Chore: Add witness hint for unit_struct_changed_kind lint

[GlitchlessCode]

Adds a witness hint for the unit_struct_changed_kind lint.

Example

unit_struct_changed_kind outputs the following witness hint for the ./test_crates/unit_struct_changed_kind/ test.

let witness = unit_struct_changed_kind::UnitStructToPlain;

This compiles on the old version, but not on the new as the struct can no longer be constructed as a unit struct;

Commit Notes

• Added witness hint for unit_struct_changed_kind

Comments

I'm using this just to make sure my workspace is all working well, which it seems to be. Excited to start working on the witness system soon!

[obi1kenobi]

I haven't looked at this lint in a while, and I'm not sure I'm convinced anymore that this lint is correct as it currently stands.

This witness fails to work if we define a const/static with the same name:

// Imagine this is the crate we're checking:
pub mod inner {
    // Imagine this is the struct that used to be a unit.
    pub struct SomeStruct {}
    
    pub const SomeStruct: SomeStruct = SomeStruct {};
}

fn witness() {
    // This works fine!
    let _x = inner::SomeStruct;
}

Try to take a look at pattern-matching the struct as the source of breakage, maybe? Make sure to consider #[non_exhaustive] structs, noting that #[non_exhaustive] structs can still be matched exhaustively from inside their own crate. A true #[non_exhaustive] test requires two crates — or a test and its doctests (which are compiled as a separate crate).

Depending on what you find, we may need to fix the lint to consider const/static values with the same name, or we may just need a different witness.

A similar close analysis like this might allow us to write a new lint for unit-to-tuple struct changes too, since we don't currently cover that case at all.

[GlitchlessCode]

Good point! Some of these are trickier than one expects XD
I'll need to look at that, I'll try to get another commit in. Admittedly, I was mostly just looking for something theoretically simple to test my setup (had some changes since I last worked on this). I guess this needs more work than I expected.

Note: This may just need to backseated for now, since I'll be more focused on the witness system

[GlitchlessCode]

Also I noticed that I accidentally commented out a line, which was not intentional.

[obi1kenobi]

I guess this needs more work than I expected.

Story of my life in cargo-semver-checks lately 😂