clippy_lints/src/mem_replace.rs

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