New lints: an enum variant changes from unit/tuple/struct to another kind.

[suaviloquence]

Describe your use case

We have lints for when a struct changes "kind":

• tuple_struct_to_plain_struct
• unit_struct_changed_kind

However, currently cargo-semver-checks does not check when an enum variant changes kind like this.

from:

pub enum Enum { 
    WasTuple(),
    WasStruct {},
    WasUnit,
    TupleItem (()),
    StructItem { a: (), },
    Unit,
}

to:

pub enum Enum {
    WasTuple {},
    WasStruct,
    WasUnit(),
    TupleItem { a: () },
    StructItem,
    Unit(()),
}

the first three variants would break with:

let _ = Enum::WasTuple;
// since it's now a tuple variant, this still gives us a value,
// but it's a `fn(()) -> Enum` tuple constructor, not an `Enum` instance.
let _: Enum = Enum::WasStruct;
let _ = Enum::WasUnit;

the second three also have breakage in a similar fashion, and cargo-semver-checks currently does not catch the
fields_missing/removed lints because these variants changed kind

Describe the solution you'd like

Thus, it would be helpful to have lint(s) that detect this change for constructible enum variants - if a tuple/struct variant is marked #[non_exhaustive] in the baseline version, this is not a breaking change because it was not possible to construct these variants in the baseline version.

Alternatives, if applicable

No response

Additional Context

No response

[obi1kenobi]

Nice catch!

Similarly to the #[non_exhaustive] case, if the variant was #[doc(hidden)] we probably don't want to lint on it since the variant isn't meant to be constructed by downstream code.

For a piece of code like:

pub enum Example {
    Plain,
    Tuple(),
    Struct {},
}

Note that #[non_exhaustive] tuple variants can't be constructed by an external crate, so witness (proof-of-breakage) code can't rely on constructing the variant.

We want the following lints:

• public API unit variant that isn't #[non_exhaustive] changes to another kind: Enum unit variant changed kind #894
  • witness: let _: Example = Example::Plain only works for unit variants
• public API tuple variant that isn't #[non_exhaustive] changes to another kind: Enum tuple variant changed kind #906
  • witness let _: Example = Example::Tuple(...) only works for tuple variants, nothing else can use parens
• public API #[non_exhaustive] tuple variant that has at least one public API field changes to another kind
  • witness: accessing the public API field, since struct variants can't declare fields with numeric keys to make .0 work
  • this covers the edge case of the previous bullet point; we add #[non_exhaustive] to this lint to ensure the two lints are completely disjoint
• public API struct variant with at least one public API field changes to another kind
  • witness: accessing the public API field, since it must have been named non-numerically (not .0) so plain and tuple structs can't have the same field
  • changing a struct variant with no fields is not breaking since {}-like instantiation is allowed for all variant kinds

Remaining questions:

• Is it a breaking change if a #[non_exhaustive] unit variant changes to another kind?
  • I think the answer is "no" since I can't come up with a witness. Is this correct?