src/tools/clippy/clippy_lints/src/needless_borrow.rs

   1 //! Checks for needless address of operations (`&`)
   2 //!
   3 //! This lint is **warn** by default
   4
   5 use clippy_utils::diagnostics::span_lint_and_then;
   6 use clippy_utils::source::{snippet_opt, snippet_with_applicability, snippet_with_context};
   7 use clippy_utils::{get_parent_expr, in_macro, path_to_local};
   8 use if_chain::if_chain;
   9 use rustc_ast::util::parser::PREC_POSTFIX;
  10 use rustc_data_structures::fx::FxIndexMap;
  11 use rustc_errors::Applicability;
  12 use rustc_hir::{BindingAnnotation, Body, BodyId, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, UnOp};
  13 use rustc_lint::{LateContext, LateLintPass};
  14 use rustc_middle::ty;
  15 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
  16 use rustc_session::{declare_tool_lint, impl_lint_pass};
  17 use rustc_span::Span;
  18
  19 declare_clippy_lint! {
  20     /// ### What it does
  21     /// Checks for address of operations (`&`) that are going to
  22     /// be dereferenced immediately by the compiler.
  23     ///
  24     /// ### Why is this bad?
  25     /// Suggests that the receiver of the expression borrows
  26     /// the expression.
  27     ///
  28     /// ### Example
  29     /// ```rust
  30     /// fn fun(_a: &i32) {}
  31     ///
  32     /// // Bad
  33     /// let x: &i32 = &&&&&&5;
  34     /// fun(&x);
  35     ///
  36     /// // Good
  37     /// let x: &i32 = &5;
  38     /// fun(x);
  39     /// ```
  40     pub NEEDLESS_BORROW,
  41     style,
  42     "taking a reference that is going to be automatically dereferenced"
  43 }
  44
  45 declare_clippy_lint! {
  46     /// ### What it does
  47     /// Checks for `ref` bindings which create a reference to a reference.
  48     ///
  49     /// ### Why is this bad?
  50     /// The address-of operator at the use site is clearer about the need for a reference.
  51     ///
  52     /// ### Example
  53     /// ```rust
  54     /// // Bad
  55     /// let x = Some("");
  56     /// if let Some(ref x) = x {
  57     ///     // use `x` here
  58     /// }
  59     ///
  60     /// // Good
  61     /// let x = Some("");
  62     /// if let Some(x) = x {
  63     ///     // use `&x` here
  64     /// }
  65     /// ```
  66     pub REF_BINDING_TO_REFERENCE,
  67     pedantic,
  68     "`ref` binding to a reference"
  69 }
  70
  71 impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW, REF_BINDING_TO_REFERENCE]);
  72 #[derive(Default)]
  73 pub struct NeedlessBorrow {
  74     /// The body the first local was found in. Used to emit lints when the traversal of the body has
  75     /// been finished. Note we can't lint at the end of every body as they can be nested within each
  76     /// other.
  77     current_body: Option<BodyId>,
  78     /// The list of locals currently being checked by the lint.
  79     /// If the value is `None`, then the binding has been seen as a ref pattern, but is not linted.
  80     /// This is needed for or patterns where one of the branches can be linted, but another can not
  81     /// be.
  82     ///
  83     /// e.g. `m!(x) | Foo::Bar(ref x)`
  84     ref_locals: FxIndexMap<HirId, Option<RefPat>>,
  85 }
  86
  87 struct RefPat {
  88     /// Whether every usage of the binding is dereferenced.
  89     always_deref: bool,
  90     /// The spans of all the ref bindings for this local.
  91     spans: Vec<Span>,
  92     /// The applicability of this suggestion.
  93     app: Applicability,
  94     /// All the replacements which need to be made.
  95     replacements: Vec<(Span, String)>,
  96 }
  97
  98 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
  99     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 100         if let Some(local) = path_to_local(e) {
 101             self.check_local_usage(cx, e, local);
 102         }
 103
 104         if e.span.from_expansion() {
 105             return;
 106         }
 107         if let ExprKind::AddrOf(BorrowKind::Ref, mutability, inner) = e.kind {
 108             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(inner).kind() {
 109                 for adj3 in cx.typeck_results().expr_adjustments(e).windows(3) {
 110                     if let [Adjustment {
 111                         kind: Adjust::Deref(_), ..
 112                     }, Adjustment {
 113                         kind: Adjust::Deref(_), ..
 114                     }, Adjustment {
 115                         kind: Adjust::Borrow(_),
 116                         ..
 117                     }] = *adj3
 118                     {
 119                         let help_msg_ty = if matches!(mutability, Mutability::Not) {
 120                             format!("&{}", ty)
 121                         } else {
 122                             format!("&mut {}", ty)
 123                         };
 124
 125                         span_lint_and_then(
 126                             cx,
 127                             NEEDLESS_BORROW,
 128                             e.span,
 129                             &format!(
 130                                 "this expression borrows a reference (`{}`) that is immediately dereferenced \
 131                              by the compiler",
 132                                 help_msg_ty
 133                             ),
 134                             |diag| {
 135                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
 136                                     diag.span_suggestion(
 137                                         e.span,
 138                                         "change this to",
 139                                         snippet,
 140                                         Applicability::MachineApplicable,
 141                                     );
 142                                 }
 143                             },
 144                         );
 145                     }
 146                 }
 147             }
 148         }
 149     }
 150
 151     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
 152         if let PatKind::Binding(BindingAnnotation::Ref, id, name, _) = pat.kind {
 153             if let Some(opt_prev_pat) = self.ref_locals.get_mut(&id) {
 154                 // This binding id has been seen before. Add this pattern to the list of changes.
 155                 if let Some(prev_pat) = opt_prev_pat {
 156                     if in_macro(pat.span) {
 157                         // Doesn't match the context of the previous pattern. Can't lint here.
 158                         *opt_prev_pat = None;
 159                     } else {
 160                         prev_pat.spans.push(pat.span);
 161                         prev_pat.replacements.push((
 162                             pat.span,
 163                             snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut prev_pat.app)
 164                                 .0
 165                                 .into(),
 166                         ));
 167                     }
 168                 }
 169                 return;
 170             }
 171
 172             if_chain! {
 173                 if !in_macro(pat.span);
 174                 if let ty::Ref(_, tam, _) = *cx.typeck_results().pat_ty(pat).kind();
 175                 // only lint immutable refs, because borrowed `&mut T` cannot be moved out
 176                 if let ty::Ref(_, _, Mutability::Not) = *tam.kind();
 177                 then {
 178                     let mut app = Applicability::MachineApplicable;
 179                     let snip = snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut app).0;
 180                     self.current_body = self.current_body.or(cx.enclosing_body);
 181                     self.ref_locals.insert(
 182                         id,
 183                         Some(RefPat {
 184                             always_deref: true,
 185                             spans: vec![pat.span],
 186                             app,
 187                             replacements: vec![(pat.span, snip.into())],
 188                         }),
 189                     );
 190                 }
 191             }
 192         }
 193     }
 194
 195     fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
 196         if Some(body.id()) == self.current_body {
 197             for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
 198                 let replacements = pat.replacements;
 199                 let app = pat.app;
 200                 span_lint_and_then(
 201                     cx,
 202                     if pat.always_deref {
 203                         NEEDLESS_BORROW
 204                     } else {
 205                         REF_BINDING_TO_REFERENCE
 206                     },
 207                     pat.spans,
 208                     "this pattern creates a reference to a reference",
 209                     |diag| {
 210                         diag.multipart_suggestion("try this", replacements, app);
 211                     },
 212                 );
 213             }
 214             self.current_body = None;
 215         }
 216     }
 217 }
 218 impl NeedlessBorrow {
 219     fn check_local_usage(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, local: HirId) {
 220         if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
 221             if let Some(pat) = outer_pat {
 222                 // Check for auto-deref
 223                 if !matches!(
 224                     cx.typeck_results().expr_adjustments(e),
 225                     [
 226                         Adjustment {
 227                             kind: Adjust::Deref(_),
 228                             ..
 229                         },
 230                         Adjustment {
 231                             kind: Adjust::Deref(_),
 232                             ..
 233                         },
 234                         ..
 235                     ]
 236                 ) {
 237                     match get_parent_expr(cx, e) {
 238                         // Field accesses are the same no matter the number of references.
 239                         Some(Expr {
 240                             kind: ExprKind::Field(..),
 241                             ..
 242                         }) => (),
 243                         Some(&Expr {
 244                             span,
 245                             kind: ExprKind::Unary(UnOp::Deref, _),
 246                             ..
 247                         }) if !in_macro(span) => {
 248                             // Remove explicit deref.
 249                             let snip = snippet_with_context(cx, e.span, span.ctxt(), "..", &mut pat.app).0;
 250                             pat.replacements.push((span, snip.into()));
 251                         },
 252                         Some(parent) if !in_macro(parent.span) => {
 253                             // Double reference might be needed at this point.
 254                             if parent.precedence().order() == PREC_POSTFIX {
 255                                 // Parentheses would be needed here, don't lint.
 256                                 *outer_pat = None;
 257                             } else {
 258                                 pat.always_deref = false;
 259                                 let snip = snippet_with_context(cx, e.span, parent.span.ctxt(), "..", &mut pat.app).0;
 260                                 pat.replacements.push((e.span, format!("&{}", snip)));
 261                             }
 262                         },
 263                         _ if !in_macro(e.span) => {
 264                             // Double reference might be needed at this point.
 265                             pat.always_deref = false;
 266                             let snip = snippet_with_applicability(cx, e.span, "..", &mut pat.app);
 267                             pat.replacements.push((e.span, format!("&{}", snip)));
 268                         },
 269                         // Edge case for macros. The span of the identifier will usually match the context of the
 270                         // binding, but not if the identifier was created in a macro. e.g. `concat_idents` and proc
 271                         // macros
 272                         _ => *outer_pat = None,
 273                     }
 274                 }
 275             }
 276         }
 277     }
 278 }