Why does generic type parameterized by "my type" violate orphan rule?

[davemilter]

Let's suppose that we have concrete type defined in one crate Lib1Foo
and generic type defined in another:

pub struct Lib2Boo<T> {
    x: T,
}

and in the third crate I want to write something like this:

struct MyType;
impl From<Lib1Foo> for Lib2Boo<MyType> {
    fn from(_: Lib1Foo) -> Self {
        unimplemented!();
    }
}

Why this is violation of

the impl does not reference only types defined in this crate
This error indicates a violation of one of Rust's orphan rules for trait
implementations

?

MyType defined in the current crate,
so Lib2Boo<MyType> technically is local crate type,
because of MyType is defined in current crate, no?

[ldesgoui]

MyType defined in the current crate,
so Lib2Boo<MyType> technically is local crate type,
because of MyType is defined in current crate, no?

Nop, the "root" (Lib2Boo) is what matters.

[ExpHP]

This impl is forbidden so that Lib2 is permitted to add something like the following in a minor semver bump:

impl<T> From<Lib1Foo> for Lib2Boo<T> {
    fn from(_: Lib1Foo) -> Self { ... }
}

Note there is an accepted RFC to weaken the orphan rules, but it won't enable this case.