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_lang_ctor, match_def_path, paths};
   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 let ExprKind::Path(ref err_fun_path) = err_fun.kind;
  31         if is_lang_ctor(cx, err_fun_path, ResultErr);
  32         if let Some(return_ty) = find_return_type(cx, &expr.kind);
  33         then {
  34             let prefix;
  35             let suffix;
  36             let err_ty;
  37
  38             if let Some(ty) = result_error_type(cx, return_ty) {
  39                 prefix = "Err(";
  40                 suffix = ")";
  41                 err_ty = ty;
  42             } else if let Some(ty) = poll_result_error_type(cx, return_ty) {
  43                 prefix = "Poll::Ready(Err(";
  44                 suffix = "))";
  45                 err_ty = ty;
  46             } else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
  47                 prefix = "Poll::Ready(Some(Err(";
  48                 suffix = ")))";
  49                 err_ty = ty;
  50             } else {
  51                 return;
  52             };
  53
  54             let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
  55             let span = hygiene::walk_chain(err_arg.span, try_arg.span.ctxt());
  56             let mut applicability = Applicability::MachineApplicable;
  57             let origin_snippet = snippet_with_applicability(cx, span, "_", &mut applicability);
  58             let ret_prefix = if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::Ret(_))) {
  59                 "" // already returns
  60             } else {
  61                 "return "
  62             };
  63             let suggestion = if err_ty == expr_err_ty {
  64                 format!("{}{}{}{}", ret_prefix, prefix, origin_snippet, suffix)
  65             } else {
  66                 format!("{}{}{}.into(){}", ret_prefix, prefix, origin_snippet, suffix)
  67             };
  68
  69             span_lint_and_sugg(
  70                 cx,
  71                 TRY_ERR,
  72                 expr.span,
  73                 "returning an `Err(_)` with the `?` operator",
  74                 "try this",
  75                 suggestion,
  76                 applicability,
  77             );
  78         }
  79     }
  80 }
  81
  82 /// Finds function return type by examining return expressions in match arms.
  83 fn find_return_type<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
  84     if let ExprKind::Match(_, arms, MatchSource::TryDesugar) = expr {
  85         for arm in arms.iter() {
  86             if let ExprKind::Ret(Some(ret)) = arm.body.kind {
  87                 return Some(cx.typeck_results().expr_ty(ret));
  88             }
  89         }
  90     }
  91     None
  92 }
  93
  94 /// Extracts the error type from Result<T, E>.
  95 fn result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
  96     if_chain! {
  97         if let ty::Adt(_, subst) = ty.kind();
  98         if is_type_diagnostic_item(cx, ty, sym::Result);
  99         then {
 100             Some(subst.type_at(1))
 101         } else {
 102             None
 103         }
 104     }
 105 }
 106
 107 /// Extracts the error type from Poll<Result<T, E>>.
 108 fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 109     if_chain! {
 110         if let ty::Adt(def, subst) = ty.kind();
 111         if match_def_path(cx, def.did(), &paths::POLL);
 112         let ready_ty = subst.type_at(0);
 113
 114         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 115         if cx.tcx.is_diagnostic_item(sym::Result, ready_def.did());
 116         then {
 117             Some(ready_subst.type_at(1))
 118         } else {
 119             None
 120         }
 121     }
 122 }
 123
 124 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 125 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 126     if_chain! {
 127         if let ty::Adt(def, subst) = ty.kind();
 128         if match_def_path(cx, def.did(), &paths::POLL);
 129         let ready_ty = subst.type_at(0);
 130
 131         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 132         if cx.tcx.is_diagnostic_item(sym::Option, ready_def.did());
 133         let some_ty = ready_subst.type_at(0);
 134
 135         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 136         if cx.tcx.is_diagnostic_item(sym::Result, some_def.did());
 137         then {
 138             Some(some_subst.type_at(1))
 139         } else {
 140             None
 141         }
 142     }
 143 }