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