E0004 — Unknown identifier

A name was referenced that isn’t bound in the current scope.

---

What triggers it

Resilient is strict about scope: every identifier read has to resolve to a let, a function parameter, a top-level fn / struct / type declaration, or a builtin. Typos and out-of- scope references land here.

Minimal example

fn main() {
    let x = 1;
    return y;
}

Output:

scratch.rs:3:12: error[E0004]: unknown identifier `y`
   return y;
          ^

Fix

Declare the name or fix the typo. If you meant a builtin, check the spelling — builtin names are case-sensitive (println, not Println).

fn main() {
    let y = 1;
    return y;
}

Source

Emitted from the typechecker (resilient/src/typechecker.rs), the interpreter (resilient/src/main.rs), and the VM compiler (resilient/src/compiler.rs).