src/tools/clippy/clippy_lints/src/non_copy_const.rs

   1 //! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
   2 //!
   3 //! This lint is **warn** by default.
   4
   5 use std::ptr;
   6
   7 use clippy_utils::diagnostics::span_lint_and_then;
   8 use clippy_utils::in_constant;
   9 use if_chain::if_chain;
  10 use rustc_hir::def::{DefKind, Res};
  11 use rustc_hir::def_id::DefId;
  12 use rustc_hir::{
  13     BodyId, Expr, ExprKind, HirId, Impl, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
  14 };
  15 use rustc_lint::{LateContext, LateLintPass, Lint};
  16 use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
  17 use rustc_middle::ty::adjustment::Adjust;
  18 use rustc_middle::ty::{self, Const, Ty};
  19 use rustc_session::{declare_lint_pass, declare_tool_lint};
  20 use rustc_span::{InnerSpan, Span, DUMMY_SP};
  21 use rustc_typeck::hir_ty_to_ty;
  22
  23 // FIXME: this is a correctness problem but there's no suitable
  24 // warn-by-default category.
  25 declare_clippy_lint! {
  26     /// ### What it does
  27     /// Checks for declaration of `const` items which is interior
  28     /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
  29     ///
  30     /// ### Why is this bad?
  31     /// Consts are copied everywhere they are referenced, i.e.,
  32     /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
  33     /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
  34     /// these types in the first place.
  35     ///
  36     /// The `const` should better be replaced by a `static` item if a global
  37     /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
  38     ///
  39     /// ### Known problems
  40     /// A "non-constant" const item is a legacy way to supply an
  41     /// initialized value to downstream `static` items (e.g., the
  42     /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
  43     /// and this lint should be suppressed.
  44     ///
  45     /// Even though the lint avoids triggering on a constant whose type has enums that have variants
  46     /// with interior mutability, and its value uses non interior mutable variants (see
  47     /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
  48     /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
  49     /// it complains about associated constants without default values only based on its types;
  50     /// which might not be preferable.
  51     /// There're other enums plus associated constants cases that the lint cannot handle.
  52     ///
  53     /// Types that have underlying or potential interior mutability trigger the lint whether
  54     /// the interior mutable field is used or not. See issues
  55     /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
  56     ///
  57     /// ### Example
  58     /// ```rust
  59     /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
  60     ///
  61     /// // Bad.
  62     /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
  63     /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
  64     /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
  65     ///
  66     /// // Good.
  67     /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
  68     /// STATIC_ATOM.store(9, SeqCst);
  69     /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
  70     /// ```
  71     #[clippy::version = "pre 1.29.0"]
  72     pub DECLARE_INTERIOR_MUTABLE_CONST,
  73     style,
  74     "declaring `const` with interior mutability"
  75 }
  76
  77 // FIXME: this is a correctness problem but there's no suitable
  78 // warn-by-default category.
  79 declare_clippy_lint! {
  80     /// ### What it does
  81     /// Checks if `const` items which is interior mutable (e.g.,
  82     /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
  83     ///
  84     /// ### Why is this bad?
  85     /// Consts are copied everywhere they are referenced, i.e.,
  86     /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
  87     /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
  88     /// these types in the first place.
  89     ///
  90     /// The `const` value should be stored inside a `static` item.
  91     ///
  92     /// ### Known problems
  93     /// When an enum has variants with interior mutability, use of its non
  94     /// interior mutable variants can generate false positives. See issue
  95     /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
  96     ///
  97     /// Types that have underlying or potential interior mutability trigger the lint whether
  98     /// the interior mutable field is used or not. See issues
  99     /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
 100     /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
 101     ///
 102     /// ### Example
 103     /// ```rust
 104     /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 105     /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
 106     ///
 107     /// // Bad.
 108     /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
 109     /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
 110     ///
 111     /// // Good.
 112     /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
 113     /// STATIC_ATOM.store(9, SeqCst);
 114     /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
 115     /// ```
 116     #[clippy::version = "pre 1.29.0"]
 117     pub BORROW_INTERIOR_MUTABLE_CONST,
 118     style,
 119     "referencing `const` with interior mutability"
 120 }
 121
 122 fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
 123     // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
 124     // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
 125     // 'unfrozen'. However, this code causes a false negative in which
 126     // a type contains a layout-unknown type, but also an unsafe cell like `const CELL: Cell<T>`.
 127     // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
 128     // since it works when a pointer indirection involves (`Cell<*const T>`).
 129     // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
 130     // but I'm not sure whether it's a decent way, if possible.
 131     cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env)
 132 }
 133
 134 fn is_value_unfrozen_raw<'tcx>(
 135     cx: &LateContext<'tcx>,
 136     result: Result<ConstValue<'tcx>, ErrorHandled>,
 137     ty: Ty<'tcx>,
 138 ) -> bool {
 139     fn inner<'tcx>(cx: &LateContext<'tcx>, val: Const<'tcx>) -> bool {
 140         match val.ty().kind() {
 141             // the fact that we have to dig into every structs to search enums
 142             // leads us to the point checking `UnsafeCell` directly is the only option.
 143             ty::Adt(ty_def, ..) if Some(ty_def.did()) == cx.tcx.lang_items().unsafe_cell_type() => true,
 144             ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
 145                 let val = cx.tcx.destructure_const(cx.param_env.and(val));
 146                 val.fields.iter().any(|field| inner(cx, *field))
 147             },
 148             _ => false,
 149         }
 150     }
 151
 152     result.map_or_else(
 153         |err| {
 154             // Consider `TooGeneric` cases as being unfrozen.
 155             // This causes a false positive where an assoc const whose type is unfrozen
 156             // have a value that is a frozen variant with a generic param (an example is
 157             // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
 158             // However, it prevents a number of false negatives that is, I think, important:
 159             // 1. assoc consts in trait defs referring to consts of themselves
 160             //    (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
 161             // 2. a path expr referring to assoc consts whose type is doesn't have
 162             //    any frozen variants in trait defs (i.e. without substitute for `Self`).
 163             //    (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
 164             // 3. similar to the false positive above;
 165             //    but the value is an unfrozen variant, or the type has no enums. (An example is
 166             //    `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
 167             //    and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
 168             // One might be able to prevent these FNs correctly, and replace this with `false`;
 169             // e.g. implementing `has_frozen_variant` described above, and not running this function
 170             // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
 171             // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
 172             // similar to 2., but with the a frozen variant) (e.g. borrowing
 173             // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
 174             // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
 175             err == ErrorHandled::TooGeneric
 176         },
 177         |val| inner(cx, Const::from_value(cx.tcx, val, ty)),
 178     )
 179 }
 180
 181 fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
 182     let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
 183     is_value_unfrozen_raw(cx, result, ty)
 184 }
 185
 186 fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
 187     let substs = cx.typeck_results().node_substs(hir_id);
 188
 189     let result = cx.tcx.const_eval_resolve(
 190         cx.param_env,
 191         ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs),
 192         None,
 193     );
 194     is_value_unfrozen_raw(cx, result, ty)
 195 }
 196
 197 #[derive(Copy, Clone)]
 198 enum Source {
 199     Item { item: Span },
 200     Assoc { item: Span },
 201     Expr { expr: Span },
 202 }
 203
 204 impl Source {
 205     #[must_use]
 206     fn lint(&self) -> (&'static Lint, &'static str, Span) {
 207         match self {
 208             Self::Item { item } | Self::Assoc { item, .. } => (
 209                 DECLARE_INTERIOR_MUTABLE_CONST,
 210                 "a `const` item should never be interior mutable",
 211                 *item,
 212             ),
 213             Self::Expr { expr } => (
 214                 BORROW_INTERIOR_MUTABLE_CONST,
 215                 "a `const` item with interior mutability should not be borrowed",
 216                 *expr,
 217             ),
 218         }
 219     }
 220 }
 221
 222 fn lint(cx: &LateContext<'_>, source: Source) {
 223     let (lint, msg, span) = source.lint();
 224     span_lint_and_then(cx, lint, span, msg, |diag| {
 225         if span.from_expansion() {
 226             return; // Don't give suggestions into macros.
 227         }
 228         match source {
 229             Source::Item { .. } => {
 230                 let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
 231                 diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
 232             },
 233             Source::Assoc { .. } => (),
 234             Source::Expr { .. } => {
 235                 diag.help("assign this const to a local or static variable, and use the variable here");
 236             },
 237         }
 238     });
 239 }
 240
 241 declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
 242
 243 impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
 244     fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
 245         if let ItemKind::Const(hir_ty, body_id) = it.kind {
 246             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
 247
 248             if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
 249                 lint(cx, Source::Item { item: it.span });
 250             }
 251         }
 252     }
 253
 254     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
 255         if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
 256             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
 257
 258             // Normalize assoc types because ones originated from generic params
 259             // bounded other traits could have their bound.
 260             let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
 261             if is_unfrozen(cx, normalized)
 262                 // When there's no default value, lint it only according to its type;
 263                 // in other words, lint consts whose value *could* be unfrozen, not definitely is.
 264                 // This feels inconsistent with how the lint treats generic types,
 265                 // which avoids linting types which potentially become unfrozen.
 266                 // One could check whether an unfrozen type have a *frozen variant*
 267                 // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
 268                 // and do the same as the case of generic types at impl items.
 269                 // Note that it isn't sufficient to check if it has an enum
 270                 // since all of that enum's variants can be unfrozen:
 271                 // i.e. having an enum doesn't necessary mean a type has a frozen variant.
 272                 // And, implementing it isn't a trivial task; it'll probably end up
 273                 // re-implementing the trait predicate evaluation specific to `Freeze`.
 274                 && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
 275             {
 276                 lint(cx, Source::Assoc { item: trait_item.span });
 277             }
 278         }
 279     }
 280
 281     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
 282         if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
 283             let item_def_id = cx.tcx.hir().get_parent_item(impl_item.hir_id());
 284             let item = cx.tcx.hir().expect_item(item_def_id);
 285
 286             match &item.kind {
 287                 ItemKind::Impl(Impl {
 288                     of_trait: Some(of_trait_ref),
 289                     ..
 290                 }) => {
 291                     if_chain! {
 292                         // Lint a trait impl item only when the definition is a generic type,
 293                         // assuming an assoc const is not meant to be an interior mutable type.
 294                         if let Some(of_trait_def_id) = of_trait_ref.trait_def_id();
 295                         if let Some(of_assoc_item) = cx
 296                             .tcx
 297                             .associated_item(impl_item.def_id)
 298                             .trait_item_def_id;
 299                         if cx
 300                             .tcx
 301                             .layout_of(cx.tcx.param_env(of_trait_def_id).and(
 302                                 // Normalize assoc types because ones originated from generic params
 303                                 // bounded other traits could have their bound at the trait defs;
 304                                 // and, in that case, the definition is *not* generic.
 305                                 cx.tcx.normalize_erasing_regions(
 306                                     cx.tcx.param_env(of_trait_def_id),
 307                                     cx.tcx.type_of(of_assoc_item),
 308                                 ),
 309                             ))
 310                             .is_err();
 311                             // If there were a function like `has_frozen_variant` described above,
 312                             // we should use here as a frozen variant is a potential to be frozen
 313                             // similar to unknown layouts.
 314                             // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
 315                         let ty = hir_ty_to_ty(cx.tcx, hir_ty);
 316                         let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
 317                         if is_unfrozen(cx, normalized);
 318                         if is_value_unfrozen_poly(cx, *body_id, normalized);
 319                         then {
 320                             lint(
 321                                cx,
 322                                Source::Assoc {
 323                                    item: impl_item.span,
 324                                 },
 325                             );
 326                         }
 327                     }
 328                 },
 329                 ItemKind::Impl(Impl { of_trait: None, .. }) => {
 330                     let ty = hir_ty_to_ty(cx.tcx, hir_ty);
 331                     // Normalize assoc types originated from generic params.
 332                     let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
 333
 334                     if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
 335                         lint(cx, Source::Assoc { item: impl_item.span });
 336                     }
 337                 },
 338                 _ => (),
 339             }
 340         }
 341     }
 342
 343     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 344         if let ExprKind::Path(qpath) = &expr.kind {
 345             // Only lint if we use the const item inside a function.
 346             if in_constant(cx, expr.hir_id) {
 347                 return;
 348             }
 349
 350             // Make sure it is a const item.
 351             let item_def_id = match cx.qpath_res(qpath, expr.hir_id) {
 352                 Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
 353                 _ => return,
 354             };
 355
 356             // Climb up to resolve any field access and explicit referencing.
 357             let mut cur_expr = expr;
 358             let mut dereferenced_expr = expr;
 359             let mut needs_check_adjustment = true;
 360             loop {
 361                 let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
 362                 if parent_id == cur_expr.hir_id {
 363                     break;
 364                 }
 365                 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
 366                     match &parent_expr.kind {
 367                         ExprKind::AddrOf(..) => {
 368                             // `&e` => `e` must be referenced.
 369                             needs_check_adjustment = false;
 370                         },
 371                         ExprKind::Field(..) => {
 372                             needs_check_adjustment = true;
 373
 374                             // Check whether implicit dereferences happened;
 375                             // if so, no need to go further up
 376                             // because of the same reason as the `ExprKind::Unary` case.
 377                             if cx
 378                                 .typeck_results()
 379                                 .expr_adjustments(dereferenced_expr)
 380                                 .iter()
 381                                 .any(|adj| matches!(adj.kind, Adjust::Deref(_)))
 382                             {
 383                                 break;
 384                             }
 385
 386                             dereferenced_expr = parent_expr;
 387                         },
 388                         ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
 389                             // `e[i]` => desugared to `*Index::index(&e, i)`,
 390                             // meaning `e` must be referenced.
 391                             // no need to go further up since a method call is involved now.
 392                             needs_check_adjustment = false;
 393                             break;
 394                         },
 395                         ExprKind::Unary(UnOp::Deref, _) => {
 396                             // `*e` => desugared to `*Deref::deref(&e)`,
 397                             // meaning `e` must be referenced.
 398                             // no need to go further up since a method call is involved now.
 399                             needs_check_adjustment = false;
 400                             break;
 401                         },
 402                         _ => break,
 403                     }
 404                     cur_expr = parent_expr;
 405                 } else {
 406                     break;
 407                 }
 408             }
 409
 410             let ty = if needs_check_adjustment {
 411                 let adjustments = cx.typeck_results().expr_adjustments(dereferenced_expr);
 412                 if let Some(i) = adjustments
 413                     .iter()
 414                     .position(|adj| matches!(adj.kind, Adjust::Borrow(_) | Adjust::Deref(_)))
 415                 {
 416                     if i == 0 {
 417                         cx.typeck_results().expr_ty(dereferenced_expr)
 418                     } else {
 419                         adjustments[i - 1].target
 420                     }
 421                 } else {
 422                     // No borrow adjustments means the entire const is moved.
 423                     return;
 424                 }
 425             } else {
 426                 cx.typeck_results().expr_ty(dereferenced_expr)
 427             };
 428
 429             if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
 430                 lint(cx, Source::Expr { expr: expr.span });
 431             }
 432         }
 433     }
 434 }