error[E0261]: use of undeclared lifetime name `'f`
  --> $DIR/span-bug-issue-121414.rs:5:22
   |
LL | impl<'a> Bar for Foo<'f> {
   |                      ^^ undeclared lifetime
   |
help: consider introducing lifetime `'f` here
   |
LL | impl<'f, 'a> Bar for Foo<'f> {
   |      +++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0261`.