warning: irrefutable `if let` pattern
  --> $DIR/issue-78720.rs:6:8
   |
LL |     if let a = "" {
   |        ^^^^^^^^^^
   |
   = note: this pattern will always match, so the `if let` is useless
   = help: consider replacing the `if let` with a `let`
   = note: `#[warn(irrefutable_let_patterns)]` on by default

warning: 1 warning emitted