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::{in_macro, is_default_equivalent, is_lang_ctor, match_def_path, meets_msrv, msrvs, paths};
   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, LintContext};
  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     pub MEM_REPLACE_OPTION_WITH_NONE,
  39     style,
  40     "replacing an `Option` with `None` instead of `take()`"
  41 }
  42
  43 declare_clippy_lint! {
  44     /// ### What it does
  45     /// Checks for `mem::replace(&mut _, mem::uninitialized())`
  46     /// and `mem::replace(&mut _, mem::zeroed())`.
  47     ///
  48     /// ### Why is this bad?
  49     /// This will lead to undefined behavior even if the
  50     /// value is overwritten later, because the uninitialized value may be
  51     /// observed in the case of a panic.
  52     ///
  53     /// ### Example
  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
  76     /// Checks for `std::mem::replace` on a value of type
  77     /// `T` with `T::default()`.
  78     ///
  79     /// ### Why is this bad?
  80     /// `std::mem` module already has the method `take` to
  81     /// take the current value and replace it with the default value of that type.
  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 is_lang_ctor(cx, replacement_qpath, OptionNone) {
 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, replaced) => {
 112                     if let ExprKind::Path(QPath::Resolved(None, replaced_path)) = replaced.kind {
 113                         replaced_path
 114                     } else {
 115                         return;
 116                     }
 117                 },
 118                 ExprKind::Path(QPath::Resolved(None, 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(repl_func, 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     // 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 let ExprKind::Path(q) = &src.kind {
 205         if is_lang_ctor(cx, q, OptionNone) {
 206             return;
 207         }
 208     }
 209     if is_default_equivalent(cx, src) && !in_external_macro(cx.tcx.sess, expr_span) {
 210         span_lint_and_then(
 211             cx,
 212             MEM_REPLACE_WITH_DEFAULT,
 213             expr_span,
 214             "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 215             |diag| {
 216                 if !in_macro(expr_span) {
 217                     let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 218
 219                     diag.span_suggestion(
 220                         expr_span,
 221                         "consider using",
 222                         suggestion,
 223                         Applicability::MachineApplicable,
 224                     );
 225                 }
 226             },
 227         );
 228     }
 229 }
 230
 231 pub struct MemReplace {
 232     msrv: Option<RustcVersion>,
 233 }
 234
 235 impl MemReplace {
 236     #[must_use]
 237     pub fn new(msrv: Option<RustcVersion>) -> Self {
 238         Self { msrv }
 239     }
 240 }
 241
 242 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 243     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 244         if_chain! {
 245             // Check that `expr` is a call to `mem::replace()`
 246             if let ExprKind::Call(func, func_args) = expr.kind;
 247             if let ExprKind::Path(ref func_qpath) = func.kind;
 248             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 249             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 250             if let [dest, src] = func_args;
 251             then {
 252                 check_replace_option_with_none(cx, src, dest, expr.span);
 253                 check_replace_with_uninit(cx, src, dest, expr.span);
 254                 if meets_msrv(self.msrv.as_ref(), &msrvs::MEM_TAKE) {
 255                     check_replace_with_default(cx, src, dest, expr.span);
 256                 }
 257             }
 258         }
 259     }
 260     extract_msrv_attr!(LateContext);
 261 }