src/tools/clippy/clippy_lints/src/dereference.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_context;
   3 use clippy_utils::ty::peel_mid_ty_refs;
   4 use clippy_utils::{get_parent_node, in_macro, is_allowed};
   5 use rustc_ast::util::parser::PREC_PREFIX;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BorrowKind, Expr, ExprKind, HirId, MatchSource, Mutability, Node, UnOp};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty::{self, Ty, TyCtxt, TyS, TypeckResults};
  10 use rustc_session::{declare_tool_lint, impl_lint_pass};
  11 use rustc_span::{symbol::sym, Span};
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for explicit `deref()` or `deref_mut()` method calls.
  15     ///
  16     /// **Why is this bad?** Dereferencing by `&*x` or `&mut *x` is clearer and more concise,
  17     /// when not part of a method chain.
  18     ///
  19     /// **Example:**
  20     /// ```rust
  21     /// use std::ops::Deref;
  22     /// let a: &mut String = &mut String::from("foo");
  23     /// let b: &str = a.deref();
  24     /// ```
  25     /// Could be written as:
  26     /// ```rust
  27     /// let a: &mut String = &mut String::from("foo");
  28     /// let b = &*a;
  29     /// ```
  30     ///
  31     /// This lint excludes
  32     /// ```rust,ignore
  33     /// let _ = d.unwrap().deref();
  34     /// ```
  35     pub EXPLICIT_DEREF_METHODS,
  36     pedantic,
  37     "Explicit use of deref or deref_mut method while not in a method chain."
  38 }
  39
  40 impl_lint_pass!(Dereferencing => [
  41     EXPLICIT_DEREF_METHODS,
  42 ]);
  43
  44 #[derive(Default)]
  45 pub struct Dereferencing {
  46     state: Option<(State, StateData)>,
  47
  48     // While parsing a `deref` method call in ufcs form, the path to the function is itself an
  49     // expression. This is to store the id of that expression so it can be skipped when
  50     // `check_expr` is called for it.
  51     skip_expr: Option<HirId>,
  52 }
  53
  54 struct StateData {
  55     /// Span of the top level expression
  56     span: Span,
  57     /// The required mutability
  58     target_mut: Mutability,
  59 }
  60
  61 enum State {
  62     // Any number of deref method calls.
  63     DerefMethod {
  64         // The number of calls in a sequence which changed the referenced type
  65         ty_changed_count: usize,
  66         is_final_ufcs: bool,
  67     },
  68 }
  69
  70 // A reference operation considered by this lint pass
  71 enum RefOp {
  72     Method(Mutability),
  73     Deref,
  74     AddrOf,
  75 }
  76
  77 impl<'tcx> LateLintPass<'tcx> for Dereferencing {
  78     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  79         // Skip path expressions from deref calls. e.g. `Deref::deref(e)`
  80         if Some(expr.hir_id) == self.skip_expr.take() {
  81             return;
  82         }
  83
  84         // Stop processing sub expressions when a macro call is seen
  85         if in_macro(expr.span) {
  86             if let Some((state, data)) = self.state.take() {
  87                 report(cx, expr, state, data);
  88             }
  89             return;
  90         }
  91
  92         let typeck = cx.typeck_results();
  93         let (kind, sub_expr) = if let Some(x) = try_parse_ref_op(cx.tcx, typeck, expr) {
  94             x
  95         } else {
  96             // The whole chain of reference operations has been seen
  97             if let Some((state, data)) = self.state.take() {
  98                 report(cx, expr, state, data);
  99             }
 100             return;
 101         };
 102
 103         match (self.state.take(), kind) {
 104             (None, kind) => {
 105                 let parent = get_parent_node(cx.tcx, expr.hir_id);
 106                 let expr_ty = typeck.expr_ty(expr);
 107
 108                 match kind {
 109                     RefOp::Method(target_mut)
 110                         if !is_allowed(cx, EXPLICIT_DEREF_METHODS, expr.hir_id)
 111                             && is_linted_explicit_deref_position(parent, expr.hir_id, expr.span) =>
 112                     {
 113                         self.state = Some((
 114                             State::DerefMethod {
 115                                 ty_changed_count: if deref_method_same_type(expr_ty, typeck.expr_ty(sub_expr)) {
 116                                     0
 117                                 } else {
 118                                     1
 119                                 },
 120                                 is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
 121                             },
 122                             StateData {
 123                                 span: expr.span,
 124                                 target_mut,
 125                             },
 126                         ));
 127                     }
 128                     _ => (),
 129                 }
 130             },
 131             (Some((State::DerefMethod { ty_changed_count, .. }, data)), RefOp::Method(_)) => {
 132                 self.state = Some((
 133                     State::DerefMethod {
 134                         ty_changed_count: if deref_method_same_type(typeck.expr_ty(expr), typeck.expr_ty(sub_expr)) {
 135                             ty_changed_count
 136                         } else {
 137                             ty_changed_count + 1
 138                         },
 139                         is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
 140                     },
 141                     data,
 142                 ));
 143             },
 144
 145             (Some((state, data)), _) => report(cx, expr, state, data),
 146         }
 147     }
 148 }
 149
 150 fn try_parse_ref_op(
 151     tcx: TyCtxt<'tcx>,
 152     typeck: &'tcx TypeckResults<'_>,
 153     expr: &'tcx Expr<'_>,
 154 ) -> Option<(RefOp, &'tcx Expr<'tcx>)> {
 155     let (def_id, arg) = match expr.kind {
 156         ExprKind::MethodCall(_, _, [arg], _) => (typeck.type_dependent_def_id(expr.hir_id)?, arg),
 157         ExprKind::Call(
 158             Expr {
 159                 kind: ExprKind::Path(path),
 160                 hir_id,
 161                 ..
 162             },
 163             [arg],
 164         ) => (typeck.qpath_res(path, *hir_id).opt_def_id()?, arg),
 165         ExprKind::Unary(UnOp::Deref, sub_expr) if !typeck.expr_ty(sub_expr).is_unsafe_ptr() => {
 166             return Some((RefOp::Deref, sub_expr));
 167         },
 168         ExprKind::AddrOf(BorrowKind::Ref, _, sub_expr) => return Some((RefOp::AddrOf, sub_expr)),
 169         _ => return None,
 170     };
 171     if tcx.is_diagnostic_item(sym::deref_method, def_id) {
 172         Some((RefOp::Method(Mutability::Not), arg))
 173     } else if tcx.trait_of_item(def_id)? == tcx.lang_items().deref_mut_trait()? {
 174         Some((RefOp::Method(Mutability::Mut), arg))
 175     } else {
 176         None
 177     }
 178 }
 179
 180 // Checks whether the type for a deref call actually changed the type, not just the mutability of
 181 // the reference.
 182 fn deref_method_same_type(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
 183     match (result_ty.kind(), arg_ty.kind()) {
 184         (ty::Ref(_, result_ty, _), ty::Ref(_, arg_ty, _)) => TyS::same_type(result_ty, arg_ty),
 185
 186         // The result type for a deref method is always a reference
 187         // Not matching the previous pattern means the argument type is not a reference
 188         // This means that the type did change
 189         _ => false,
 190     }
 191 }
 192
 193 // Checks whether the parent node is a suitable context for switching from a deref method to the
 194 // deref operator.
 195 fn is_linted_explicit_deref_position(parent: Option<Node<'_>>, child_id: HirId, child_span: Span) -> bool {
 196     let parent = match parent {
 197         Some(Node::Expr(e)) if e.span.ctxt() == child_span.ctxt() => e,
 198         _ => return true,
 199     };
 200     match parent.kind {
 201         // Leave deref calls in the middle of a method chain.
 202         // e.g. x.deref().foo()
 203         ExprKind::MethodCall(_, _, [self_arg, ..], _) if self_arg.hir_id == child_id => false,
 204
 205         // Leave deref calls resulting in a called function
 206         // e.g. (x.deref())()
 207         ExprKind::Call(func_expr, _) if func_expr.hir_id == child_id => false,
 208
 209         // Makes an ugly suggestion
 210         // e.g. *x.deref() => *&*x
 211         ExprKind::Unary(UnOp::Deref, _)
 212         // Postfix expressions would require parens
 213         | ExprKind::Match(_, _, MatchSource::TryDesugar | MatchSource::AwaitDesugar)
 214         | ExprKind::Field(..)
 215         | ExprKind::Index(..)
 216         | ExprKind::Err => false,
 217
 218         ExprKind::Box(..)
 219         | ExprKind::ConstBlock(..)
 220         | ExprKind::Array(_)
 221         | ExprKind::Call(..)
 222         | ExprKind::MethodCall(..)
 223         | ExprKind::Tup(..)
 224         | ExprKind::Binary(..)
 225         | ExprKind::Unary(..)
 226         | ExprKind::Lit(..)
 227         | ExprKind::Cast(..)
 228         | ExprKind::Type(..)
 229         | ExprKind::DropTemps(..)
 230         | ExprKind::If(..)
 231         | ExprKind::Loop(..)
 232         | ExprKind::Match(..)
 233         | ExprKind::Closure(..)
 234         | ExprKind::Block(..)
 235         | ExprKind::Assign(..)
 236         | ExprKind::AssignOp(..)
 237         | ExprKind::Path(..)
 238         | ExprKind::AddrOf(..)
 239         | ExprKind::Break(..)
 240         | ExprKind::Continue(..)
 241         | ExprKind::Ret(..)
 242         | ExprKind::InlineAsm(..)
 243         | ExprKind::LlvmInlineAsm(..)
 244         | ExprKind::Struct(..)
 245         | ExprKind::Repeat(..)
 246         | ExprKind::Yield(..) => true,
 247     }
 248 }
 249
 250 #[allow(clippy::needless_pass_by_value)]
 251 fn report(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data: StateData) {
 252     match state {
 253         State::DerefMethod {
 254             ty_changed_count,
 255             is_final_ufcs,
 256         } => {
 257             let mut app = Applicability::MachineApplicable;
 258             let (expr_str, expr_is_macro_call) = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app);
 259             let ty = cx.typeck_results().expr_ty(expr);
 260             let (_, ref_count) = peel_mid_ty_refs(ty);
 261             let deref_str = if ty_changed_count >= ref_count && ref_count != 0 {
 262                 // a deref call changing &T -> &U requires two deref operators the first time
 263                 // this occurs. One to remove the reference, a second to call the deref impl.
 264                 "*".repeat(ty_changed_count + 1)
 265             } else {
 266                 "*".repeat(ty_changed_count)
 267             };
 268             let addr_of_str = if ty_changed_count < ref_count {
 269                 // Check if a reborrow from &mut T -> &T is required.
 270                 if data.target_mut == Mutability::Not && matches!(ty.kind(), ty::Ref(_, _, Mutability::Mut)) {
 271                     "&*"
 272                 } else {
 273                     ""
 274                 }
 275             } else if data.target_mut == Mutability::Mut {
 276                 "&mut "
 277             } else {
 278                 "&"
 279             };
 280
 281             let expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence().order() < PREC_PREFIX {
 282                 format!("({})", expr_str)
 283             } else {
 284                 expr_str.into_owned()
 285             };
 286
 287             span_lint_and_sugg(
 288                 cx,
 289                 EXPLICIT_DEREF_METHODS,
 290                 data.span,
 291                 match data.target_mut {
 292                     Mutability::Not => "explicit `deref` method call",
 293                     Mutability::Mut => "explicit `deref_mut` method call",
 294                 },
 295                 "try this",
 296                 format!("{}{}{}", addr_of_str, deref_str, expr_str),
 297                 app,
 298             );
 299         },
 300     }
 301 }