src/tools/clippy/clippy_lints/src/mem_replace.rs

   1 use crate::utils::{
   2     in_macro, match_def_path, match_qpath, meets_msrv, 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, LintContext};
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_semver::RustcVersion;
  11 use rustc_session::{declare_tool_lint, impl_lint_pass};
  12 use rustc_span::source_map::Span;
  13 use rustc_span::symbol::sym;
  14
  15 declare_clippy_lint! {
  16     /// **What it does:** Checks for `mem::replace()` on an `Option` with
  17     /// `None`.
  18     ///
  19     /// **Why is this bad?** `Option` already has the method `take()` for
  20     /// taking its current value (Some(..) or None) and replacing it with
  21     /// `None`.
  22     ///
  23     /// **Known problems:** None.
  24     ///
  25     /// **Example:**
  26     /// ```rust
  27     /// use std::mem;
  28     ///
  29     /// let mut an_option = Some(0);
  30     /// let replaced = mem::replace(&mut an_option, None);
  31     /// ```
  32     /// Is better expressed with:
  33     /// ```rust
  34     /// let mut an_option = Some(0);
  35     /// let taken = an_option.take();
  36     /// ```
  37     pub MEM_REPLACE_OPTION_WITH_NONE,
  38     style,
  39     "replacing an `Option` with `None` instead of `take()`"
  40 }
  41
  42 declare_clippy_lint! {
  43     /// **What it does:** Checks for `mem::replace(&mut _, mem::uninitialized())`
  44     /// and `mem::replace(&mut _, mem::zeroed())`.
  45     ///
  46     /// **Why is this bad?** This will lead to undefined behavior even if the
  47     /// value is overwritten later, because the uninitialized value may be
  48     /// observed in the case of a panic.
  49     ///
  50     /// **Known problems:** None.
  51     ///
  52     /// **Example:**
  53     ///
  54     /// ```
  55     /// use std::mem;
  56     ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
  57     ///
  58     /// #[allow(deprecated, invalid_value)]
  59     /// fn myfunc (v: &mut Vec<i32>) {
  60     ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
  61     ///     let new_v = may_panic(taken_v); // undefined behavior on panic
  62     ///     mem::forget(mem::replace(v, new_v));
  63     /// }
  64     /// ```
  65     ///
  66     /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
  67     /// at the cost of either lazily creating a replacement value or aborting
  68     /// on panic, to ensure that the uninitialized value cannot be observed.
  69     pub MEM_REPLACE_WITH_UNINIT,
  70     correctness,
  71     "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
  72 }
  73
  74 declare_clippy_lint! {
  75     /// **What it does:** Checks for `std::mem::replace` on a value of type
  76     /// `T` with `T::default()`.
  77     ///
  78     /// **Why is this bad?** `std::mem` module already has the method `take` to
  79     /// take the current value and replace it with the default value of that type.
  80     ///
  81     /// **Known problems:** None.
  82     ///
  83     /// **Example:**
  84     /// ```rust
  85     /// let mut text = String::from("foo");
  86     /// let replaced = std::mem::replace(&mut text, String::default());
  87     /// ```
  88     /// Is better expressed with:
  89     /// ```rust
  90     /// let mut text = String::from("foo");
  91     /// let taken = std::mem::take(&mut text);
  92     /// ```
  93     pub MEM_REPLACE_WITH_DEFAULT,
  94     style,
  95     "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
  96 }
  97
  98 impl_lint_pass!(MemReplace =>
  99     [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);
 100
 101 fn check_replace_option_with_none(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 102     if let ExprKind::Path(ref replacement_qpath) = src.kind {
 103         // Check that second argument is `Option::None`
 104         if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
 105             // Since this is a late pass (already type-checked),
 106             // and we already know that the second argument is an
 107             // `Option`, we do not need to check the first
 108             // argument's type. All that's left is to get
 109             // replacee's path.
 110             let replaced_path = match dest.kind {
 111                 ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, ref replaced) => {
 112                     if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
 113                         replaced_path
 114                     } else {
 115                         return;
 116                     }
 117                 },
 118                 ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
 119                 _ => return,
 120             };
 121
 122             let mut applicability = Applicability::MachineApplicable;
 123             span_lint_and_sugg(
 124                 cx,
 125                 MEM_REPLACE_OPTION_WITH_NONE,
 126                 expr_span,
 127                 "replacing an `Option` with `None`",
 128                 "consider `Option::take()` instead",
 129                 format!(
 130                     "{}.take()",
 131                     snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
 132                 ),
 133                 applicability,
 134             );
 135         }
 136     }
 137 }
 138
 139 fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 140     if_chain! {
 141         // check if replacement is mem::MaybeUninit::uninit().assume_init()
 142         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
 143         if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
 144         then {
 145             let mut applicability = Applicability::MachineApplicable;
 146             span_lint_and_sugg(
 147                 cx,
 148                 MEM_REPLACE_WITH_UNINIT,
 149                 expr_span,
 150                 "replacing with `mem::MaybeUninit::uninit().assume_init()`",
 151                 "consider using",
 152                 format!(
 153                     "std::ptr::read({})",
 154                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
 155                 ),
 156                 applicability,
 157             );
 158             return;
 159         }
 160     }
 161
 162     if_chain! {
 163         if let ExprKind::Call(ref repl_func, ref repl_args) = src.kind;
 164         if repl_args.is_empty();
 165         if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 166         if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 167         then {
 168             if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
 169                 let mut applicability = Applicability::MachineApplicable;
 170                 span_lint_and_sugg(
 171                     cx,
 172                     MEM_REPLACE_WITH_UNINIT,
 173                     expr_span,
 174                     "replacing with `mem::uninitialized()`",
 175                     "consider using",
 176                     format!(
 177                         "std::ptr::read({})",
 178                         snippet_with_applicability(cx, dest.span, "", &mut applicability)
 179                     ),
 180                     applicability,
 181                 );
 182             } else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
 183                     !cx.typeck_results().expr_ty(src).is_primitive() {
 184                 span_lint_and_help(
 185                     cx,
 186                     MEM_REPLACE_WITH_UNINIT,
 187                     expr_span,
 188                     "replacing with `mem::zeroed()`",
 189                     None,
 190                     "consider using a default value or the `take_mut` crate instead",
 191                 );
 192             }
 193         }
 194     }
 195 }
 196
 197 fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 198     if let ExprKind::Call(ref repl_func, _) = src.kind {
 199         if_chain! {
 200             if !in_external_macro(cx.tcx.sess, expr_span);
 201             if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 202             if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 203             if match_def_path(cx, repl_def_id, &paths::DEFAULT_TRAIT_METHOD);
 204             then {
 205                 span_lint_and_then(
 206                     cx,
 207                     MEM_REPLACE_WITH_DEFAULT,
 208                     expr_span,
 209                     "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 210                     |diag| {
 211                         if !in_macro(expr_span) {
 212                             let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 213
 214                             diag.span_suggestion(
 215                                 expr_span,
 216                                 "consider using",
 217                                 suggestion,
 218                                 Applicability::MachineApplicable
 219                             );
 220                         }
 221                     }
 222                 );
 223             }
 224         }
 225     }
 226 }
 227
 228 const MEM_REPLACE_WITH_DEFAULT_MSRV: RustcVersion = RustcVersion::new(1, 40, 0);
 229
 230 pub struct MemReplace {
 231     msrv: Option<RustcVersion>,
 232 }
 233
 234 impl MemReplace {
 235     #[must_use]
 236     pub fn new(msrv: Option<RustcVersion>) -> Self {
 237         Self { msrv }
 238     }
 239 }
 240
 241 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 242     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 243         if_chain! {
 244             // Check that `expr` is a call to `mem::replace()`
 245             if let ExprKind::Call(ref func, ref func_args) = expr.kind;
 246             if let ExprKind::Path(ref func_qpath) = func.kind;
 247             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 248             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 249             if let [dest, src] = &**func_args;
 250             then {
 251                 check_replace_option_with_none(cx, src, dest, expr.span);
 252                 check_replace_with_uninit(cx, src, dest, expr.span);
 253                 if meets_msrv(self.msrv.as_ref(), &MEM_REPLACE_WITH_DEFAULT_MSRV) {
 254                     check_replace_with_default(cx, src, dest, expr.span);
 255                 }
 256             }
 257         }
 258     }
 259     extract_msrv_attr!(LateContext);
 260 }