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::def_id::DefId;
   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 use clippy_utils::is_diagnostic_assoc_item;
  17
  18 declare_clippy_lint! {
  19     /// **What it does:** Checks for `mem::replace()` on an `Option` with
  20     /// `None`.
  21     ///
  22     /// **Why is this bad?** `Option` already has the method `take()` for
  23     /// taking its current value (Some(..) or None) and replacing it with
  24     /// `None`.
  25     ///
  26     /// **Known problems:** None.
  27     ///
  28     /// **Example:**
  29     /// ```rust
  30     /// use std::mem;
  31     ///
  32     /// let mut an_option = Some(0);
  33     /// let replaced = mem::replace(&mut an_option, None);
  34     /// ```
  35     /// Is better expressed with:
  36     /// ```rust
  37     /// let mut an_option = Some(0);
  38     /// let taken = an_option.take();
  39     /// ```
  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:** Checks for `mem::replace(&mut _, mem::uninitialized())`
  47     /// and `mem::replace(&mut _, mem::zeroed())`.
  48     ///
  49     /// **Why is this bad?** 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     /// **Known problems:** None.
  54     ///
  55     /// **Example:**
  56     ///
  57     /// ```
  58     /// use std::mem;
  59     ///# fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
  60     ///
  61     /// #[allow(deprecated, invalid_value)]
  62     /// fn myfunc (v: &mut Vec<i32>) {
  63     ///     let taken_v = unsafe { mem::replace(v, mem::uninitialized()) };
  64     ///     let new_v = may_panic(taken_v); // undefined behavior on panic
  65     ///     mem::forget(mem::replace(v, new_v));
  66     /// }
  67     /// ```
  68     ///
  69     /// The [take_mut](https://docs.rs/take_mut) crate offers a sound solution,
  70     /// at the cost of either lazily creating a replacement value or aborting
  71     /// on panic, to ensure that the uninitialized value cannot be observed.
  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:** Checks for `std::mem::replace` on a value of type
  79     /// `T` with `T::default()`.
  80     ///
  81     /// **Why is this bad?** `std::mem` module already has the method `take` to
  82     /// take the current value and replace it with the default value of that type.
  83     ///
  84     /// **Known problems:** None.
  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     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 match_qpath(replacement_qpath, &paths::OPTION_NONE) {
 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, ref replaced) => {
 115                     if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
 116                         replaced_path
 117                     } else {
 118                         return;
 119                     }
 120                 },
 121                 ExprKind::Path(QPath::Resolved(None, ref 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(ref repl_func, ref 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 /// Returns true if the `def_id` associated with the `path` is recognized as a "default-equivalent"
 201 /// constructor from the std library
 202 fn is_default_equivalent_ctor(cx: &LateContext<'_>, def_id: DefId, path: &QPath<'_>) -> bool {
 203     let std_types_symbols = &[
 204         sym::string_type,
 205         sym::vec_type,
 206         sym::vecdeque_type,
 207         sym::LinkedList,
 208         sym::hashmap_type,
 209         sym::BTreeMap,
 210         sym::hashset_type,
 211         sym::BTreeSet,
 212         sym::BinaryHeap,
 213     ];
 214
 215     if std_types_symbols
 216         .iter()
 217         .any(|symbol| is_diagnostic_assoc_item(cx, def_id, *symbol))
 218     {
 219         if let QPath::TypeRelative(_, ref method) = path {
 220             if method.ident.name == sym::new {
 221                 return true;
 222             }
 223         }
 224     }
 225
 226     false
 227 }
 228
 229 fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 230     if let ExprKind::Call(ref repl_func, _) = src.kind {
 231         if_chain! {
 232             if !in_external_macro(cx.tcx.sess, expr_span);
 233             if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 234             if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 235             if is_diagnostic_assoc_item(cx, repl_def_id, sym::Default)
 236                 || is_default_equivalent_ctor(cx, repl_def_id, repl_func_qpath);
 237
 238             then {
 239                 span_lint_and_then(
 240                     cx,
 241                     MEM_REPLACE_WITH_DEFAULT,
 242                     expr_span,
 243                     "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 244                     |diag| {
 245                         if !in_macro(expr_span) {
 246                             let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 247
 248                             diag.span_suggestion(
 249                                 expr_span,
 250                                 "consider using",
 251                                 suggestion,
 252                                 Applicability::MachineApplicable
 253                             );
 254                         }
 255                     }
 256                 );
 257             }
 258         }
 259     }
 260 }
 261
 262 const MEM_REPLACE_WITH_DEFAULT_MSRV: RustcVersion = RustcVersion::new(1, 40, 0);
 263
 264 pub struct MemReplace {
 265     msrv: Option<RustcVersion>,
 266 }
 267
 268 impl MemReplace {
 269     #[must_use]
 270     pub fn new(msrv: Option<RustcVersion>) -> Self {
 271         Self { msrv }
 272     }
 273 }
 274
 275 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 276     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 277         if_chain! {
 278             // Check that `expr` is a call to `mem::replace()`
 279             if let ExprKind::Call(ref func, ref func_args) = expr.kind;
 280             if let ExprKind::Path(ref func_qpath) = func.kind;
 281             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 282             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 283             if let [dest, src] = &**func_args;
 284             then {
 285                 check_replace_option_with_none(cx, src, dest, expr.span);
 286                 check_replace_with_uninit(cx, src, dest, expr.span);
 287                 if meets_msrv(self.msrv.as_ref(), &MEM_REPLACE_WITH_DEFAULT_MSRV) {
 288                     check_replace_with_default(cx, src, dest, expr.span);
 289                 }
 290             }
 291         }
 292     }
 293     extract_msrv_attr!(LateContext);
 294 }