clippy_lints/src/try_err.rs

   1 use crate::utils::{
   2     differing_macro_contexts, in_macro, match_def_path, match_qpath, paths, snippet, snippet_with_macro_callsite,
   3     span_lint_and_sugg,
   4 };
   5 use clippy_utils::ty::is_type_diagnostic_item;
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   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::sym;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for usages of `Err(x)?`.
  17     ///
  18     /// **Why is this bad?** The `?` operator is designed to allow calls that
  19     /// can fail to be easily chained. For example, `foo()?.bar()` or
  20     /// `foo(bar()?)`. Because `Err(x)?` can't be used that way (it will
  21     /// always return), it is more clear to write `return Err(x)`.
  22     ///
  23     /// **Known problems:** None.
  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(ref match_arg, _, MatchSource::TryDesugar) = expr.kind;
  65             if let ExprKind::Call(ref match_fun, ref 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::TryIntoResult, _));
  68             if let Some(ref try_arg) = try_args.get(0);
  69             if let ExprKind::Call(ref err_fun, ref err_args) = try_arg.kind;
  70             if let Some(ref err_arg) = err_args.get(0);
  71             if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
  72             if match_qpath(err_fun_path, &paths::RESULT_ERR);
  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 differing_contexts = differing_macro_contexts(expr.span, err_arg.span);
  97
  98                 let origin_snippet = if in_macro(expr.span) && in_macro(err_arg.span) && differing_contexts {
  99                     snippet(cx, err_arg.span.ctxt().outer_expn_data().call_site, "_")
 100                 } else if err_arg.span.from_expansion() && !in_macro(expr.span) {
 101                     snippet_with_macro_callsite(cx, err_arg.span, "_")
 102                 } else {
 103                     snippet(cx, err_arg.span, "_")
 104                 };
 105                 let suggestion = if err_ty == expr_err_ty {
 106                     format!("return {}{}{}", prefix, origin_snippet, suffix)
 107                 } else {
 108                     format!("return {}{}.into(){}", 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::MachineApplicable
 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(_, ref arms, MatchSource::TryDesugar) = expr {
 128         for arm in arms.iter() {
 129             if let ExprKind::Ret(Some(ref 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_type);
 142         let err_ty = subst.type_at(1);
 143         then {
 144             Some(err_ty)
 145         } else {
 146             None
 147         }
 148     }
 149 }
 150
 151 /// Extracts the error type from Poll<Result<T, E>>.
 152 fn poll_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 153     if_chain! {
 154         if let ty::Adt(def, subst) = ty.kind();
 155         if match_def_path(cx, def.did, &paths::POLL);
 156         let ready_ty = subst.type_at(0);
 157
 158         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 159         if cx.tcx.is_diagnostic_item(sym::result_type, ready_def.did);
 160         let err_ty = ready_subst.type_at(1);
 161
 162         then {
 163             Some(err_ty)
 164         } else {
 165             None
 166         }
 167     }
 168 }
 169
 170 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 171 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 172     if_chain! {
 173         if let ty::Adt(def, subst) = ty.kind();
 174         if match_def_path(cx, def.did, &paths::POLL);
 175         let ready_ty = subst.type_at(0);
 176
 177         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 178         if cx.tcx.is_diagnostic_item(sym::option_type, ready_def.did);
 179         let some_ty = ready_subst.type_at(0);
 180
 181         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 182         if cx.tcx.is_diagnostic_item(sym::result_type, some_def.did);
 183         let err_ty = some_subst.type_at(1);
 184
 185         then {
 186             Some(err_ty)
 187         } else {
 188             None
 189         }
 190     }
 191 }