error[E0308]: mismatched types
  --> $DIR/issue-6458-4.rs:1:20
   |
LL | fn foo(b: bool) -> Result<bool,String> { //~ ERROR mismatched types
   |    ---             ^^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found ()
   |    |
   |    this function's body doesn't return
LL |     Err("bar".to_string());
   |                           - help: consider removing this semicolon
   |
   = note: expected type `std::result::Result<bool, std::string::String>`
              found type `()`

error: aborting due to previous error

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