Add Cargo.toml context to exported-private-dependencies lints

[epage]

Problem

Right now the lint just says

warning: trait `FooTrait` from private dependency 'foo' in public interface
  --> src/lib.rs:25:1
   |
25 | pub fn use_foo_trait<F: foo::FooTrait>(_: Local) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

rustc has no knowledge of Cargo.toml to provide additional context

Proposed Solution

Build on #13095 and #12235 to inject a note in the lint to highlight the dependency in Cargo.toml, saying the dependency is private

Notes

No response

[ShoyuVanilla]

Hi, I'd like to work on this issue.
I believe this issue should be addressed by providing the Cargo.toml path and the span for the problematic dependency as metadata in cargo, then using this data to construct the actual warning message in rustc. Does this approach seem correct?

[epage]

If I'm understanding correctly, you are suggesting Cargo proactively get the span for all dependencies, rustc add a new CLI flag for getting dependency location and span information, and for Cargo to pass that flag to rustc?

That would lead to a very specialized interface in rustc and would make #13095 much harder.

[ShoyuVanilla]

That would lead to a very specialized interface in rustc and would make #13095 much harder.

That sounds problematic 🤔 What would be most appropriate approach? I think that there would be two directions for implementing this as rustc doesn't know about Cargo.toml

1. rustc queries the Cargo.toml span to cargo: My original suggestion, which seems problematic
2. cargo intercepts rustc's warning message and modify it to include the Cargo.toml span: Not so robust, but implementable

[epage]

in #13095, I suggested the latter.

[ShoyuVanilla]

Okay, I'll try in that way