Better help for `pub(in parent)` in edition 2018 and beyond

[zackw]

Code

// Compile with edition 2018 or later
mod foo {
    pub(in parent) fn bar() {
        println!("hello from bar");
    }
}

fn main() {
    bar();
}

Current output

error: relative paths are not supported in visibilities in 2018 edition or later
 --> src/lib.rs:2:12
  |
2 |     pub(in parent) fn bar() {
  |            ^^^^^^ help: try: `crate::parent`

Desired output

error: relative paths are not supported in visibilities in 2018 edition or later
 --> src/lib.rs:2:12
  |
2 |     pub(in parent) fn bar() {
  |            ^^^^^^ help: try: `crate::parent`
  |         ^^^^^^^^ help: did you mean? `super`

Rationale and extra context

If the programmer wrote pub(in parent) and there is no module named parent in the crate (at the location where edition 2015 would have resolved the path), it is likely that what they were actually trying to do is make the item visible to "the parent module", which is properly spelled super.

It would also probably be helpful to refer the programmer to https://doc.rust-lang.org/reference/visibility-and-privacy.html#pubin-path-pubcrate-pubsuper-and-pubself either directly or via --explain.

Note also that this error is missing an E-code (and therefore there is nothing to use --explain on).

[zackw]

Thinking about this a bit more, "relative paths are not supported" is a bad error message whenever the path resolves to a nonexistent module. rustc should lead off with E0433 in all such cases. Something like

error[E0433]: failed to resolve: could not find `parent` using relative path
error[Exxxx]: relative paths are not supported in visibilities in 2018 edition or later
 --> src/main.rs:2:19
  |
2 |     pub(in parent) fn bar() {
  |            ^^^^^^ could not find `parent` relative to `foo`
  |                 \ this should have been `[2018-compatible path here]` but that module doesn't exist
  |         ^^^^^^^^^ help: did you mean: `super`?

"Did you mean: super" is an additional suggestion only for when the module name is literally "parent", because the thing that super refers to is called "the parent module" in lots of Rust documentation, so it's natural for people to expect pub(parent) and/or pub(in parent) to do what pub(super) does.