src/tools/clippy/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     #[clippy::version = "1.38.0"]
  45     pub TRY_ERR,
  46     style,
  47     "return errors explicitly rather than hiding them behind a `?`"
  48 }
  49
  50 declare_lint_pass!(TryErr => [TRY_ERR]);
  51
  52 impl<'tcx> LateLintPass<'tcx> for TryErr {
  53     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  54         // Looks for a structure like this:
  55         // match ::std::ops::Try::into_result(Err(5)) {
  56         //     ::std::result::Result::Err(err) =>
  57         //         #[allow(unreachable_code)]
  58         //         return ::std::ops::Try::from_error(::std::convert::From::from(err)),
  59         //     ::std::result::Result::Ok(val) =>
  60         //         #[allow(unreachable_code)]
  61         //         val,
  62         // };
  63         if_chain! {
  64             if !in_external_macro(cx.tcx.sess, expr.span);
  65             if let ExprKind::Match(match_arg, _, MatchSource::TryDesugar) = expr.kind;
  66             if let ExprKind::Call(match_fun, try_args) = match_arg.kind;
  67             if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
  68             if matches!(match_fun_path, QPath::LangItem(LangItem::TryTraitBranch, _));
  69             if let Some(try_arg) = try_args.get(0);
  70             if let ExprKind::Call(err_fun, err_args) = try_arg.kind;
  71             if let Some(err_arg) = err_args.get(0);
  72             if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
  73             if is_lang_ctor(cx, err_fun_path, ResultErr);
  74             if let Some(return_ty) = find_return_type(cx, &expr.kind);
  75             then {
  76                 let prefix;
  77                 let suffix;
  78                 let err_ty;
  79
  80                 if let Some(ty) = result_error_type(cx, return_ty) {
  81                     prefix = "Err(";
  82                     suffix = ")";
  83                     err_ty = ty;
  84                 } else if let Some(ty) = poll_result_error_type(cx, return_ty) {
  85                     prefix = "Poll::Ready(Err(";
  86                     suffix = "))";
  87                     err_ty = ty;
  88                 } else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
  89                     prefix = "Poll::Ready(Some(Err(";
  90                     suffix = ")))";
  91                     err_ty = ty;
  92                 } else {
  93                     return;
  94                 };
  95
  96                 let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
  97                 let span = hygiene::walk_chain(err_arg.span, try_arg.span.ctxt());
  98                 let mut applicability = Applicability::MachineApplicable;
  99                 let origin_snippet = snippet_with_applicability(cx, span, "_", &mut applicability);
 100                 let ret_prefix = if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::Ret(_))) {
 101                     "" // already returns
 102                 } else {
 103                     "return "
 104                 };
 105                 let suggestion = if err_ty == expr_err_ty {
 106                     format!("{}{}{}{}", ret_prefix, prefix, origin_snippet, suffix)
 107                 } else {
 108                     format!("{}{}{}.into(){}", ret_prefix, prefix, origin_snippet, suffix)
 109                 };
 110
 111                 span_lint_and_sugg(
 112                     cx,
 113                     TRY_ERR,
 114                     expr.span,
 115                     "returning an `Err(_)` with the `?` operator",
 116                     "try this",
 117                     suggestion,
 118                     applicability,
 119                 );
 120             }
 121         }
 122     }
 123 }
 124
 125 /// Finds function return type by examining return expressions in match arms.
 126 fn find_return_type<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx ExprKind<'_>) -> Option<Ty<'tcx>> {
 127     if let ExprKind::Match(_, arms, MatchSource::TryDesugar) = expr {
 128         for arm in arms.iter() {
 129             if let ExprKind::Ret(Some(ret)) = arm.body.kind {
 130                 return Some(cx.typeck_results().expr_ty(ret));
 131             }
 132         }
 133     }
 134     None
 135 }
 136
 137 /// Extracts the error type from Result<T, E>.
 138 fn result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 139     if_chain! {
 140         if let ty::Adt(_, subst) = ty.kind();
 141         if is_type_diagnostic_item(cx, ty, sym::Result);
 142         then {
 143             Some(subst.type_at(1))
 144         } else {
 145             None
 146         }
 147     }
 148 }
 149
 150 /// Extracts the error type from Poll<Result<T, E>>.
 151 fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 152     if_chain! {
 153         if let ty::Adt(def, subst) = ty.kind();
 154         if match_def_path(cx, def.did, &paths::POLL);
 155         let ready_ty = subst.type_at(0);
 156
 157         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 158         if cx.tcx.is_diagnostic_item(sym::Result, ready_def.did);
 159         then {
 160             Some(ready_subst.type_at(1))
 161         } else {
 162             None
 163         }
 164     }
 165 }
 166
 167 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 168 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 169     if_chain! {
 170         if let ty::Adt(def, subst) = ty.kind();
 171         if match_def_path(cx, def.did, &paths::POLL);
 172         let ready_ty = subst.type_at(0);
 173
 174         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 175         if cx.tcx.is_diagnostic_item(sym::Option, ready_def.did);
 176         let some_ty = ready_subst.type_at(0);
 177
 178         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 179         if cx.tcx.is_diagnostic_item(sym::Result, some_def.did);
 180         then {
 181             Some(some_subst.type_at(1))
 182         } else {
 183             None
 184         }
 185     }
 186 }