抽象特征中的“检查可见性时检测到循环”

Question

我试图让工作成为一种依赖于多个层面的其他抽象特征的特征。我遇到了这个错误：

error[E0391]: cycle detected when checking effective visibilities
   |
note: ...which requires computing type of `wavefront::<impl at src/wavefront.rs:28:1: 47:46>::pos_iterator::{opaque#0}`...
  --> src/wavefront.rs:54:34
   |
54 |     fn pos_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires computing type of opaque `wavefront::<impl at src/wavefront.rs:28:1: 47:46>::pos_iterator::{opaque#0}`...
  --> src/wavefront.rs:54:34
   |
54 |     fn pos_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires type-checking `wavefront::<impl at src/wavefront.rs:28:1: 47:46>::pos_iterator`...
  --> src/wavefront.rs:54:5
   |
54 |     fn pos_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...which again requires checking effective visibilities, completing the cycle
note: cycle used when checking that `wavefront::<impl at src/wavefront.rs:28:1: 47:46>` is well-formed
  --> src/wavefront.rs:28:1
   |
28 | / impl<'a, V2, V3, V, E, F, VI, EI, FI> WaveFrontCompatible<'a>
29 | |     for dyn HalfMeshLike<
30 | |         VertData = V,
31 | |         EdgeData = E,
...  |
46 | |     EI: Iterator<Item = HEdgeHandle<V, E, F>>,
47 | |     FI: Iterator<Item = FaceHandle<V, E, F>>,
   | |_____________________________________________^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

我尝试更改我的类型，尝试添加更多特征，专门化返回类型...我只是不明白，为什么编译器试图在这里实例化类型，为什么它不等到我尝试应用一个具体类型的特征来知道它是否可以推导出类型？

代码：

pub trait WaveFrontCompatible<'a>
{
    type Scalar: num_traits::Float + Debug + AddAssign + Display;
    type Vert2D: VectorLike<Scalar = Self::Scalar>;
    type Vert3D: VectorLike<Scalar = Self::Scalar>;
    type Index: num_traits::int::PrimInt + Display;

    fn pos_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>;
    fn uv_iterator(&'a self) -> impl Iterator<Item = Self::Vert2D>;
    fn norm_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>;

    fn segment_iterator(&'a self) -> impl Iterator<Item = [Self::Index; 2]>;
    fn pos_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>;
    fn uv_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>;
    fn norm_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>;
}

trait FaceDataGetters
{
    type Normal;
    type Uv;
    fn normal_count(&self) -> usize;
    fn uv_count(&self) -> usize;
    fn normal(&self, id: usize);
    fn uv(&self, id: usize);
}

trait VertDataGetters
{
    type V3;
    fn position(&self) -> Self::V3;
}

impl<'a, V2, V3, V, E, F, VI, EI, FI> WaveFrontCompatible<'a>
    for dyn HalfMeshLike<
        VertData = V,
        EdgeData = E,
        FaceData = F,
        VertIterator = VI,
        EdgeIterator = EI,
        FaceIterator = FI,
    >
where
    V: VertDataGetters + Debug + Clone,
    V2: VectorLike + Debug + Clone,
    V2::Scalar: Display + Debug,
    V3: VectorLike + Debug + Clone,
    V3::Scalar: Display + Debug,
    E: Debug + Clone,
    F: Debug + Clone + FaceDataGetters<Normal = V3, Uv = V2>,
    VI: Iterator<Item = VertHandle<V, E, F>>,
    EI: Iterator<Item = HEdgeHandle<V, E, F>>,
    FI: Iterator<Item = FaceHandle<V, E, F>>,
{
    type Index = usize;
    type Scalar = V3::Scalar;
    type Vert2D = V2;
    type Vert3D = V3;

    fn pos_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>
    {
        self.iter_verts().map(|v| v.data().position())
    }

    fn norm_iterator(&'a self) -> impl Iterator<Item = Self::Vert3D>
    {
        self.iter_faces()
            .map(|face| face.data().iter_normals())
            .flattent()
    }

    fn uv_iterator(&'a self) -> impl Iterator<Item = Self::Vert2D>
    {
        self.iter_faces()
            .map(|face| face.data().iter_uvs())
            .flatten()
    }

    fn pos_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>
    {
        self.iter_faces()
            .map(|f| f.vertex_handles().map(|v| v.id().0 as usize))
    }

    fn norm_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>
    {
        self.norm_iterator().enumerate().map(|i| i.0)
    }

    fn uv_index_iterator(
        &'a self,
    ) -> impl Iterator<Item = impl Iterator<Item = Self::Index>>
    {
        self.norm_uv().enumerate().map(|i| i.0)
    }

    fn segment_iterator(&'a self) -> impl Iterator<Item = [Self::Index; 2]>
    {
        std::iter::empty()
    }
}

Answer 1

我认为this稍微解释了这个问题。如果您可以使用夜间 Rust 进行编译，您应该会得到更好的错误。