error: extern location for foo is of an unknown type: $DIR/issue-104621-extern-bad-file.rs
  --> $DIR/issue-104621-extern-bad-file.rs:4:1
   |
LL | extern crate foo;
   | ^^^^^^^^^^^^^^^^^

error: file name should be lib*.rlib or lib*.so
  --> $DIR/issue-104621-extern-bad-file.rs:4:1
   |
LL | extern crate foo;
   | ^^^^^^^^^^^^^^^^^

error[E0463]: can't find crate for `foo`
  --> $DIR/issue-104621-extern-bad-file.rs:4:1
   |
LL | extern crate foo;
   | ^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0463`.