rust-lang/rust-clippy · error

called `.err().expect()` on a `Result` value

Error message

called `.err().expect()` on a `Result` value

What it means

Clippy lint ERR_EXPECT: calling .err() on a Result<T, E> converts it into an Option<E>, discarding the Ok(T) payload; .expect(msg) on that Option then panics with the caller-supplied message whenever the Result was Ok. The panic therefore fires on success and reports a misleading generic message instead of the actual value. The lint only fires when T: Debug (so expect_err is expressible) and the MSRV supports EXPECT_ERR.

Solutions

1. Replace .err().expect("...") with .expect_err("...") so the panic message includes the debug-printed Ok value
2. Use .unwrap_err() when no custom message is needed
3. Handle both cases explicitly with a match or if let! to avoid panicking entirely

When it happens

Trigger: Thrown at clippy_lints/src/methods/err_expect.rs:30 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rust-lang/rust-clippy@13aece1138 (2026-09-07). Data as JSON: /api/errors/879fcb31541e15eb. Report an issue: GitHub.

Appendix: source

Thrown at clippy_lints/src/methods/err_expect.rs:30

    cx: &LateContext<'_>,
    _expr: &rustc_hir::Expr<'_>,
    recv: &rustc_hir::Expr<'_>,
    expect_span: Span,
    err_span: Span,
    msrv: Msrv,
) {
    let result_ty = cx.typeck_results().expr_ty(recv);
    // Grabs the `Result<T, E>` type
    if let Some(data_type) = get_data_type(cx, result_ty)
        // Tests if the T type in a `Result<T, E>` implements Debug
        && has_debug_impl(cx, data_type)
        && msrv.meets(cx, msrvs::EXPECT_ERR)
    {
        span_lint_and_sugg(
            cx,
            ERR_EXPECT,
            err_span.to(expect_span),
            "called `.err().expect()` on a `Result` value",
            "try",
            "expect_err".to_string(),
            Applicability::MachineApplicable,
        );
    }
}

/// Given a `Result<T, E>` type, return its data (`T`).
fn get_data_type<'a>(cx: &LateContext<'_>, ty: Ty<'a>) -> Option<Ty<'a>> {
    match ty.kind() {
        ty::Adt(adt, args) if cx.tcx.is_diagnostic_item(sym::Result, adt.did()) => args.types().next(),
        _ => None,
    }
}

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