Semver breaking change not listed in book: Relaxing trait type bounds

[mysteriouslyseeing]

Relaxing a supertrait is a breaking change not listed in the book's section on SemVer, because other crates may rely on supertraits for generic code. Note that this also applies to trait bounds on associated types:

trait Foo: Default {}

fn make_a_foo<A: Foo>() -> A {
    A::default()
}

trait Bar {
    type Baz: Default;
}

fn make_a_baz<A: Bar>() -> A::Baz {
    A::Baz::default()
}

That code fails to compile if either Default is removed. Playground

[weihanglo]

cc @obi1kenobi though you might already know it :)

[obi1kenobi]

Thanks! cargo-semver-checks has a lint for the supertrait bound already, and I'll make an issue for the associated type's bound too.

The lints won't catch changes in generics currently (e.g. changing the bound from Foo<A> to Foo<B> for unrelated A and B), unfortunately, but something is always better than nothing :)

[obi1kenobi]

I opened obi1kenobi/cargo-semver-checks#1364

While writing it up, I noticed that where Self: Sized bounds on trait associated types and functions are particularly interesting as well.

We need to work out the rules with respect to both regular trait bound + sealed trait rules, and also keeping in mind that items with where Self: Sized bounds cannot be used in a dyn Trait context. That was a bit too tricky for me to work out right in the moment, and given the trickiness would probably be good to have explained in the reference as well.