error[E0308]: mismatched types
  --> $DIR/universal-two-impl-traits.rs:5:9
   |
LL | fn foo(x: impl Debug, y: impl Debug) -> String {
   |           ----------     ---------- found type parameter
   |           |
   |           expected type parameter
LL |     let mut a = x;
   |                 - expected due to this value
LL |     a = y;
   |         ^ expected type parameter `impl Debug`, found a different type parameter `impl Debug`
   |
   = note: expected type parameter `impl Debug` (type parameter `impl Debug`)
              found type parameter `impl Debug` (type parameter `impl Debug`)
   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.