src/tools/clippy/clippy_lints/src/matches/try_err.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_applicability;
   3 use clippy_utils::ty::is_type_diagnostic_item;
   4 use clippy_utils::{get_parent_expr, is_res_lang_ctor, path_res};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::LangItem::ResultErr;
   8 use rustc_hir::{Expr, ExprKind, LangItem, MatchSource, QPath};
   9 use rustc_lint::LateContext;
  10 use rustc_middle::ty::{self, Ty};
  11 use rustc_span::{hygiene, sym};
  12
  13 use super::TRY_ERR;
  14
  15 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, scrutinee: &'tcx Expr<'_>) {
  16     // Looks for a structure like this:
  17     // match ::std::ops::Try::into_result(Err(5)) {
  18     //     ::std::result::Result::Err(err) =>
  19     //         #[allow(unreachable_code)]
  20     //         return ::std::ops::Try::from_error(::std::convert::From::from(err)),
  21     //     ::std::result::Result::Ok(val) =>
  22     //         #[allow(unreachable_code)]
  23     //         val,
  24     // };
  25     if_chain! {
  26         if let ExprKind::Call(match_fun, [try_arg, ..]) = scrutinee.kind;
  27         if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
  28         if matches!(match_fun_path, QPath::LangItem(LangItem::TryTraitBranch, ..));
  29         if let ExprKind::Call(err_fun, [err_arg, ..]) = try_arg.kind;
  30         if is_res_lang_ctor(cx, path_res(cx, err_fun), ResultErr);
  31         if let Some(return_ty) = find_return_type(cx, &expr.kind);
  32         then {
  33             let prefix;
  34             let suffix;
  35             let err_ty;
  36
  37             if let Some(ty) = result_error_type(cx, return_ty) {
  38                 prefix = "Err(";
  39                 suffix = ")";
  40                 err_ty = ty;
  41             } else if let Some(ty) = poll_result_error_type(cx, return_ty) {
  42                 prefix = "Poll::Ready(Err(";
  43                 suffix = "))";
  44                 err_ty = ty;
  45             } else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
  46                 prefix = "Poll::Ready(Some(Err(";
  47                 suffix = ")))";
  48                 err_ty = ty;
  49             } else {
  50                 return;
  51             };
  52
  53             let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
  54             let span = hygiene::walk_chain(err_arg.span, try_arg.span.ctxt());
  55             let mut applicability = Applicability::MachineApplicable;
  56             let origin_snippet = snippet_with_applicability(cx, span, "_", &mut applicability);
  57             let ret_prefix = if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::Ret(_))) {
  58                 "" // already returns
  59             } else {
  60                 "return "
  61             };
  62             let suggestion = if err_ty == expr_err_ty {
  63                 format!("{ret_prefix}{prefix}{origin_snippet}{suffix}")
  64             } else {
  65                 format!("{ret_prefix}{prefix}{origin_snippet}.into(){suffix}")
  66             };
  67
  68             span_lint_and_sugg(
  69                 cx,
  70                 TRY_ERR,
  71                 expr.span,
  72                 "returning an `Err(_)` with the `?` operator",
  73                 "try this",
  74                 suggestion,
  75                 applicability,
  76             );
  77         }
  78     }
  79 }
  80
  81 /// Finds function return type by examining return expressions in match arms.
  82 fn find_return_type<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
  83     if let ExprKind::Match(_, arms, MatchSource::TryDesugar) = expr {
  84         for arm in arms.iter() {
  85             if let ExprKind::Ret(Some(ret)) = arm.body.kind {
  86                 return Some(cx.typeck_results().expr_ty(ret));
  87             }
  88         }
  89     }
  90     None
  91 }
  92
  93 /// Extracts the error type from Result<T, E>.
  94 fn result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
  95     if_chain! {
  96         if let ty::Adt(_, subst) = ty.kind();
  97         if is_type_diagnostic_item(cx, ty, sym::Result);
  98         then {
  99             Some(subst.type_at(1))
 100         } else {
 101             None
 102         }
 103     }
 104 }
 105
 106 /// Extracts the error type from Poll<Result<T, E>>.
 107 fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 108     if_chain! {
 109         if let ty::Adt(def, subst) = ty.kind();
 110         if cx.tcx.lang_items().get(LangItem::Poll) == Some(def.did());
 111         let ready_ty = subst.type_at(0);
 112
 113         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 114         if cx.tcx.is_diagnostic_item(sym::Result, ready_def.did());
 115         then {
 116             Some(ready_subst.type_at(1))
 117         } else {
 118             None
 119         }
 120     }
 121 }
 122
 123 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 124 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 125     if_chain! {
 126         if let ty::Adt(def, subst) = ty.kind();
 127         if cx.tcx.lang_items().get(LangItem::Poll) == Some(def.did());
 128         let ready_ty = subst.type_at(0);
 129
 130         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 131         if cx.tcx.is_diagnostic_item(sym::Option, ready_def.did());
 132         let some_ty = ready_subst.type_at(0);
 133
 134         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 135         if cx.tcx.is_diagnostic_item(sym::Result, some_def.did());
 136         then {
 137             Some(some_subst.type_at(1))
 138         } else {
 139             None
 140         }
 141     }
 142 }