Issue #919: rustc’s dead-code warnings reach the user in Rust’s words; Nikaia has to give them itself

Open and answer this issue on GitHub.

ready · opened 2026-10-28 · updated 2026-10-28 · 0 comments

Seen in the probe of #918, on develop at 642b0d9.

Probe

enum Bad { Neg, Big }
impl Error for Bad { fn message(ref self) -> String { return "bad" } }

fn risky(n: i64) -> i64 throws {
    if n < 0 { throw Bad::Neg }
    return n
}

fn main() throws {
    let v = risky(-1) catch { 0 }
    println(f"{v}")
}

nikaia run shows rustc’s warning, with Rust’s vocabulary:

warning: Variant `Big` is never constructed.
  --> f2.nika:1:1
   = note: `Bad` has derived impls for the traits `Clone` and `Debug`, but these are intentionally ignored during dead code analysis.

Why it matters

• Part III C.1, the Iron Rule: an untranslated backend message reaching the user is a compiler bug. C.2 also rules out Rust’s vocabulary (derived impls, traits).
• The warning itself is wanted (owner, 2026-10-28): Nikaia sets out to rule out classes of mistakes Rust lets through; it cannot let through one Rust catches. A variant never made, a function never called, a name never read are such mistakes. Today they are seen only because rustc happens to say so, in its words, and they vanish wherever the lowering silences rustc.

To build

1. Inventory, with a probe each: which rustc warnings a program can bring out today (a variant never constructed, a function never called, a field never read, a local never read, mut never needed, unreachable code, an unused use, …). Write the list into this issue as a comment, with which of them Nikaia already says itself.
2. Nikaia’s own warning for each, in the checker, on the .nika span, in Nikaia’s words with a way out (C.2): a code per class from a block the supervisor reserves, the specification text and Part III’s code table written with it (a short ADR if a class needs a decision, e.g. what counts as used for a pub item or across a package).
3. rustc’s lint for that class switched off in the emitted program once Nikaia gives it, so the user is told once, in Nikaia’s words; a class not yet owned stays visible until it is.
4. Tests in Nikaia (tests/compiler/tests/, .stderr): each class warned once, in Nikaia’s words, with no rustc text.

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