clippy_lints/src/mem_replace.rs

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