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