error[E0308]: mismatched types
  --> $DIR/chain-method-call-mutation-in-place.rs:3:5
   |
LL | fn foo(mut s: String) -> String {
   |                          ------ expected `String` because of return type
LL |     s.push_str("asdf")
   |     ^^^^^^^^^^^^^^^^^^ expected struct `String`, found `()`
   |
note: method `push_str` modifies its receiver in-place
  --> $DIR/chain-method-call-mutation-in-place.rs:3:7
   |
LL |     s.push_str("asdf")
   |     - ^^^^^^^^ this call modifies `s` in-place
   |     |
   |     you probably want to use this value after calling the method...
   = note: ...instead of the `()` output of method `push_str`

error: aborting due to previous error

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