error[E0609]: no field `sleep` on type `&mut S`
  --> $DIR/issue-67971.rs:5:9
   |
LL |     ctx.sleep = 0;
   |         ^^^^^ unknown field

error[E0308]: mismatched types
  --> $DIR/issue-67971.rs:3:24
   |
LL | fn foo(ctx: &mut S) -> String {
   |    ---                 ^^^^^^ expected struct `std::string::String`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0308, E0609.
For more information about an error, try `rustc --explain E0308`.