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

   1 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg, span_lint_and_then};
   2 use clippy_utils::source::{snippet, snippet_with_applicability};
   3 use clippy_utils::ty::is_non_aggregate_primitive_type;
   4 use clippy_utils::{is_default_equivalent, is_lang_ctor, meets_msrv, msrvs};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::LangItem::OptionNone;
   8 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_semver::RustcVersion;
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::source_map::Span;
  14 use rustc_span::symbol::sym;
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// Checks for `mem::replace()` on an `Option` with
  19     /// `None`.
  20     ///
  21     /// ### Why is this bad?
  22     /// `Option` already has the method `take()` for
  23     /// taking its current value (Some(..) or None) and replacing it with
  24     /// `None`.
  25     ///
  26     /// ### Example
  27     /// ```rust
  28     /// use std::mem;
  29     ///
  30     /// let mut an_option = Some(0);
  31     /// let replaced = mem::replace(&mut an_option, None);
  32     /// ```
  33     /// Is better expressed with:
  34     /// ```rust
  35     /// let mut an_option = Some(0);
  36     /// let taken = an_option.take();
  37     /// ```
  38     #[clippy::version = "1.31.0"]
  39     pub MEM_REPLACE_OPTION_WITH_NONE,
  40     style,
  41     "replacing an `Option` with `None` instead of `take()`"
  42 }
  43
  44 declare_clippy_lint! {
  45     /// ### What it does
  46     /// Checks for `mem::replace(&mut _, mem::uninitialized())`
  47     /// and `mem::replace(&mut _, mem::zeroed())`.
  48     ///
  49     /// ### Why is this bad?
  50     /// This will lead to undefined behavior even if the
  51     /// value is overwritten later, because the uninitialized value may be
  52     /// observed in the case of a panic.
  53     ///
  54     /// ### Example
  55     /// ```
  56     /// use std::mem;
  57     ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
  58     ///
  59     /// #[allow(deprecated, invalid_value)]
  60     /// fn myfunc (v: &mut Vec<i32>) {
  61     ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
  62     ///     let new_v = may_panic(taken_v); // undefined behavior on panic
  63     ///     mem::forget(mem::replace(v, new_v));
  64     /// }
  65     /// ```
  66     ///
  67     /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
  68     /// at the cost of either lazily creating a replacement value or aborting
  69     /// on panic, to ensure that the uninitialized value cannot be observed.
  70     #[clippy::version = "1.39.0"]
  71     pub MEM_REPLACE_WITH_UNINIT,
  72     correctness,
  73     "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
  74 }
  75
  76 declare_clippy_lint! {
  77     /// ### What it does
  78     /// Checks for `std::mem::replace` on a value of type
  79     /// `T` with `T::default()`.
  80     ///
  81     /// ### Why is this bad?
  82     /// `std::mem` module already has the method `take` to
  83     /// take the current value and replace it with the default value of that type.
  84     ///
  85     /// ### Example
  86     /// ```rust
  87     /// let mut text = String::from("foo");
  88     /// let replaced = std::mem::replace(&mut text, String::default());
  89     /// ```
  90     /// Is better expressed with:
  91     /// ```rust
  92     /// let mut text = String::from("foo");
  93     /// let taken = std::mem::take(&mut text);
  94     /// ```
  95     #[clippy::version = "1.42.0"]
  96     pub MEM_REPLACE_WITH_DEFAULT,
  97     style,
  98     "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
  99 }
 100
 101 impl_lint_pass!(MemReplace =>
 102     [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);
 103
 104 fn check_replace_option_with_none(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 105     if let ExprKind::Path(ref replacement_qpath) = src.kind {
 106         // Check that second argument is `Option::None`
 107         if is_lang_ctor(cx, replacement_qpath, OptionNone) {
 108             // Since this is a late pass (already type-checked),
 109             // and we already know that the second argument is an
 110             // `Option`, we do not need to check the first
 111             // argument's type. All that's left is to get
 112             // replacee's path.
 113             let replaced_path = match dest.kind {
 114                 ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, replaced) => {
 115                     if let ExprKind::Path(QPath::Resolved(None, replaced_path)) = replaced.kind {
 116                         replaced_path
 117                     } else {
 118                         return;
 119                     }
 120                 },
 121                 ExprKind::Path(QPath::Resolved(None, replaced_path)) => replaced_path,
 122                 _ => return,
 123             };
 124
 125             let mut applicability = Applicability::MachineApplicable;
 126             span_lint_and_sugg(
 127                 cx,
 128                 MEM_REPLACE_OPTION_WITH_NONE,
 129                 expr_span,
 130                 "replacing an `Option` with `None`",
 131                 "consider `Option::take()` instead",
 132                 format!(
 133                     "{}.take()",
 134                     snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
 135                 ),
 136                 applicability,
 137             );
 138         }
 139     }
 140 }
 141
 142 fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 143     if_chain! {
 144         // check if replacement is mem::MaybeUninit::uninit().assume_init()
 145         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
 146         if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
 147         then {
 148             let mut applicability = Applicability::MachineApplicable;
 149             span_lint_and_sugg(
 150                 cx,
 151                 MEM_REPLACE_WITH_UNINIT,
 152                 expr_span,
 153                 "replacing with `mem::MaybeUninit::uninit().assume_init()`",
 154                 "consider using",
 155                 format!(
 156                     "std::ptr::read({})",
 157                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
 158                 ),
 159                 applicability,
 160             );
 161             return;
 162         }
 163     }
 164
 165     if_chain! {
 166         if let ExprKind::Call(repl_func, repl_args) = src.kind;
 167         if repl_args.is_empty();
 168         if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 169         if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 170         then {
 171             if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
 172                 let mut applicability = Applicability::MachineApplicable;
 173                 span_lint_and_sugg(
 174                     cx,
 175                     MEM_REPLACE_WITH_UNINIT,
 176                     expr_span,
 177                     "replacing with `mem::uninitialized()`",
 178                     "consider using",
 179                     format!(
 180                         "std::ptr::read({})",
 181                         snippet_with_applicability(cx, dest.span, "", &mut applicability)
 182                     ),
 183                     applicability,
 184                 );
 185             } else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
 186                     !cx.typeck_results().expr_ty(src).is_primitive() {
 187                 span_lint_and_help(
 188                     cx,
 189                     MEM_REPLACE_WITH_UNINIT,
 190                     expr_span,
 191                     "replacing with `mem::zeroed()`",
 192                     None,
 193                     "consider using a default value or the `take_mut` crate instead",
 194                 );
 195             }
 196         }
 197     }
 198 }
 199
 200 fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 201     // disable lint for primitives
 202     let expr_type = cx.typeck_results().expr_ty_adjusted(src);
 203     if is_non_aggregate_primitive_type(expr_type) {
 204         return;
 205     }
 206     // disable lint for Option since it is covered in another lint
 207     if let ExprKind::Path(q) = &src.kind {
 208         if is_lang_ctor(cx, q, OptionNone) {
 209             return;
 210         }
 211     }
 212     if is_default_equivalent(cx, src) && !in_external_macro(cx.tcx.sess, expr_span) {
 213         span_lint_and_then(
 214             cx,
 215             MEM_REPLACE_WITH_DEFAULT,
 216             expr_span,
 217             "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 218             |diag| {
 219                 if !expr_span.from_expansion() {
 220                     let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 221
 222                     diag.span_suggestion(
 223                         expr_span,
 224                         "consider using",
 225                         suggestion,
 226                         Applicability::MachineApplicable,
 227                     );
 228                 }
 229             },
 230         );
 231     }
 232 }
 233
 234 pub struct MemReplace {
 235     msrv: Option<RustcVersion>,
 236 }
 237
 238 impl MemReplace {
 239     #[must_use]
 240     pub fn new(msrv: Option<RustcVersion>) -> Self {
 241         Self { msrv }
 242     }
 243 }
 244
 245 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 246     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 247         if_chain! {
 248             // Check that `expr` is a call to `mem::replace()`
 249             if let ExprKind::Call(func, func_args) = expr.kind;
 250             if let ExprKind::Path(ref func_qpath) = func.kind;
 251             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 252             if cx.tcx.is_diagnostic_item(sym::mem_replace, def_id);
 253             if let [dest, src] = func_args;
 254             then {
 255                 check_replace_option_with_none(cx, src, dest, expr.span);
 256                 check_replace_with_uninit(cx, src, dest, expr.span);
 257                 if meets_msrv(self.msrv.as_ref(), &msrvs::MEM_TAKE) {
 258                     check_replace_with_default(cx, src, dest, expr.span);
 259                 }
 260             }
 261         }
 262     }
 263     extract_msrv_attr!(LateContext);
 264 }