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::Not, 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                         span_lint_and_then(
 120                             cx,
 121                             NEEDLESS_BORROW,
 122                             e.span,
 123                             &format!(
 124                                 "this expression borrows a reference (`&{}`) that is immediately dereferenced \
 125                              by the compiler",
 126                                 ty
 127                             ),
 128                             |diag| {
 129                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
 130                                     diag.span_suggestion(
 131                                         e.span,
 132                                         "change this to",
 133                                         snippet,
 134                                         Applicability::MachineApplicable,
 135                                     );
 136                                 }
 137                             },
 138                         );
 139                     }
 140                 }
 141             }
 142         }
 143     }
 144
 145     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
 146         if let PatKind::Binding(BindingAnnotation::Ref, id, name, _) = pat.kind {
 147             if let Some(opt_prev_pat) = self.ref_locals.get_mut(&id) {
 148                 // This binding id has been seen before. Add this pattern to the list of changes.
 149                 if let Some(prev_pat) = opt_prev_pat {
 150                     if in_macro(pat.span) {
 151                         // Doesn't match the context of the previous pattern. Can't lint here.
 152                         *opt_prev_pat = None;
 153                     } else {
 154                         prev_pat.spans.push(pat.span);
 155                         prev_pat.replacements.push((
 156                             pat.span,
 157                             snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut prev_pat.app)
 158                                 .0
 159                                 .into(),
 160                         ));
 161                     }
 162                 }
 163                 return;
 164             }
 165
 166             if_chain! {
 167                 if !in_macro(pat.span);
 168                 if let ty::Ref(_, tam, _) = *cx.typeck_results().pat_ty(pat).kind();
 169                 // only lint immutable refs, because borrowed `&mut T` cannot be moved out
 170                 if let ty::Ref(_, _, Mutability::Not) = *tam.kind();
 171                 then {
 172                     let mut app = Applicability::MachineApplicable;
 173                     let snip = snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut app).0;
 174                     self.current_body = self.current_body.or(cx.enclosing_body);
 175                     self.ref_locals.insert(
 176                         id,
 177                         Some(RefPat {
 178                             always_deref: true,
 179                             spans: vec![pat.span],
 180                             app,
 181                             replacements: vec![(pat.span, snip.into())],
 182                         }),
 183                     );
 184                 }
 185             }
 186         }
 187     }
 188
 189     fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
 190         if Some(body.id()) == self.current_body {
 191             for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
 192                 let replacements = pat.replacements;
 193                 let app = pat.app;
 194                 span_lint_and_then(
 195                     cx,
 196                     if pat.always_deref {
 197                         NEEDLESS_BORROW
 198                     } else {
 199                         REF_BINDING_TO_REFERENCE
 200                     },
 201                     pat.spans,
 202                     "this pattern creates a reference to a reference",
 203                     |diag| {
 204                         diag.multipart_suggestion("try this", replacements, app);
 205                     },
 206                 );
 207             }
 208             self.current_body = None;
 209         }
 210     }
 211 }
 212 impl NeedlessBorrow {
 213     fn check_local_usage(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, local: HirId) {
 214         if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
 215             if let Some(pat) = outer_pat {
 216                 // Check for auto-deref
 217                 if !matches!(
 218                     cx.typeck_results().expr_adjustments(e),
 219                     [
 220                         Adjustment {
 221                             kind: Adjust::Deref(_),
 222                             ..
 223                         },
 224                         Adjustment {
 225                             kind: Adjust::Deref(_),
 226                             ..
 227                         },
 228                         ..
 229                     ]
 230                 ) {
 231                     match get_parent_expr(cx, e) {
 232                         // Field accesses are the same no matter the number of references.
 233                         Some(Expr {
 234                             kind: ExprKind::Field(..),
 235                             ..
 236                         }) => (),
 237                         Some(&Expr {
 238                             span,
 239                             kind: ExprKind::Unary(UnOp::Deref, _),
 240                             ..
 241                         }) if !in_macro(span) => {
 242                             // Remove explicit deref.
 243                             let snip = snippet_with_context(cx, e.span, span.ctxt(), "..", &mut pat.app).0;
 244                             pat.replacements.push((span, snip.into()));
 245                         },
 246                         Some(parent) if !in_macro(parent.span) => {
 247                             // Double reference might be needed at this point.
 248                             if parent.precedence().order() == PREC_POSTFIX {
 249                                 // Parentheses would be needed here, don't lint.
 250                                 *outer_pat = None;
 251                             } else {
 252                                 pat.always_deref = false;
 253                                 let snip = snippet_with_context(cx, e.span, parent.span.ctxt(), "..", &mut pat.app).0;
 254                                 pat.replacements.push((e.span, format!("&{}", snip)));
 255                             }
 256                         },
 257                         _ if !in_macro(e.span) => {
 258                             // Double reference might be needed at this point.
 259                             pat.always_deref = false;
 260                             let snip = snippet_with_applicability(cx, e.span, "..", &mut pat.app);
 261                             pat.replacements.push((e.span, format!("&{}", snip)));
 262                         },
 263                         // Edge case for macros. The span of the identifier will usually match the context of the
 264                         // binding, but not if the identifier was created in a macro. e.g. `concat_idents` and proc
 265                         // macros
 266                         _ => *outer_pat = None,
 267                     }
 268                 }
 269             }
 270         }
 271     }
 272 }