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_res_lang_ctor, meets_msrv, msrvs, path_res};
   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     // Check that second argument is `Option::None`
 106     if is_res_lang_ctor(cx, path_res(cx, src), OptionNone) {
 107         // Since this is a late pass (already type-checked),
 108         // and we already know that the second argument is an
 109         // `Option`, we do not need to check the first
 110         // argument's type. All that's left is to get
 111         // replacee's path.
 112         let replaced_path = match dest.kind {
 113             ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, replaced) => {
 114                 if let ExprKind::Path(QPath::Resolved(None, replaced_path)) = replaced.kind {
 115                     replaced_path
 116                 } else {
 117                     return;
 118                 }
 119             },
 120             ExprKind::Path(QPath::Resolved(None, replaced_path)) => replaced_path,
 121             _ => return,
 122         };
 123
 124         let mut applicability = Applicability::MachineApplicable;
 125         span_lint_and_sugg(
 126             cx,
 127             MEM_REPLACE_OPTION_WITH_NONE,
 128             expr_span,
 129             "replacing an `Option` with `None`",
 130             "consider `Option::take()` instead",
 131             format!(
 132                 "{}.take()",
 133                 snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
 134             ),
 135             applicability,
 136         );
 137     }
 138 }
 139
 140 fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 141     if_chain! {
 142         // check if replacement is mem::MaybeUninit::uninit().assume_init()
 143         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
 144         if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
 145         then {
 146             let mut applicability = Applicability::MachineApplicable;
 147             span_lint_and_sugg(
 148                 cx,
 149                 MEM_REPLACE_WITH_UNINIT,
 150                 expr_span,
 151                 "replacing with `mem::MaybeUninit::uninit().assume_init()`",
 152                 "consider using",
 153                 format!(
 154                     "std::ptr::read({})",
 155                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
 156                 ),
 157                 applicability,
 158             );
 159             return;
 160         }
 161     }
 162
 163     if_chain! {
 164         if let ExprKind::Call(repl_func, []) = src.kind;
 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     // disable lint for primitives
 199     let expr_type = cx.typeck_results().expr_ty_adjusted(src);
 200     if is_non_aggregate_primitive_type(expr_type) {
 201         return;
 202     }
 203     // disable lint for Option since it is covered in another lint
 204     if is_res_lang_ctor(cx, path_res(cx, src), OptionNone) {
 205         return;
 206     }
 207     if is_default_equivalent(cx, src) && !in_external_macro(cx.tcx.sess, expr_span) {
 208         span_lint_and_then(
 209             cx,
 210             MEM_REPLACE_WITH_DEFAULT,
 211             expr_span,
 212             "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 213             |diag| {
 214                 if !expr_span.from_expansion() {
 215                     let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 216
 217                     diag.span_suggestion(
 218                         expr_span,
 219                         "consider using",
 220                         suggestion,
 221                         Applicability::MachineApplicable,
 222                     );
 223                 }
 224             },
 225         );
 226     }
 227 }
 228
 229 pub struct MemReplace {
 230     msrv: Option<RustcVersion>,
 231 }
 232
 233 impl MemReplace {
 234     #[must_use]
 235     pub fn new(msrv: Option<RustcVersion>) -> Self {
 236         Self { msrv }
 237     }
 238 }
 239
 240 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 241     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 242         if_chain! {
 243             // Check that `expr` is a call to `mem::replace()`
 244             if let ExprKind::Call(func, [dest, src]) = expr.kind;
 245             if let ExprKind::Path(ref func_qpath) = func.kind;
 246             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 247             if cx.tcx.is_diagnostic_item(sym::mem_replace, def_id);
 248             then {
 249                 check_replace_option_with_none(cx, src, dest, expr.span);
 250                 check_replace_with_uninit(cx, src, dest, expr.span);
 251                 if meets_msrv(self.msrv, msrvs::MEM_TAKE) {
 252                     check_replace_with_default(cx, src, dest, expr.span);
 253                 }
 254             }
 255         }
 256     }
 257     extract_msrv_attr!(LateContext);
 258 }