The comparable crate defines the trait Comparable, along with a derive macro for auto-generating instances of this trait for most data types. Primarily the purpose of this trait is to offer a method, Comparable::comparison, by which two values of any type supporting that trait can yield a summary of the differences between them.

Note that unlike other crates that do data differencing (primarily between scalars and collections), comparable has been written primarily with testing in mind. That is, the purpose of generating such change descriptions is to enable writing tests that assert the set of expected changes after some operation between an initial state and the resulting state. This goal also means that some types, like HashMap, must be differenced after ordering the keys first, so that the set of changes produced can be made deterministic and thus expressible as a test expectation.

To these ends, the macro assert_changes! is also provided, taking two values of the same type along with an expected "change description" as returned by foo.comparison(&bar). This function uses the pretty_assertions crate under the hood so that minute differences within deep structures can be easily seen in the failure output.

Quickstart

If you want to get started quickly with the Comparable crate to enhance unit testing, do the following:

1. Add the comparable crate as a dependency, enabling features = ["derive"].
2. Derive the Comparable trait on as many structs and enums as needed.
3. Structure your unit tests to follow these three phases: a. Create the initial state or dataset you intend to test and make a copy of it. b. Apply your operations and changes to this state. c. Use assert_changes! between the initial state and the resulting state to assert that whatever happened is exactly what you expected to happen.

The main benefit of this approach over the usual method of "probing" the resulting state -- to ensure it changed as you expected it to-- is that it asserts against the exhaustive set of changes to ensure that no unintended side-effects occurred beyond what you expected to happen. In this way, it is both a positive and a negative test: checking for what you expect to see as well as what you don't expect to see.

The Comparable trait

The Comparable trait has two associated types and two methods, one pair corresponding to value descriptions and the other to value changes:

pub trait Comparable {
    type Desc: std::cmp::PartialEq + std::fmt::Debug;
    fn describe(&self) -> Self::Desc;

    type Change: std::cmp::PartialEq + std::fmt::Debug;
    fn comparison(&self, other: &Self) -> comparable::Changed<Self::Change>;
}

Descriptions: the Comparable::Desc associated type

Value descriptions (the Comparable::Desc associated type) are needed because value hierarchies can involve many types. Perhaps some of these types implement PartialEq and Debug, but not all. To work around this limitation, the Comparable derive macro creates a "mirror" of your data structure with all the same constructors ands field, but using the Comparable::Desc associated type for each of its contained types.

# use comparable_derive::*;
#[derive(Comparable)]
struct MyStruct {
  bar: u32,
  baz: u32
}

This generates a description that mirrors the original type, but using type descriptions rather than the types themselves:

struct MyStructDesc {
  bar: <u32 as comparable::Comparable>::Desc,
  baz: <u32 as comparable::Comparable>::Desc
}

You may also choose an alternate description type, such as a reduced form of a value or some other type entirely. For example, complex structures could describe themselves by the set of changes they represent from a Default value. This is so common, that it's supported via a compare_default macro attribute provided by comparable:

# use comparable_derive::*;
#[derive(Comparable)]
#[compare_default]
struct MyStruct { /* ...lots of fields... */ }

impl Default for MyStruct {
    fn default() -> Self { MyStruct {} }
}

For scalars, the Comparable::Desc type is the same as the type it's describing, and these are called "self-describing".

There are other macro attributes provided for customizing things even further, which are covered below, beginning at the section on Structures.

Changes: the Comparable::Change associated type

When two values of a type differ, this difference gets represented using the associated type Comparable::Change. Such values are produced by the Comparable::comparison method, which actually returns Changed<Change> since the result may be either Changed::Unchanged or Changed::Changed(_changes_).[^option]

[^option] Changed is just a different flavor of the Option type, created to make changesets clearer than just seeing Some in various places.

The primary purpose of a Comparable::Change value is to compare it to a set of changes you expected to see, so design choices have been made to optimize for clarity and printing rather than, say, the ability to transform one value into another by applying a changeset. This is entirely possible give a dataset and a change description, but no work has been done to achieve this goal.

How changes are represented can differ greatly between scalars, collections, structs and enums, so more detail is given below in the section discussing each of these types.

Scalars

Comparable traits have been implemented for all of the basic scalar types. These are self-describing, and use a Comparable::Change structure named after the type that holds the previous and changed values. For example, the following assertions hold:

# use comparable::*;
assert_changes!(&100, &100, Changed::Unchanged);
assert_changes!(&100, &200, Changed::Changed(I32Change(100, 200)));
assert_changes!(&true, &false, Changed::Changed(BoolChange(true, false)));
assert_changes!(
    &"foo",
    &"bar",
    Changed::Changed(StringChange("foo".to_string(), "bar".to_string())),
);

Vec and Set Collections

The set collections for which Comparable has been implemented are: Vec, HashSet, and BTreeSet.

The Vec uses Vec<VecChange> to report all of the indices at which changes happened. Note that it cannot detect insertions in the middle, and so will likely report every item as changed from there until the end of the vector, at which point it will report an added member.

HashSet and BTreeSet types both report changes the same way, using the SetChange type. Note that in order for HashSet change results to be deterministic, the values in a HashSet must support the Ord trait so they can be sorted prior to comparison. Sets cannot tell when specific members have change, and so only report changes in terms of SetChange::Added and SetChange::Removed.

Here are a few examples, taken from the comparable_test test suite:

# use comparable::*;
# use std::collections::HashSet;
// Vectors
assert_changes!(
    &vec![1 as i32, 2],
    &vec![1 as i32, 2, 3],
    Changed::Changed(vec![VecChange::Added(2, 3)]),
);
assert_changes!(
    &vec![1 as i32, 3],
    &vec![1 as i32, 2, 3],
    Changed::Changed(vec![
        VecChange::Changed(1, I32Change(3, 2)),
        VecChange::Added(2, 3),
    ]),
);
assert_changes!(
    &vec![1 as i32, 2, 3],
    &vec![1 as i32, 3],
    Changed::Changed(vec![
        VecChange::Changed(1, I32Change(2, 3)),
        VecChange::Removed(2, 3),
    ]),
);
assert_changes!(
    &vec![1 as i32, 2, 3],
    &vec![1 as i32, 4, 3],
    Changed::Changed(vec![VecChange::Changed(1, I32Change(2, 4))]),
);

// Sets
assert_changes!(
    &vec![1 as i32, 2].into_iter().collect::<HashSet<_>>(),
    &vec![1 as i32, 2, 3].into_iter().collect::<HashSet<_>>(),
    Changed::Changed(vec![SetChange::Added(3)]),
);
assert_changes!(
    &vec![1 as i32, 3].into_iter().collect::<HashSet<_>>(),
    &vec![1 as i32, 2, 3].into_iter().collect::<HashSet<_>>(),
    Changed::Changed(vec![SetChange::Added(2)]),
);
assert_changes!(
    &vec![1 as i32, 2, 3].into_iter().collect::<HashSet<_>>(),
    &vec![1 as i32, 3].into_iter().collect::<HashSet<_>>(),
    Changed::Changed(vec![SetChange::Removed(2)]),
);
assert_changes!(
    &vec![1 as i32, 2, 3].into_iter().collect::<HashSet<_>>(),
    &vec![1 as i32, 4, 3].into_iter().collect::<HashSet<_>>(),
    Changed::Changed(vec![SetChange::Added(4), SetChange::Removed(2)]),
);

Note that if the first VecChange::Change above had used an index of 1 instead of 0, the resulting failure would look something like this:

running 1 test
test test_comparable_bar ... FAILED

failures:

---- test_comparable_bar stdout ----
thread 'test_comparable_bar' panicked at 'assertion failed: `(left == right)`

Diff < left / right > :
 Changed(
     [
         Change(
<            1,
>            0,
             I32Change(
                 100,
                 200,
             ),
         ),
     ],
 )

', /Users/johnw/src/comparable/comparable/src/lib.rs:19:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_comparable_bar

Map Collections

The map collections for which Comparable has been implemented are: HashMap, and BTreeMap.

Both report changes the same way, using the MapChange type. Note that in order for HashMap change results to be deterministic, the keys in a HashMap must support the Ord trait so they can be sorted prior to comparison. Changes are reported in terms of MapChange::Added, MapChange::Removed and MapChange::Changed, exactly like VecChange above.

Here are a few examples, taken from the comparable_test test suite:

# use comparable::*;
# use std::collections::HashMap;
// HashMaps
assert_changes!(
    &vec![(0, 1 as i32), (1, 2)].into_iter().collect::<HashMap<_, _>>(),
    &vec![(0, 1 as i32), (1, 2), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    Changed::Changed(vec![MapChange::Added(2, 3)]),
);
assert_changes!(
    &vec![(0, 1 as i32), (1, 2), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    &vec![(0, 1 as i32), (1, 2)].into_iter().collect::<HashMap<_, _>>(),
    Changed::Changed(vec![MapChange::Removed(2)]),
);
assert_changes!(
    &vec![(0, 1 as i32), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    &vec![(0, 1 as i32), (1, 2), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    Changed::Changed(vec![MapChange::Added(1, 2)]),
);
assert_changes!(
    &vec![(0, 1 as i32), (1, 2), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    &vec![(0, 1 as i32), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    Changed::Changed(vec![MapChange::Removed(1)]),
);
assert_changes!(
    &vec![(0, 1 as i32), (1, 2), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    &vec![(0, 1 as i32), (1, 4), (2, 3)].into_iter().collect::<HashMap<_, _>>(),
    Changed::Changed(vec![MapChange::Changed(1, I32Change(2, 4))]),
);

Structures

Differencing arbitrary structures was the original motive for creating comparable. This is made feasible using a Comparable derive macro that auto-generates code needed for such comparisons. The purpose of this section is to explain how this macro works, and the various attribute macros that can be used to guide the process. If all else fails, manual trait implementations are always an alternative.

Here is what deriving Change for a structure with multiple fields typically produces:

# use comparable_derive::*;
# use comparable::*;
struct MyStruct {
  bar: u32,
  baz: u32,
}

// The following would be generated by `#[derive(Comparable)]`:

#[derive(PartialEq, Debug)]
struct MyStructDesc {
    bar: <u32 as Comparable>::Desc,
    baz: <u32 as Comparable>::Desc,
}

#[derive(PartialEq, Debug)]
enum MyStructChange {
    Bar(<u32 as Comparable>::Change),
    Baz(<u32 as Comparable>::Change),
}

impl Comparable for MyStruct {
    type Desc = MyStructDesc;

    fn describe(&self) -> Self::Desc {
        MyStructDesc {
            bar: self.bar.describe(),
            baz: self.baz.describe(),
        }
    }

    type Change = Vec<MyStructChange>;

    fn comparison(&self, other: &Self) -> Changed<Self::Change> {
        let changes: Self::Change = vec![
            self.bar.comparison(&other.bar).map(MyStructChange::Bar),
            self.baz.comparison(&other.baz).map(MyStructChange::Baz),
        ]
            .into_iter()
            .flatten()
            .collect();
        if changes.is_empty() {
            Changed::Unchanged
        } else {
            Changed::Changed(changes)
        }
    }
}

For structs with one field or no fields, see the related section below.

Field attribute: comparable_ignore

The first attribute macro you'll notice that can be applied to individual fields is #[comparable_ignore], which must be used if the type in question cannot be compared for differences.

Field attribute: comparable_synthetic

The #[comparable_synthetic { <BINDINGS...> }] attribute allows you to attach one or more "synthetic properties" to a field, which are then considered in both descriptions and change sets, as if they were actual fields with the computed value. Here is an example:

# use comparable_derive::*;
#[derive(Comparable)]
pub struct Synthetics {
    #[comparable_synthetic {
        let full_value = |x: &Self| -> u8 { x.ensemble.iter().sum() };
    }]
    #[comparable_ignore]
    pub ensemble: Vec<u8>,
}

This structure has an ensemble field containing a vector of u8 values. However, in tests we may not care if the vector's contents change, so long as the final sum remains the same. This is done by ignoring the ensemble field so that it's not generated or described at all, while creating a synthetic field derived from the full object that yields the sum.

Note that the syntax for the comparable_synthetic attribute is rather specific: a series of simply-named let bindings, where the value in each case is a fully typed closure that takes a reference to the object containing the original field (&Self), and yields a value of some type for which Comparable has been implemented or derived.

Deriving Comparable for structs: the Desc type

By default, deriving Comparable for a structure will create a "mirror" of that structure, with all the same fields, but replacing every type T with <T as Comparable>::Desc:

# use comparable::*;
struct MyStructDesc {
  bar: <u32 as Comparable>::Desc,
  baz: <u32 as Comparable>::Desc
}

This process can be influenced using several attribute macros.

Macro attribute: self_describing

If the self_describing attribute is used, the Comparable::Desc type is set to be the type itself, and the Comparable::describe method return a clone of the value.

Note the following traits are required for self-describing types: Clone, Debug and PartialEq.

Macro attribute: no_description

If you want no description at all for a type, since you only care about how it has changed and never want to report a description of the value in any other context, then you can use #[no_description]. This sets the Comparable::Desc type to be unit, and the Comparable::describe method accordingly:

type Desc = ();

fn describe(&self) -> Self::Desc {
    ()
}

It is assumed that when this is appropriate, such values will never appear in any change output, so consider a different approach if you see lots of units turning up.

Macro attribute: describe_type and describe_body

You can have more control over description by specifying exactly the text that should appear for the Comparable::Desc type and the body of the Comparable::describe function. Basically, for the following definition:

# use comparable_derive::*;
#[derive(Comparable)]
#[describe_type(T)]
#[describe_body(B)]
struct MyStruct {
  bar: u32,
  baz: u32
}

The following is generated:

type Desc = T;

fn describe(&self) -> Self::Desc {
    B
}

This also means that the expression argument passed to describe_body may reference the self parameter. Here is a real-world use case:

# use comparable_derive::*;
#[cfg_attr(feature = "comparable",
           derive(comparable::Comparable),
           describe_type(String),
           describe_body(self.to_string()))]
struct MyStruct {}

This same approach could be used to represent large blobs of data by their checksum hash, for example, or large data structures that you don't need to ever display by their Merkle root hash.

Macro attribute: compare_default

When the #[compare_default] attribute macro is used, the Comparable::Desc type is defined to be the same as the Comparable::Change type, with the Comparable::describe method being implemented as a comparison against the value of Default::default():

# use comparable::*;
impl comparable::Comparable for MyStruct {
    type Desc = Self::Change;

    fn describe(&self) -> Self::Desc {
        MyStruct::default().comparison(self).unwrap_or_default()
    }

    type Change = Vec<MyStructChange>;

    /* ... */
}

Note that changes for structures are always a vector, since this allows changes to be reported separately for each field. More on this in the following section.

Macro attribute: comparable_public and comparable_private

By default, the auto-generated Comparable::Desc and Comparable::Change types have the same visibility as their parent. This may not be appropriate, however, if you want to keep the original data type private but allow exporting of descriptions and change sets. To support this -- and the converse -- you can use #[comparable_public] and #[comparable_private] to be explicit about the visibility of these generated types.

Special case: Unit structs

If a struct has no fields it can never change, and so only a unitary Comparable::Desc type is generated.

Special case: Singleton structs

If a struct has only one fields, whether named or unnamed, it no longer makes sense to use a vector of enum values to record what has changed. In this case the derivation becomes much simpler:

# use comparable_derive::*;
# use comparable::*;
struct MyStruct {
  bar: u32,
}

// The following would be generated by `#[derive(Comparable)]`:

#[derive(PartialEq, Debug)]
struct MyStructDesc {
    bar: <u32 as Comparable>::Desc,
}

#[derive(PartialEq, Debug)]
struct MyStructChange {
    bar: <u32 as Comparable>::Change,
}

impl Comparable for MyStruct {
    type Desc = MyStructDesc;

    fn describe(&self) -> Self::Desc {
        MyStructDesc { bar: self.bar.describe() }
    }

    type Change = MyStructChange;

    fn comparison(&self, other: &Self) -> Changed<Self::Change> {
        self.bar.comparison(&other.bar).map(|x| MyStructChange { bar: x })
    }
}

Deriving Comparable for structs: the Change type

By default for structs, deriving Comparable creates an enum with variants for each field in the struct, and it represents changes using a vector of such values. This means that for the following definition:

# use comparable_derive::*;
#[derive(Comparable)]
struct MyStruct {
  bar: u32,
  baz: u32
}

The Comparable::Change type is defined to be Vec<MyStructChange>, with MyStructChange as follows:

#[derive(PartialEq, Debug)]
enum MyStructChange {
    Bar(<u32 as Comparable>::Change),
    Baz(<u32 as Comparable>::Change),
}

impl comparable::Comparable for MyStruct {
    type Desc = MyStructDesc;
    type Change = Vec<MyStructChange>;
}

Note that if a struct has only one field, there is no reason to specify changes using a vector, since either the struct is unchanged or just that one field has changed. For this reason, singleton structs optimize away the vector and use type Change = [type]Change in their Comparable derivation, rather than type Change = Vec<[type]Change> as for multi-field structs.

Here is an abbreviated example of how this looks when asserting changes for a struct with multiple fields:

assert_changes!(
    &initial_foo, &later_foo,
    Changed::Changed(vec![
        MyStructChange::Bar(...),
        MyStructChange::Baz(...),
    ]));

If the field hasn't been changed it won't appear in the vector, and each field appears at most once. The reason for taking this approach is that structures with many, many fields can be represented by a small change set if most of the other fields were left untouched.

Enumerations

Enumerations are handled quite differently from structures, for the reason that while a struct is always a product of fields, an enum can be more than a sum of variants -- but also a sum of products.

To unpack that a bit: By "a product of fields", this means that a struct is a simple grouping of typed fields, where the same fields are available for every value of such a structure.

Meanwhile an enum is a sum, or choice, among variants. However, some of these variants can themselves contain groups of fields, as though there were an unnamed structure embedded in the variant. Consider the following enum:

# use comparable_derive::*;
#[derive(Comparable)]
enum MyEnum {
    One(bool),
    Two { two: Vec<bool>, two_more: u32 },
    Three,
}

Here we see variant that has a variant with no fields (Three), one with unnamed fields (One), and one with named fields like a usual structure (Two). The problem, though, is that these embedded structures are never represented as independent types, so we can't define Comparable for them and just compute the differences between the enum arguments. Nor can we just create a copy of the field type with a real name and generate Comparable for it, because not every value is copyable or clonable, and it gets very tricky to auto-generate a new hierarchy built out fields with reference types all the way down...

Instead, the following gets generated, which can end up being a bit verbose, but captures the full nature of any differences:

enum MyEnumChange {
    BothOne(<bool as comparable::Comparable>::Change),
    BothTwo {
        two: Changed<<Vec<bool> as comparable::Comparable>::Change>,
        two_more: Changed<Baz as comparable::Comparable>::Change
    },
    BothThree,
    Different(
        <MyEnum as comparable::Comparable>::Desc,
        <MyEnum as comparable::Comparable>::Desc
    ),
}

Note that variants with singleton fields do not use Comparable::Change, since that information is already reflected when the variant is reported as having changed at all using, for example, BothOne. In the case of BothTwo, each of the field types is wrapped in Changed because it's possible that either one or both of the fields may changed.

Below is a full example of what gets derived for the enum above:

# use comparable_derive::*;
# use comparable::*;
enum MyEnum {
    One(bool),
    Two { two: Vec<bool>, two_more: u32 },
    Three,
}

// The following would be generated by `#[derive(Comparable)]`:

#[derive(PartialEq, Debug)]
enum MyEnumDesc {
    One(<bool as Comparable>::Desc),
    Two { two: <Vec<bool> as Comparable>::Desc,
          two_more: <u32 as Comparable>::Desc },
    Three,
}

#[derive(PartialEq, Debug)]
enum MyEnumChange {
    BothOne(<bool as Comparable>::Change),
    BothTwo { two: Changed<<Vec<bool> as Comparable>::Change>,
              two_more: Changed<<u32 as Comparable>::Change> },
    BothThree,
    Different(MyEnumDesc, MyEnumDesc),
}

impl Comparable for MyEnum {
    type Desc = MyEnumDesc;

    fn describe(&self) -> Self::Desc {
        match self {
            MyEnum::One(x) => MyEnumDesc::One(x.describe()),
            MyEnum::Two { two: x, two_more: y } =>
                MyEnumDesc::Two { two: x.describe(),
                                  two_more: y.describe() },
            MyEnum::Three => MyEnumDesc::Three,
        }
    }

    type Change = MyEnumChange;

    fn comparison(&self, other: &Self) -> Changed<Self::Change> {
        match (self, other) {
            (MyEnum::One(x), MyEnum::One(y)) =>
                x.comparison(&y).map(MyEnumChange::BothOne),
            (MyEnum::Two { two: x0, two_more: x1 },
             MyEnum::Two { two: y0, two_more: y1 }) => {
                let c0 = x0.comparison(&y0);
                let c1 = x1.comparison(&y1);
                if c0.is_unchanged() && c1.is_unchanged() {
                    Changed::Unchanged
                } else {
                    Changed::Changed(MyEnumChange::BothTwo {
                        two: c0, two_more: c1
                    })
                }
            }
            (MyEnum::Three, MyEnum::Three) => Changed::Unchanged,
            (_, _) => Changed::Changed(
                MyEnumChange::Different(self.describe(), other.describe()))
        }
    }
}

Field attribute: comparable_ignore

Similarly to structs, #[comparable_ignore] can be applied to enum variant fields that cannot be compared for differences.

#[derive(Comparable)]
enum MyEnumWithNamedFields {
    Variant1{ some_u8: u8},
    Variant2 {
        some_u16: u16,
        #[comparable_ignore]
        random_value: u64,
    },
}

#[derive(Comparable)]
enum MyEnumWithUnnamedFields {
    Variant1(u8),
    Variant2 (u16, #[comparable_ignore] u64),
}

Deriving Comparable for enums: the Desc type

By default for enums, deriving Comparable creates a "mirror" of that structure, with all the same variants and fields, but replacing every type T with <T as Comparable>::Desc:

# use comparable::*;
enum MyEnumDesc {
  Bar(<u32 as Comparable>::Desc),
  Baz { some_field: <u32 as Comparable>::Desc }
}

This process can be influenced using the same attribute macros as for structs, with the exception that synthetic properties are not yet supported on fields of enum variants. Use of this attribute in that context is silently ignored at present.

TODO: jww (2021-11-01): Allow for synthetic fields in enum variants.

Deriving Comparable for enums: the Change type

By default for enums, deriving Comparable create a related enum where each variant from the original is represented by a Both<Name> variant in the Change type, and a new variant named Different is added that takes two description of the original enum.

Whenever two enum values are compared and they have different variants, the Different variant of the Change type is used to represent a description of the differing values. If the values share the same variant, then Both<Variant> is used.

Note that Both<Variant> has two forms: For variant with a single named or unnamed field, it is simply the Change type associated with the original field type; for variants with multiple named or unnamed fields, each Change type is also wrapped in a Changed structure, to reflect whether that field of the variant changed or not.

Field attribute: variant_struct_fields

Note that it is possible to treat variant fields as though they were structs, and then to compare them exactly the same way as for structs above. This is not the default because enum variants with named fields typically contain fewer fields on average than structs, and it would increase verbosity in the change description to always have to name these implied structs. However, in cases where the number of fields found in variants is large, it can be just as benifical as for structs.

For this reason, the macro attribute variant_struct_fields is provided to derive such transformations. For example, it would cause the following code to be generated, with the main difference between the new MyEnumTwoChange type and how it is used:

# use comparable_derive::*;
# use comparable::*;
enum MyEnum {
    One(bool),
    Two { two: Vec<bool>, two_more: u32 },
    Three,
}

// The following would be generated by `#[derive(Comparable)]`:

#[derive(PartialEq, Debug)]
enum MyEnumDesc {
    One(<bool as Comparable>::Desc),
    Two { two: <Vec<bool> as Comparable>::Desc,
          two_more: <u32 as Comparable>::Desc },
    Three,
}

#[derive(PartialEq, Debug)]
enum MyEnumChange {
    BothOne(<bool as Comparable>::Change),
    BothTwo(Vec<MyEnumTwoChange>),
    BothThree,
    Different(MyEnumDesc, MyEnumDesc),
}

#[derive(PartialEq, Debug)]
enum MyEnumTwoChange {
    Two(<Vec<bool> as Comparable>::Change),
    TwoMore(<u32 as Comparable>::Change),
}

impl Comparable for MyEnum {
    type Desc = MyEnumDesc;

    fn describe(&self) -> Self::Desc {
        match self {
            MyEnum::One(x) => MyEnumDesc::One(x.describe()),
            MyEnum::Two { two: x, two_more: y } =>
                MyEnumDesc::Two { two: x.describe(),
                                  two_more: y.describe() },
            MyEnum::Three => MyEnumDesc::Three,
        }
    }

    type Change = MyEnumChange;

    fn comparison(&self, other: &Self) -> Changed<Self::Change> {
        match (self, other) {
            (MyEnum::One(x), MyEnum::One(y)) =>
                x.comparison(&y).map(MyEnumChange::BothOne),
            (MyEnum::Two { two: x0, two_more: x1 },
             MyEnum::Two { two: y0, two_more: y1 }) => {
                let c0 = x0.comparison(&y0);
                let c1 = x1.comparison(&y1);
                let changes: Vec<MyEnumTwoChange> = vec![
                    c0.map(MyEnumTwoChange::Two),
                    c1.map(MyEnumTwoChange::TwoMore),
                ].into_iter().flatten().collect();
                if changes.is_empty() {
                    Changed::Unchanged
                } else {
                    Changed::Changed(MyEnumChange::BothTwo(changes))
                }
            }
            (MyEnum::Three, MyEnum::Three) => Changed::Unchanged,
            (_, _) => Changed::Changed(
                MyEnumChange::Different(self.describe(), other.describe()))
        }
    }
}

Special case: Empty enums

If a enum has no variants it cannot be constructed, so both the Comparable::Desc or Comparable::Change types are omitted and it is always reported as unchanged.

Unions

Unions cannot derive Comparable instances at the present time.