Attribute rustc_dump_variances 

Dumps the variances of the annotated item.

See also the variances_of query and ty::Variance.

See AttributeKind::RustcDumpVariances for the internal representation of this attribute.

Example

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]

#[rustc_dump_variances]
struct Ref<'a, T> {
    r: &'a T,
}

#[rustc_dump_variances]
struct RefMut<'a, T> {
    r: &'a mut T,
}

#[rustc_dump_variances]
struct CellRef<'a, T> {
    r: &'a core::cell::UnsafeCell<T>,
}

#[rustc_dump_variances]
fn x<T, U>(_t: T) -> U {
    todo!()
}

produces:

error: ['a: +, T: +]
--> $DIR/rustc_dump_variances.rs:7:1
 |
7 | struct Ref<'a, T> {
 | ^^^^^^^^^^^^^^^^^

error: ['a: +, T: o]
 --> $DIR/rustc_dump_variances.rs:12:1
  |
12 | struct RefMut<'a, T> {
  | ^^^^^^^^^^^^^^^^^^^^

error: ['a: +, T: o]
 --> $DIR/rustc_dump_variances.rs:17:1
  |
17 | struct CellRef<'a, T> {
  | ^^^^^^^^^^^^^^^^^^^^^

error: [T: -, U: +]
 --> $DIR/rustc_dump_variances.rs:22:1
  |
22 | fn x<T, U>(_t: T) -> U {
  | ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 4 previous errors