Private fields in aliased type should not be shown

[Urgau]

I tried this code:

use std::collections::BTreeMap;

pub type FooBar = BTreeMap<u32, String>;

with rustdoc and --document-private-items.

I expected to see this happen: No internal fields of the BTreeMap should be displayed as users won't be able to use the fields, making their appearance confusing and useless.

Instead, this happened: The internal fields of the BTreeMap are displayed

Real impact as can be seen here.

Meta

rustdoc --version --verbose:

rustdoc 1.80.0-nightly (7d83a4c13 2024-05-06)
binary: rustdoc
commit-hash: 7d83a4c131ab9ae81a74c6fd825c827d74a2881d
commit-date: 2024-05-06
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4

[Urgau]

@rustbot labels +T-rustdoc +A-rustdoc-ui -needs-triage
@rustbot claim

[lqd]

Dupe of #123860 and more discussion there