src/tools/clippy/clippy_lints/src/methods/map_err_ignore.rs

use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::ty::is_type_diagnostic_item;
use rustc_hir::{CaptureBy, Closure, Expr, ExprKind, PatKind};
use rustc_lint::LateContext;
use rustc_span::sym;

use super::MAP_ERR_IGNORE;

pub(super) fn check(cx: &LateContext<'_>, e: &Expr<'_>, arg: &Expr<'_>) {
    if let Some(method_id) = cx.typeck_results().type_dependent_def_id(e.hir_id)
        && let Some(impl_id) = cx.tcx.impl_of_method(method_id)
        && is_type_diagnostic_item(cx, cx.tcx.type_of(impl_id).instantiate_identity(), sym::Result)
        && let ExprKind::Closure(&Closure {
            capture_clause: CaptureBy::Ref,
            body,
            fn_decl_span,
            ..
        }) = arg.kind
        && let closure_body = cx.tcx.hir().body(body)
        && let [param] = closure_body.params
        && let PatKind::Wild = param.pat.kind
    {
        // span the area of the closure capture and warn that the
        // original error will be thrown away
        span_lint_and_help(
            cx,
            MAP_ERR_IGNORE,
            fn_decl_span,
            "`map_err(|_|...` wildcard pattern discards the original error",
            None,
            "consider storing the original error as a source in the new error, or silence this warning using an ignored identifier (`.map_err(|_foo| ...`)",
        );
    }
}