clippy_lints/src/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, LateLintPass};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_middle::ty::{self, Ty};
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::{hygiene, sym};
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for usages of `Err(x)?`.
  18     ///
  19     /// ### Why is this bad?
  20     /// The `?` operator is designed to allow calls that
  21     /// can fail to be easily chained. For example, `foo()?.bar()` or
  22     /// `foo(bar()?)`. Because `Err(x)?` can't be used that way (it will
  23     /// always return), it is more clear to write `return Err(x)`.
  24     ///
  25     /// ### Example
  26     /// ```rust
  27     /// fn foo(fail: bool) -> Result<i32, String> {
  28     ///     if fail {
  29     ///       Err("failed")?;
  30     ///     }
  31     ///     Ok(0)
  32     /// }
  33     /// ```
  34     /// Could be written:
  35     ///
  36     /// ```rust
  37     /// fn foo(fail: bool) -> Result<i32, String> {
  38     ///     if fail {
  39     ///       return Err("failed".into());
  40     ///     }
  41     ///     Ok(0)
  42     /// }
  43     /// ```
  44     pub TRY_ERR,
  45     style,
  46     "return errors explicitly rather than hiding them behind a `?`"
  47 }
  48
  49 declare_lint_pass!(TryErr => [TRY_ERR]);
  50
  51 impl<'tcx> LateLintPass<'tcx> for TryErr {
  52     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  53         // Looks for a structure like this:
  54         // match ::std::ops::Try::into_result(Err(5)) {
  55         //     ::std::result::Result::Err(err) =>
  56         //         #[allow(unreachable_code)]
  57         //         return ::std::ops::Try::from_error(::std::convert::From::from(err)),
  58         //     ::std::result::Result::Ok(val) =>
  59         //         #[allow(unreachable_code)]
  60         //         val,
  61         // };
  62         if_chain! {
  63             if !in_external_macro(cx.tcx.sess, expr.span);
  64             if let ExprKind::Match(match_arg, _, MatchSource::TryDesugar) = expr.kind;
  65             if let ExprKind::Call(match_fun, try_args) = match_arg.kind;
  66             if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
  67             if matches!(match_fun_path, QPath::LangItem(LangItem::TryTraitBranch, _));
  68             if let Some(try_arg) = try_args.get(0);
  69             if let ExprKind::Call(err_fun, err_args) = try_arg.kind;
  70             if let Some(err_arg) = err_args.get(0);
  71             if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
  72             if is_lang_ctor(cx, err_fun_path, ResultErr);
  73             if let Some(return_ty) = find_return_type(cx, &expr.kind);
  74             then {
  75                 let prefix;
  76                 let suffix;
  77                 let err_ty;
  78
  79                 if let Some(ty) = result_error_type(cx, return_ty) {
  80                     prefix = "Err(";
  81                     suffix = ")";
  82                     err_ty = ty;
  83                 } else if let Some(ty) = poll_result_error_type(cx, return_ty) {
  84                     prefix = "Poll::Ready(Err(";
  85                     suffix = "))";
  86                     err_ty = ty;
  87                 } else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
  88                     prefix = "Poll::Ready(Some(Err(";
  89                     suffix = ")))";
  90                     err_ty = ty;
  91                 } else {
  92                     return;
  93                 };
  94
  95                 let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
  96                 let span = hygiene::walk_chain(err_arg.span, try_arg.span.ctxt());
  97                 let mut applicability = Applicability::MachineApplicable;
  98                 let origin_snippet = snippet_with_applicability(cx, span, "_", &mut applicability);
  99                 let ret_prefix = if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::Ret(_))) {
 100                     "" // already returns
 101                 } else {
 102                     "return "
 103                 };
 104                 let suggestion = if err_ty == expr_err_ty {
 105                     format!("{}{}{}{}", ret_prefix, prefix, origin_snippet, suffix)
 106                 } else {
 107                     format!("{}{}{}.into(){}", ret_prefix, prefix, origin_snippet, suffix)
 108                 };
 109
 110                 span_lint_and_sugg(
 111                     cx,
 112                     TRY_ERR,
 113                     expr.span,
 114                     "returning an `Err(_)` with the `?` operator",
 115                     "try this",
 116                     suggestion,
 117                     applicability,
 118                 );
 119             }
 120         }
 121     }
 122 }
 123
 124 /// Finds function return type by examining return expressions in match arms.
 125 fn find_return_type<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
 126     if let ExprKind::Match(_, arms, MatchSource::TryDesugar) = expr {
 127         for arm in arms.iter() {
 128             if let ExprKind::Ret(Some(ret)) = arm.body.kind {
 129                 return Some(cx.typeck_results().expr_ty(ret));
 130             }
 131         }
 132     }
 133     None
 134 }
 135
 136 /// Extracts the error type from Result<T, E>.
 137 fn result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 138     if_chain! {
 139         if let ty::Adt(_, subst) = ty.kind();
 140         if is_type_diagnostic_item(cx, ty, sym::Result);
 141         then {
 142             Some(subst.type_at(1))
 143         } else {
 144             None
 145         }
 146     }
 147 }
 148
 149 /// Extracts the error type from Poll<Result<T, E>>.
 150 fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 151     if_chain! {
 152         if let ty::Adt(def, subst) = ty.kind();
 153         if match_def_path(cx, def.did, &paths::POLL);
 154         let ready_ty = subst.type_at(0);
 155
 156         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 157         if cx.tcx.is_diagnostic_item(sym::Result, ready_def.did);
 158         then {
 159             Some(ready_subst.type_at(1))
 160         } else {
 161             None
 162         }
 163     }
 164 }
 165
 166 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 167 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 168     if_chain! {
 169         if let ty::Adt(def, subst) = ty.kind();
 170         if match_def_path(cx, def.did, &paths::POLL);
 171         let ready_ty = subst.type_at(0);
 172
 173         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 174         if cx.tcx.is_diagnostic_item(sym::Option, ready_def.did);
 175         let some_ty = ready_subst.type_at(0);
 176
 177         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 178         if cx.tcx.is_diagnostic_item(sym::Result, some_def.did);
 179         then {
 180             Some(some_subst.type_at(1))
 181         } else {
 182             None
 183         }
 184     }
 185 }