Function rustc_typeck::check::wfcheck::check_opaque_types

pub(in check::wfcheck) fn check_opaque_types<'fcx, 'tcx>(
    tcx: TyCtxt<'tcx>, 
    fcx: &FnCtxt<'fcx, 'tcx>, 
    fn_def_id: LocalDefId, 
    span: Span, 
    ty: Ty<'tcx>
)

Checks "defining uses" of opaque impl Trait types to ensure that they meet the restrictions laid for "higher-order pattern unification". This ensures that inference is tractable. In particular, definitions of opaque types can only use other generics as arguments, and they cannot repeat an argument. Example:

type Foo<A, B> = impl Bar<A, B>;

// Okay -- `Foo` is applied to two distinct, generic types.
fn a<T, U>() -> Foo<T, U> { .. }

// Not okay -- `Foo` is applied to `T` twice.
fn b<T>() -> Foo<T, T> { .. }

// Not okay -- `Foo` is applied to a non-generic type.
fn b<T>() -> Foo<T, u32> { .. }