src/tools/clippy/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<'_>, dest: &Expr<'_>, expr_span: Span) {
 139     if_chain! {
 140         // check if replacement is mem::MaybeUninit::uninit().assume_init()
 141         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
 142         if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
 143         then {
 144             let mut applicability = Applicability::MachineApplicable;
 145             span_lint_and_sugg(
 146                 cx,
 147                 MEM_REPLACE_WITH_UNINIT,
 148                 expr_span,
 149                 "replacing with `mem::MaybeUninit::uninit().assume_init()`",
 150                 "consider using",
 151                 format!(
 152                     "std::ptr::read({})",
 153                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
 154                 ),
 155                 applicability,
 156             );
 157             return;
 158         }
 159     }
 160
 161     if_chain! {
 162         if let ExprKind::Call(ref repl_func, ref repl_args) = src.kind;
 163         if repl_args.is_empty();
 164         if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 165         if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 166         then {
 167             if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
 168                 let mut applicability = Applicability::MachineApplicable;
 169                 span_lint_and_sugg(
 170                     cx,
 171                     MEM_REPLACE_WITH_UNINIT,
 172                     expr_span,
 173                     "replacing with `mem::uninitialized()`",
 174                     "consider using",
 175                     format!(
 176                         "std::ptr::read({})",
 177                         snippet_with_applicability(cx, dest.span, "", &mut applicability)
 178                     ),
 179                     applicability,
 180                 );
 181             } else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
 182                     !cx.typeck_results().expr_ty(src).is_primitive() {
 183                 span_lint_and_help(
 184                     cx,
 185                     MEM_REPLACE_WITH_UNINIT,
 186                     expr_span,
 187                     "replacing with `mem::zeroed()`",
 188                     None,
 189                     "consider using a default value or the `take_mut` crate instead",
 190                 );
 191             }
 192         }
 193     }
 194 }
 195
 196 fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 197     if let ExprKind::Call(ref repl_func, _) = src.kind {
 198         if_chain! {
 199             if !in_external_macro(cx.tcx.sess, expr_span);
 200             if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 201             if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 202             if match_def_path(cx, repl_def_id, &paths::DEFAULT_TRAIT_METHOD);
 203             then {
 204                 span_lint_and_then(
 205                     cx,
 206                     MEM_REPLACE_WITH_DEFAULT,
 207                     expr_span,
 208                     "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 209                     |diag| {
 210                         if !in_macro(expr_span) {
 211                             let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 212
 213                             diag.span_suggestion(
 214                                 expr_span,
 215                                 "consider using",
 216                                 suggestion,
 217                                 Applicability::MachineApplicable
 218                             );
 219                         }
 220                     }
 221                 );
 222             }
 223         }
 224     }
 225 }
 226
 227 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 228     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 229         if_chain! {
 230             // Check that `expr` is a call to `mem::replace()`
 231             if let ExprKind::Call(ref func, ref func_args) = expr.kind;
 232             if let ExprKind::Path(ref func_qpath) = func.kind;
 233             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 234             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 235             if let [dest, src] = &**func_args;
 236             then {
 237                 check_replace_option_with_none(cx, src, dest, expr.span);
 238                 check_replace_with_uninit(cx, src, dest, expr.span);
 239                 check_replace_with_default(cx, src, dest, expr.span);
 240             }
 241         }
 242     }
 243 }