clippy_lints/src/mem_replace.rs

   1 use crate::utils::{
   2     in_macro, match_def_path, match_qpath, paths, snippet, snippet_with_applicability, span_lint_and_help,
   3     span_lint_and_sugg, span_lint_and_then,
   4 };
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Span;
  12 use rustc_span::symbol::sym;
  13
  14 declare_clippy_lint! {
  15     /// **What it does:** Checks for `mem::replace()` on an `Option` with
  16     /// `None`.
  17     ///
  18     /// **Why is this bad?** `Option` already has the method `take()` for
  19     /// taking its current value (Some(..) or None) and replacing it with
  20     /// `None`.
  21     ///
  22     /// **Known problems:** None.
  23     ///
  24     /// **Example:**
  25     /// ```rust
  26     /// use std::mem;
  27     ///
  28     /// let mut an_option = Some(0);
  29     /// let replaced = mem::replace(&mut an_option, None);
  30     /// ```
  31     /// Is better expressed with:
  32     /// ```rust
  33     /// let mut an_option = Some(0);
  34     /// let taken = an_option.take();
  35     /// ```
  36     pub MEM_REPLACE_OPTION_WITH_NONE,
  37     style,
  38     "replacing an `Option` with `None` instead of `take()`"
  39 }
  40
  41 declare_clippy_lint! {
  42     /// **What it does:** Checks for `mem::replace(&mut _, mem::uninitialized())`
  43     /// and `mem::replace(&mut _, mem::zeroed())`.
  44     ///
  45     /// **Why is this bad?** This will lead to undefined behavior even if the
  46     /// value is overwritten later, because the uninitialized value may be
  47     /// observed in the case of a panic.
  48     ///
  49     /// **Known problems:** None.
  50     ///
  51     /// **Example:**
  52     ///
  53     /// ```
  54     /// use std::mem;
  55     ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
  56     ///
  57     /// #[allow(deprecated, invalid_value)]
  58     /// fn myfunc (v: &mut Vec<i32>) {
  59     ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
  60     ///     let new_v = may_panic(taken_v); // undefined behavior on panic
  61     ///     mem::forget(mem::replace(v, new_v));
  62     /// }
  63     /// ```
  64     ///
  65     /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
  66     /// at the cost of either lazily creating a replacement value or aborting
  67     /// on panic, to ensure that the uninitialized value cannot be observed.
  68     pub MEM_REPLACE_WITH_UNINIT,
  69     correctness,
  70     "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
  71 }
  72
  73 declare_clippy_lint! {
  74     /// **What it does:** Checks for `std::mem::replace` on a value of type
  75     /// `T` with `T::default()`.
  76     ///
  77     /// **Why is this bad?** `std::mem` module already has the method `take` to
  78     /// take the current value and replace it with the default value of that type.
  79     ///
  80     /// **Known problems:** None.
  81     ///
  82     /// **Example:**
  83     /// ```rust
  84     /// let mut text = String::from("foo");
  85     /// let replaced = std::mem::replace(&mut text, String::default());
  86     /// ```
  87     /// Is better expressed with:
  88     /// ```rust
  89     /// let mut text = String::from("foo");
  90     /// let taken = std::mem::take(&mut text);
  91     /// ```
  92     pub MEM_REPLACE_WITH_DEFAULT,
  93     style,
  94     "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
  95 }
  96
  97 declare_lint_pass!(MemReplace =>
  98     [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);
  99
 100 fn check_replace_option_with_none(cx: &LateContext<'_, '_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 101     if let ExprKind::Path(ref replacement_qpath) = src.kind {
 102         // Check that second argument is `Option::None`
 103         if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
 104             // Since this is a late pass (already type-checked),
 105             // and we already know that the second argument is an
 106             // `Option`, we do not need to check the first
 107             // argument's type. All that's left is to get
 108             // replacee's path.
 109             let replaced_path = match dest.kind {
 110                 ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, ref replaced) => {
 111                     if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
 112                         replaced_path
 113                     } else {
 114                         return;
 115                     }
 116                 },
 117                 ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
 118                 _ => return,
 119             };
 120
 121             let mut applicability = Applicability::MachineApplicable;
 122             span_lint_and_sugg(
 123                 cx,
 124                 MEM_REPLACE_OPTION_WITH_NONE,
 125                 expr_span,
 126                 "replacing an `Option` with `None`",
 127                 "consider `Option::take()` instead",
 128                 format!(
 129                     "{}.take()",
 130                     snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
 131                 ),
 132                 applicability,
 133             );
 134         }
 135     }
 136 }
 137
 138 fn check_replace_with_uninit(cx: &LateContext<'_, '_>, src: &Expr<'_>, expr_span: Span) {
 139     if let ExprKind::Call(ref repl_func, ref repl_args) = src.kind {
 140         if_chain! {
 141             if repl_args.is_empty();
 142             if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 143             if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 144             then {
 145                 if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
 146                     span_lint_and_help(
 147                         cx,
 148                         MEM_REPLACE_WITH_UNINIT,
 149                         expr_span,
 150                         "replacing with `mem::uninitialized()`",
 151                         "consider using the `take_mut` crate instead",
 152                     );
 153                 } else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
 154                         !cx.tables.expr_ty(src).is_primitive() {
 155                     span_lint_and_help(
 156                         cx,
 157                         MEM_REPLACE_WITH_UNINIT,
 158                         expr_span,
 159                         "replacing with `mem::zeroed()`",
 160                         "consider using a default value or the `take_mut` crate instead",
 161                     );
 162                 }
 163             }
 164         }
 165     }
 166 }
 167
 168 fn check_replace_with_default(cx: &LateContext<'_, '_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 169     if let ExprKind::Call(ref repl_func, _) = src.kind {
 170         if_chain! {
 171             if !in_external_macro(cx.tcx.sess, expr_span);
 172             if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 173             if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 174             if match_def_path(cx, repl_def_id, &paths::DEFAULT_TRAIT_METHOD);
 175             then {
 176                 span_lint_and_then(
 177                     cx,
 178                     MEM_REPLACE_WITH_DEFAULT,
 179                     expr_span,
 180                     "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 181                     |diag| {
 182                         if !in_macro(expr_span) {
 183                             let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 184
 185                             diag.span_suggestion(
 186                                 expr_span,
 187                                 "consider using",
 188                                 suggestion,
 189                                 Applicability::MachineApplicable
 190                             );
 191                         }
 192                     }
 193                 );
 194             }
 195         }
 196     }
 197 }
 198
 199 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
 200     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
 201         if_chain! {
 202             // Check that `expr` is a call to `mem::replace()`
 203             if let ExprKind::Call(ref func, ref func_args) = expr.kind;
 204             if let ExprKind::Path(ref func_qpath) = func.kind;
 205             if let Some(def_id) = cx.tables.qpath_res(func_qpath, func.hir_id).opt_def_id();
 206             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 207             if let [dest, src] = &**func_args;
 208             then {
 209                 check_replace_option_with_none(cx, src, dest, expr.span);
 210                 check_replace_with_uninit(cx, src, expr.span);
 211                 check_replace_with_default(cx, src, dest, expr.span);
 212             }
 213         }
 214     }
 215 }