clippy_lints/src/try_err.rs

   1 use crate::utils::{
   2     differing_macro_contexts, in_macro, is_type_diagnostic_item, match_def_path, match_qpath, paths, snippet,
   3     snippet_with_macro_callsite, span_lint_and_sugg,
   4 };
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{Expr, ExprKind, LangItem, MatchSource, QPath};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_middle::ty::{self, Ty};
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use rustc_span::sym;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for usages of `Err(x)?`.
  16     ///
  17     /// **Why is this bad?** The `?` operator is designed to allow calls that
  18     /// can fail to be easily chained. For example, `foo()?.bar()` or
  19     /// `foo(bar()?)`. Because `Err(x)?` can't be used that way (it will
  20     /// always return), it is more clear to write `return Err(x)`.
  21     ///
  22     /// **Known problems:** None.
  23     ///
  24     /// **Example:**
  25     /// ```rust
  26     /// fn foo(fail: bool) -> Result<i32, String> {
  27     ///     if fail {
  28     ///       Err("failed")?;
  29     ///     }
  30     ///     Ok(0)
  31     /// }
  32     /// ```
  33     /// Could be written:
  34     ///
  35     /// ```rust
  36     /// fn foo(fail: bool) -> Result<i32, String> {
  37     ///     if fail {
  38     ///       return Err("failed".into());
  39     ///     }
  40     ///     Ok(0)
  41     /// }
  42     /// ```
  43     pub TRY_ERR,
  44     style,
  45     "return errors explicitly rather than hiding them behind a `?`"
  46 }
  47
  48 declare_lint_pass!(TryErr => [TRY_ERR]);
  49
  50 impl<'tcx> LateLintPass<'tcx> for TryErr {
  51     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  52         // Looks for a structure like this:
  53         // match ::std::ops::Try::into_result(Err(5)) {
  54         //     ::std::result::Result::Err(err) =>
  55         //         #[allow(unreachable_code)]
  56         //         return ::std::ops::Try::from_error(::std::convert::From::from(err)),
  57         //     ::std::result::Result::Ok(val) =>
  58         //         #[allow(unreachable_code)]
  59         //         val,
  60         // };
  61         if_chain! {
  62             if !in_external_macro(cx.tcx.sess, expr.span);
  63             if let ExprKind::Match(ref match_arg, _, MatchSource::TryDesugar) = expr.kind;
  64             if let ExprKind::Call(ref match_fun, ref try_args) = match_arg.kind;
  65             if let ExprKind::Path(ref match_fun_path) = match_fun.kind;
  66             if matches!(match_fun_path, QPath::LangItem(LangItem::TryIntoResult, _));
  67             if let Some(ref try_arg) = try_args.get(0);
  68             if let ExprKind::Call(ref err_fun, ref err_args) = try_arg.kind;
  69             if let Some(ref err_arg) = err_args.get(0);
  70             if let ExprKind::Path(ref err_fun_path) = err_fun.kind;
  71             if match_qpath(err_fun_path, &paths::RESULT_ERR);
  72             if let Some(return_ty) = find_return_type(cx, &expr.kind);
  73             then {
  74                 let prefix;
  75                 let suffix;
  76                 let err_ty;
  77
  78                 if let Some(ty) = result_error_type(cx, return_ty) {
  79                     prefix = "Err(";
  80                     suffix = ")";
  81                     err_ty = ty;
  82                 } else if let Some(ty) = poll_result_error_type(cx, return_ty) {
  83                     prefix = "Poll::Ready(Err(";
  84                     suffix = "))";
  85                     err_ty = ty;
  86                 } else if let Some(ty) = poll_option_result_error_type(cx, return_ty) {
  87                     prefix = "Poll::Ready(Some(Err(";
  88                     suffix = ")))";
  89                     err_ty = ty;
  90                 } else {
  91                     return;
  92                 };
  93
  94                 let expr_err_ty = cx.typeck_results().expr_ty(err_arg);
  95                 let differing_contexts = differing_macro_contexts(expr.span, err_arg.span);
  96
  97                 let origin_snippet = if in_macro(expr.span) && in_macro(err_arg.span) && differing_contexts {
  98                     snippet(cx, err_arg.span.ctxt().outer_expn_data().call_site, "_")
  99                 } else if err_arg.span.from_expansion() && !in_macro(expr.span) {
 100                     snippet_with_macro_callsite(cx, err_arg.span, "_")
 101                 } else {
 102                     snippet(cx, err_arg.span, "_")
 103                 };
 104                 let suggestion = if err_ty == expr_err_ty {
 105                     format!("return {}{}{}", prefix, origin_snippet, suffix)
 106                 } else {
 107                     format!("return {}{}.into(){}", 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::MachineApplicable
 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(_, ref arms, MatchSource::TryDesugar) = expr {
 127         for arm in arms.iter() {
 128             if let ExprKind::Ret(Some(ref 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_type);
 141         let err_ty = subst.type_at(1);
 142         then {
 143             Some(err_ty)
 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_type, ready_def.did);
 159         let err_ty = ready_subst.type_at(1);
 160
 161         then {
 162             Some(err_ty)
 163         } else {
 164             None
 165         }
 166     }
 167 }
 168
 169 /// Extracts the error type from Poll<Option<Result<T, E>>>.
 170 fn poll_option_result_error_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 171     if_chain! {
 172         if let ty::Adt(def, subst) = ty.kind();
 173         if match_def_path(cx, def.did, &paths::POLL);
 174         let ready_ty = subst.type_at(0);
 175
 176         if let ty::Adt(ready_def, ready_subst) = ready_ty.kind();
 177         if cx.tcx.is_diagnostic_item(sym::option_type, ready_def.did);
 178         let some_ty = ready_subst.type_at(0);
 179
 180         if let ty::Adt(some_def, some_subst) = some_ty.kind();
 181         if cx.tcx.is_diagnostic_item(sym::result_type, some_def.did);
 182         let err_ty = some_subst.type_at(1);
 183
 184         then {
 185             Some(err_ty)
 186         } else {
 187             None
 188         }
 189     }
 190 }