Function rustc_trait_selection::opaque_types::may_define_opaque_type

pub fn may_define_opaque_type(
    tcx: TyCtxt<'_>, 
    def_id: LocalDefId, 
    opaque_hir_id: HirId
) -> bool

Returns true if opaque_hir_id is a sibling or a child of a sibling of def_id.

Example:

pub mod foo {
    pub mod bar {
        pub trait Bar { .. }

        pub type Baz = impl Bar;

        fn f1() -> Baz { .. }
    }

    fn f2() -> bar::Baz { .. }
}

Here, def_id is the LocalDefId of the defining use of the opaque type (e.g., f1 or f2), and opaque_hir_id is the HirId of the definition of the opaque type Baz. For the above example, this function returns true for f1 and false for f2.