Set struct/union/enum fields/variants as reachable when item is

[lambinoo]

Fixes #92755

[rust-highfive]

r? @wesleywiser

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

[lambinoo]

This fixes the issue, but I'd like to add a test for this before hand, but I still have to pinpoint the "minimal code" that triggers this.
If it's urgent (as it seems to be the last item in the 1.60 milestone), I can the require tests in a different PR.

[camelid]

If it's urgent (as it seems to be the last item in the 1.60 milestone)

It is somewhat urgent since it broke real-world code (though people can stay on older nightlies for now), but it being the only item in the milestone has nothing to do with it. The milestone is just used to track regressions in that version. Development of 1.60 started just days ago, so there's not much there.

If you end up adding a test in a later PR instead, make sure to remove the "Fixes" wording from the PR description—we leave issues open and add the E-needs-test label when they are fixed but need a test.

[tmiasko]

A reproducer you can turn into a test case.

$ cat a.rs 
mod machine {
        pub struct A {
            pub b: B,
        }
        pub struct B {}
        impl B {
            pub fn f(&self) {}
        }
}

pub struct Context {
    pub a: machine::A,
}

pub fn ctx() -> Context {
    todo!();
}
$ cat b.rs 
fn main() {
    a::ctx().a.b.f();
}
$ rustc +stage1 --edition=2018 --crate-type=lib -O a.rs
warning: associated function is never used: `f`
 --> a.rs:7:20
  |
7 |             pub fn f(&self) {}
  |                    ^
  |
  = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted

$ rustc +stage1 --edition=2018 --crate-type=bin -O b.rs --extern a -L.
error: internal compiler error: compiler/rustc_monomorphize/src/collector.rs:966:9: no MIR available for DefId(20:8 ~ a[c3e7]::machine::{impl#0}::f)

[camelid]

Perhaps worth adding the test now then?

[petrochenkov]

@lambinoo
It was probably a mistake to merge code from "// Update levels of nested things." to "// Mark all items in interfaces of reachable items as reachable." in https://github.com/rust-lang/rust/pull/87487/files#diff-9a9bf53a67d6c204294aa9ab4ec8fd1265306998e0eefc593f004e053513b823.

Could you move it back and make sure it's all preserved compared to the state before #87487?
All the "// Update levels of nested things." logic is still relevant, it just can mark nodes as Reachable now instead of Public/Exported.

[lambinoo]

@camelid @petrochenkov Will do in a few hours, right after work!

[inquisitivecrystal]

When this is merged, it might be worth giving it increased bors priority. The linked issue is P-critical, in part because there are a lot of reports coming in with no real workaround.

[lambinoo]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[lambinoo]

I tested the test with the current master and it does fail properly so all good on this front!

[petrochenkov]

r? @petrochenkov @bors p=1
r=me after addressing #92781 (comment)

[lambinoo]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[petrochenkov]

@bors r+ just to land this sooner and fix the regression.

[bors]

📌 Commit 11532a362b9fdf139ea24bcc6e1cba695aa996fe has been approved by petrochenkov

[petrochenkov]

@bors r+

[bors]

📌 Commit 1b2c64d has been approved by petrochenkov

[bors]

⌛ Testing commit 1b2c64d with merge 86f7f78...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 86f7f78 to master...

[rust-timer]

Finished benchmarking commit (86f7f78): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression