Cycle error when accidentally comparing a function inside itself

[RalfJung]

The following code leads to an inscrutable error message:

fn up() {
    up == 1
}

error[E0391]: cycle detected when processing `up`
 --> src/lib.rs:1:1
  |
1 | fn up() {
  | ^^^^^^^
  |
  = note: ...which again requires processing `up`, completing the cycle
  = note: cycle used when type-checking all item bodies

error: aborting due to previous error

I had this happen in a 10-line function and it took me a bit to realize that I typo'd the function instead of a variable name somewhere. There is no indication where inside the function the bad use is.