impl Trait for S1 {
        fn m1(&self) { println!("Hello from S1::m1"); }
        fn m2(&self) { println!("Hello from S1::m2"); }
    }

    mod e {
        fn f() {
            impl ::Trait for ::S2 {
                fn m1(&self) { println!("Hello from S2::m1"); }
                fn m2(&self) { println!("Hello from S2::m2"); }
            }
        }
    }

    println!("Hello from main");
}

[bors]

⌛ Testing commit 7e77d19 with merge aa1e6db...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing aa1e6db to master...