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::{in_macro, match_def_path, meets_msrv, paths};
   4 use clippy_utils::{is_diagnostic_assoc_item, is_lang_ctor};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_hir::LangItem::OptionNone;
   9 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
  10 use rustc_lint::{LateContext, LateLintPass, LintContext};
  11 use rustc_middle::lint::in_external_macro;
  12 use rustc_semver::RustcVersion;
  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:** Checks for `mem::replace()` on an `Option` with
  19     /// `None`.
  20     ///
  21     /// **Why is this bad?** `Option` already has the method `take()` for
  22     /// taking its current value (Some(..) or None) and replacing it with
  23     /// `None`.
  24     ///
  25     /// **Known problems:** 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     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:** Checks for `mem::replace(&mut _, mem::uninitialized())`
  46     /// and `mem::replace(&mut _, mem::zeroed())`.
  47     ///
  48     /// **Why is this bad?** This will lead to undefined behavior even if the
  49     /// value is overwritten later, because the uninitialized value may be
  50     /// observed in the case of a panic.
  51     ///
  52     /// **Known problems:** None.
  53     ///
  54     /// **Example:**
  55     ///
  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     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:** Checks for `std::mem::replace` on a value of type
  78     /// `T` with `T::default()`.
  79     ///
  80     /// **Why is this bad?** `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     /// **Known problems:** None.
  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     pub MEM_REPLACE_WITH_DEFAULT,
  96     style,
  97     "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
  98 }
  99
 100 impl_lint_pass!(MemReplace =>
 101     [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);
 102
 103 fn check_replace_option_with_none(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 104     if let ExprKind::Path(ref replacement_qpath) = src.kind {
 105         // Check that second argument is `Option::None`
 106         if is_lang_ctor(cx, replacement_qpath, 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
 141 fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 142     if_chain! {
 143         // check if replacement is mem::MaybeUninit::uninit().assume_init()
 144         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id);
 145         if cx.tcx.is_diagnostic_item(sym::assume_init, method_def_id);
 146         then {
 147             let mut applicability = Applicability::MachineApplicable;
 148             span_lint_and_sugg(
 149                 cx,
 150                 MEM_REPLACE_WITH_UNINIT,
 151                 expr_span,
 152                 "replacing with `mem::MaybeUninit::uninit().assume_init()`",
 153                 "consider using",
 154                 format!(
 155                     "std::ptr::read({})",
 156                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
 157                 ),
 158                 applicability,
 159             );
 160             return;
 161         }
 162     }
 163
 164     if_chain! {
 165         if let ExprKind::Call(repl_func, repl_args) = src.kind;
 166         if repl_args.is_empty();
 167         if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 168         if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 169         then {
 170             if cx.tcx.is_diagnostic_item(sym::mem_uninitialized, repl_def_id) {
 171                 let mut applicability = Applicability::MachineApplicable;
 172                 span_lint_and_sugg(
 173                     cx,
 174                     MEM_REPLACE_WITH_UNINIT,
 175                     expr_span,
 176                     "replacing with `mem::uninitialized()`",
 177                     "consider using",
 178                     format!(
 179                         "std::ptr::read({})",
 180                         snippet_with_applicability(cx, dest.span, "", &mut applicability)
 181                     ),
 182                     applicability,
 183                 );
 184             } else if cx.tcx.is_diagnostic_item(sym::mem_zeroed, repl_def_id) &&
 185                     !cx.typeck_results().expr_ty(src).is_primitive() {
 186                 span_lint_and_help(
 187                     cx,
 188                     MEM_REPLACE_WITH_UNINIT,
 189                     expr_span,
 190                     "replacing with `mem::zeroed()`",
 191                     None,
 192                     "consider using a default value or the `take_mut` crate instead",
 193                 );
 194             }
 195         }
 196     }
 197 }
 198
 199 /// Returns true if the `def_id` associated with the `path` is recognized as a "default-equivalent"
 200 /// constructor from the std library
 201 fn is_default_equivalent_ctor(cx: &LateContext<'_>, def_id: DefId, path: &QPath<'_>) -> bool {
 202     let std_types_symbols = &[
 203         sym::string_type,
 204         sym::vec_type,
 205         sym::vecdeque_type,
 206         sym::LinkedList,
 207         sym::hashmap_type,
 208         sym::BTreeMap,
 209         sym::hashset_type,
 210         sym::BTreeSet,
 211         sym::BinaryHeap,
 212     ];
 213
 214     if std_types_symbols
 215         .iter()
 216         .any(|symbol| is_diagnostic_assoc_item(cx, def_id, *symbol))
 217     {
 218         if let QPath::TypeRelative(_, method) = path {
 219             if method.ident.name == sym::new {
 220                 return true;
 221             }
 222         }
 223     }
 224
 225     false
 226 }
 227
 228 fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
 229     if_chain! {
 230         if let ExprKind::Call(repl_func, _) = src.kind;
 231         if !in_external_macro(cx.tcx.sess, expr_span);
 232         if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
 233         if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
 234         if is_diagnostic_assoc_item(cx, repl_def_id, sym::Default)
 235             || is_default_equivalent_ctor(cx, repl_def_id, repl_func_qpath);
 236
 237         then {
 238             span_lint_and_then(
 239                 cx,
 240                 MEM_REPLACE_WITH_DEFAULT,
 241                 expr_span,
 242                 "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
 243                 |diag| {
 244                     if !in_macro(expr_span) {
 245                         let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 246
 247                         diag.span_suggestion(
 248                             expr_span,
 249                             "consider using",
 250                             suggestion,
 251                             Applicability::MachineApplicable
 252                         );
 253                     }
 254                 }
 255             );
 256         }
 257     }
 258 }
 259
 260 const MEM_REPLACE_WITH_DEFAULT_MSRV: RustcVersion = RustcVersion::new(1, 40, 0);
 261
 262 pub struct MemReplace {
 263     msrv: Option<RustcVersion>,
 264 }
 265
 266 impl MemReplace {
 267     #[must_use]
 268     pub fn new(msrv: Option<RustcVersion>) -> Self {
 269         Self { msrv }
 270     }
 271 }
 272
 273 impl<'tcx> LateLintPass<'tcx> for MemReplace {
 274     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 275         if_chain! {
 276             // Check that `expr` is a call to `mem::replace()`
 277             if let ExprKind::Call(func, func_args) = expr.kind;
 278             if let ExprKind::Path(ref func_qpath) = func.kind;
 279             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
 280             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 281             if let [dest, src] = func_args;
 282             then {
 283                 check_replace_option_with_none(cx, src, dest, expr.span);
 284                 check_replace_with_uninit(cx, src, dest, expr.span);
 285                 if meets_msrv(self.msrv.as_ref(), &MEM_REPLACE_WITH_DEFAULT_MSRV) {
 286                     check_replace_with_default(cx, src, dest, expr.span);
 287                 }
 288             }
 289         }
 290     }
 291     extract_msrv_attr!(LateContext);
 292 }