error[E0308]: mismatched types
  --> $DIR/issue-6458-4.rs:1:20
   |
LL | fn foo(b: bool) -> Result<bool,String> {
   |    ---             ^^^^^^^^^^^^^^^^^^^ expected `Result<bool, String>`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
LL |     Err("bar".to_string());
   |                           - help: remove this semicolon to return this value
   |
   = note:   expected enum `Result<bool, String>`
           found unit type `()`

error: aborting due to previous error

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