`unnameable_types` should show where the type is reachable

[kpreid]

Code

// This is just the documentation sample for the lint
#![deny(unnameable_types)]

mod m {
    pub struct S;
}

pub fn get_unnameable() -> m::S { m::S }

Current output

error: struct `S` is reachable but cannot be named
 --> src/lib.rs:5:5
  |
5 |     pub struct S;
  |     ^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
  |
note: the lint level is defined here
 --> src/lib.rs:2:9
  |
2 | #![deny(unnameable_types)]
  |         ^^^^^^^^^^^^^^^^

Desired output

error: struct `S` is reachable but cannot be named
 --> src/lib.rs:5:5
  |
5 |     pub struct S;
  |     ^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
  |
note: `S` is reachable from here
  |
8 | pub fn get_unnameable() -> m::S { m::S }
  |                            ^^^^ returns a value of type `S`
  |
note: the lint level is defined here
 --> src/lib.rs:2:9
  |
2 | #![deny(unnameable_types)]
  |         ^^^^^^^^^^^^^^^^

Rationale and extra context

The output should give at least one example of how the type is reachable. This will:

• Help understand why the lint is not a false-positive (a reaction I had when first enabling it — “surely these can't be…”).
• If the desired outcome is that the type is not reachable, help the user make it not reachable.

Right now, this is a pure “hidden action at a distance” situation: there is no way to discover where the type is reachable other than by searching for uses of it (which can be difficult).

Other cases

No response

Rust Version

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-apple-darwin
release: 1.79.0
LLVM version: 18.1.7

Anything else?

@rustbot label +A-lint +A-visibility +L-unnameable_types

[traviscross]

cc @petrochenkov

[petrochenkov]

where the type is reachable

We do not collect this information right now, collecting it may be costly (or may not, need to check).

[kornelski]

Marking the item as pub(crate) helps find the culprit, since it will fire private_interfaces/private_bounds lint where used.