fix a FP in `interior_mutable_const`

fix a false positive in two `interior_mutable_const` lints where a constant with enums gets linted
even if it uses a clearly unfrozen variant. Note that the code uses the MIR interpreter, which
the author of #3962 thought unlikely to be a solution. This might be over-engineering;
but, I think it's important to be able to work with the 'http' crate (#3825).

6 files changed, 539 insertions, 35 deletions

diff --git a/clippy_lints/src/non_copy_const.rs b/clippy_lints/src/non_copy_const.rs
index 7b662eae775..6b0d198edcf 100644
--- a/clippy_lints/src/non_copy_const.rs
+++ b/clippy_lints/src/non_copy_const.rs
@@ -5,11 +5,15 @@
 use std::ptr;
 
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::{Expr, ExprKind, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp};
+use rustc_hir::def_id::DefId;
+use rustc_hir::{
+    BodyId, Expr, ExprKind, HirId, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
+};
 use rustc_infer::traits::specialization_graph;
 use rustc_lint::{LateContext, LateLintPass, Lint};
+use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
 use rustc_middle::ty::adjustment::Adjust;
-use rustc_middle::ty::{AssocKind, Ty};
+use rustc_middle::ty::{self, AssocKind, Const, Ty};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::{InnerSpan, Span, DUMMY_SP};
 use rustc_typeck::hir_ty_to_ty;
@@ -36,14 +40,17 @@ declare_clippy_lint! {
     /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
     /// and this lint should be suppressed.
     ///
-    /// When an enum has variants with interior mutability, use of its non interior mutable
-    /// variants can generate false positives. See issue
-    /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
+    /// Even though the lint avoids triggering on a constant whose type has enums that have variants
+    /// with interior mutability, and its value uses non interior mutable variants (see
+    /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
+    /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
+    /// it complains about associated constants without default values only based on its types;
+    /// which might not be preferable.
+    /// There're other enums plus associated constants cases that the lint cannot handle.
     ///
     /// Types that have underlying or potential interior mutability trigger the lint whether
     /// the interior mutable field is used or not. See issues
     /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
-    /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
     ///
     /// **Example:**
     /// ```rust
@@ -105,6 +112,79 @@ declare_clippy_lint! {
     "referencing `const` with interior mutability"
 }
 
+fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
+    // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
+    // 'unfrozen'. However, this code causes a false negative in which
+    // a type contains a layout-unknown type, but also a unsafe cell like `const CELL: Cell<T>`.
+    // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
+    // since it works when a pointer indirection involves (`Cell<*const T>`).
+    // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
+    // but I'm not sure whether it's a decent way, if possible.
+    cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env)
+}
+
+fn is_value_unfrozen_raw<'tcx>(
+    cx: &LateContext<'tcx>,
+    result: Result<ConstValue<'tcx>, ErrorHandled>,
+    ty: Ty<'tcx>,
+) -> bool {
+    fn inner<'tcx>(cx: &LateContext<'tcx>, val: &'tcx Const<'tcx>) -> bool {
+        match val.ty.kind() {
+            // the fact that we have to dig into every structs to search enums
+            // leads us to the point checking `UnsafeCell` directly is the only option.
+            ty::Adt(ty_def, ..) if Some(ty_def.did) == cx.tcx.lang_items().unsafe_cell_type() => true,
+            ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
+                let val = cx.tcx.destructure_const(cx.param_env.and(val));
+                val.fields.iter().any(|field| inner(cx, field))
+            },
+            _ => false,
+        }
+    }
+
+    result.map_or_else(
+        |err| {
+            // Consider `TooGeneric` cases as being unfrozen.
+            // This causes a false positive where an assoc const whose type is unfrozen
+            // have a value that is a frozen variant with a generic param (an example is
+            // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
+            // However, it prevents a number of false negatives that is, I think, important:
+            // 1. assoc consts in trait defs referring to consts of themselves
+            //    (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
+            // 2. a path expr referring to assoc consts whose type is doesn't have
+            //    any frozen variants in trait defs (i.e. without substitute for `Self`).
+            //    (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
+            // 3. similar to the false positive above;
+            //    but the value is an unfrozen variant, or the type has no enums. (An example is
+            //    `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
+            //    and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
+            // One might be able to prevent these FNs correctly, and replace this with `false`;
+            // e.g. implementing `has_frozen_variant` described above, and not running this function
+            // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
+            // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
+            // similar to 2., but with the a frozen variant) (e.g. borrowing
+            // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
+            // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
+            err == ErrorHandled::TooGeneric
+        },
+        |val| inner(cx, Const::from_value(cx.tcx, val, ty)),
+    )
+}
+
+fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
+    let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
+    is_value_unfrozen_raw(cx, result, ty)
+}
+
+fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
+    let substs = cx.typeck_results().node_substs(hir_id);
+
+    let result = cx
+        .tcx
+        .const_eval_resolve(cx.param_env, ty::WithOptConstParam::unknown(def_id), substs, None, None);
+    is_value_unfrozen_raw(cx, result, ty)
+}
+
 #[derive(Copy, Clone)]
 enum Source {
     Item { item: Span },
@@ -130,19 +210,7 @@ impl Source {
     }
 }
 
-fn verify_ty_bound<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, source: Source) {
-    // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
-    // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
-    // 'unfrozen'. However, this code causes a false negative in which
-    // a type contains a layout-unknown type, but also a unsafe cell like `const CELL: Cell<T>`.
-    // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
-    // since it works when a pointer indirection involves (`Cell<*const T>`).
-    // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
-    // but I'm not sure whether it's a decent way, if possible.
-    if cx.tcx.layout_of(cx.param_env.and(ty)).is_err() || ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env) {
-        return;
-    }
-
+fn lint(cx: &LateContext<'_>, source: Source) {
     let (lint, msg, span) = source.lint();
     span_lint_and_then(cx, lint, span, msg, |diag| {
         if span.from_expansion() {
@@ -165,24 +233,44 @@ declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTER
 
 impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
     fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
-        if let ItemKind::Const(hir_ty, ..) = &it.kind {
+        if let ItemKind::Const(hir_ty, body_id) = it.kind {
             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
-            verify_ty_bound(cx, ty, Source::Item { item: it.span });
+
+            if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
+                lint(cx, Source::Item { item: it.span });
+            }
         }
     }
 
     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
-        if let TraitItemKind::Const(hir_ty, ..) = &trait_item.kind {
+        if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
+
             // Normalize assoc types because ones originated from generic params
             // bounded other traits could have their bound.
             let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
-            verify_ty_bound(cx, normalized, Source::Assoc { item: trait_item.span });
+            if is_unfrozen(cx, normalized)
+                // When there's no default value, lint it only according to its type;
+                // in other words, lint consts whose value *could* be unfrozen, not definitely is.
+                // This feels inconsistent with how the lint treats generic types,
+                // which avoids linting types which potentially become unfrozen.
+                // One could check whether a unfrozen type have a *frozen variant*
+                // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
+                // and do the same as the case of generic types at impl items.
+                // Note that it isn't sufficient to check if it has an enum
+                // since all of that enum's variants can be unfrozen:
+                // i.e. having an enum doesn't necessary mean a type has a frozen variant.
+                // And, implementing it isn't a trivial task; it'll probably end up
+                // re-implementing the trait predicate evaluation specific to `Freeze`.
+                && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
+            {
+                lint(cx, Source::Assoc { item: trait_item.span });
+            }
         }
     }
 
     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
-        if let ImplItemKind::Const(hir_ty, ..) = &impl_item.kind {
+        if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
             let item_hir_id = cx.tcx.hir().get_parent_node(impl_item.hir_id);
             let item = cx.tcx.hir().expect_item(item_hir_id);
 
@@ -209,16 +297,23 @@ impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
                                 ),
                             ))
                             .is_err();
+                            // If there were a function like `has_frozen_variant` described above,
+                            // we should use here as a frozen variant is a potential to be frozen
+                            // similar to unknown layouts.
+                            // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
                         then {
                             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
                             let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
-                            verify_ty_bound(
-                                cx,
-                                normalized,
-                                Source::Assoc {
-                                    item: impl_item.span,
-                                },
-                            );
+                            if is_unfrozen(cx, normalized)
+                                && is_value_unfrozen_poly(cx, *body_id, normalized)
+                            {
+                                lint(
+                                   cx,
+                                   Source::Assoc {
+                                       item: impl_item.span,
+                                    },
+                                );
+                            }
                         }
                     }
                 },
@@ -226,7 +321,10 @@ impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
                     let ty = hir_ty_to_ty(cx.tcx, hir_ty);
                     // Normalize assoc types originated from generic params.
                     let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
-                    verify_ty_bound(cx, normalized, Source::Assoc { item: impl_item.span });
+
+                    if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
+                        lint(cx, Source::Assoc { item: impl_item.span });
+                    }
                 },
                 _ => (),
             }
@@ -241,8 +339,8 @@ impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
             }
 
             // Make sure it is a const item.
-            match qpath_res(cx, qpath, expr.hir_id) {
-                Res::Def(DefKind::Const | DefKind::AssocConst, _) => {},
+            let item_def_id = match qpath_res(cx, qpath, expr.hir_id) {
+                Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
                 _ => return,
             };
 
@@ -319,7 +417,9 @@ impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
                 cx.typeck_results().expr_ty(dereferenced_expr)
             };
 
-            verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });
+            if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
+                lint(cx, Source::Expr { expr: expr.span });
+            }
         }
     }
 }
diff --git a/tests/ui/borrow_interior_mutable_const/auxiliary/helper.rs b/tests/ui/borrow_interior_mutable_const/auxiliary/helper.rs
new file mode 100644
index 00000000000..2289f7875f0
--- /dev/null
+++ b/tests/ui/borrow_interior_mutable_const/auxiliary/helper.rs
@@ -0,0 +1,16 @@
+// this file solely exists to test constants defined in foreign crates.
+// As the most common case is the `http` crate, it replicates `http::HeadewrName`'s structure.
+
+#![allow(clippy::declare_interior_mutable_const)]
+
+use std::sync::atomic::AtomicUsize;
+
+enum Private<T> {
+    ToBeUnfrozen(T),
+    Frozen(usize),
+}
+
+pub struct Wrapper(Private<AtomicUsize>);
+
+pub const WRAPPED_PRIVATE_UNFROZEN_VARIANT: Wrapper = Wrapper(Private::ToBeUnfrozen(AtomicUsize::new(6)));
+pub const WRAPPED_PRIVATE_FROZEN_VARIANT: Wrapper = Wrapper(Private::Frozen(7));
diff --git a/tests/ui/borrow_interior_mutable_const/enums.rs b/tests/ui/borrow_interior_mutable_const/enums.rs
new file mode 100644
index 00000000000..5027db44561
--- /dev/null
+++ b/tests/ui/borrow_interior_mutable_const/enums.rs
@@ -0,0 +1,101 @@
+// aux-build:helper.rs
+
+#![warn(clippy::borrow_interior_mutable_const)]
+#![allow(clippy::declare_interior_mutable_const)]
+
+// this file (mostly) replicates its `declare` counterpart. Please see it for more discussions.
+
+extern crate helper;
+
+use std::cell::Cell;
+use std::sync::atomic::AtomicUsize;
+
+enum OptionalCell {
+    Unfrozen(Cell<bool>),
+    Frozen,
+}
+
+const UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(true));
+const FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+
+fn borrow_optional_cell() {
+    let _ = &UNFROZEN_VARIANT; //~ ERROR interior mutability
+    let _ = &FROZEN_VARIANT;
+}
+
+trait AssocConsts {
+    const TO_BE_UNFROZEN_VARIANT: OptionalCell;
+    const TO_BE_FROZEN_VARIANT: OptionalCell;
+
+    const DEFAULTED_ON_UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false));
+    const DEFAULTED_ON_FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+
+    fn function() {
+        // This is the "suboptimal behavior" mentioned in `is_value_unfrozen`
+        // caused by a similar reason to unfrozen types without any default values
+        // get linted even if it has frozen variants'.
+        let _ = &Self::TO_BE_FROZEN_VARIANT; //~ ERROR interior mutable
+
+        // The lint ignores default values because an impl of this trait can set
+        // an unfrozen variant to `DEFAULTED_ON_FROZEN_VARIANT` and use the default impl for `function`.
+        let _ = &Self::DEFAULTED_ON_FROZEN_VARIANT; //~ ERROR interior mutable
+    }
+}
+
+impl AssocConsts for u64 {
+    const TO_BE_UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false));
+    const TO_BE_FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+
+    fn function() {
+        let _ = &<Self as AssocConsts>::TO_BE_UNFROZEN_VARIANT; //~ ERROR interior mutable
+        let _ = &<Self as AssocConsts>::TO_BE_FROZEN_VARIANT;
+        let _ = &Self::DEFAULTED_ON_UNFROZEN_VARIANT; //~ ERROR interior mutable
+        let _ = &Self::DEFAULTED_ON_FROZEN_VARIANT;
+    }
+}
+
+trait AssocTypes {
+    type ToBeUnfrozen;
+
+    const TO_BE_UNFROZEN_VARIANT: Option<Self::ToBeUnfrozen>;
+    const TO_BE_FROZEN_VARIANT: Option<Self::ToBeUnfrozen>;
+
+    // there's no need to test here because it's the exactly same as `trait::AssocTypes`
+    fn function();
+}
+
+impl AssocTypes for u64 {
+    type ToBeUnfrozen = AtomicUsize;
+
+    const TO_BE_UNFROZEN_VARIANT: Option<Self::ToBeUnfrozen> = Some(Self::ToBeUnfrozen::new(4)); //~ ERROR interior mutable
+    const TO_BE_FROZEN_VARIANT: Option<Self::ToBeUnfrozen> = None;
+
+    fn function() {
+        let _ = &<Self as AssocTypes>::TO_BE_UNFROZEN_VARIANT; //~ ERROR interior mutable
+        let _ = &<Self as AssocTypes>::TO_BE_FROZEN_VARIANT;
+    }
+}
+
+enum BothOfCellAndGeneric<T> {
+    Unfrozen(Cell<*const T>),
+    Generic(*const T),
+    Frozen(usize),
+}
+
+impl<T> BothOfCellAndGeneric<T> {
+    const UNFROZEN_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Unfrozen(Cell::new(std::ptr::null())); //~ ERROR interior mutable
+    const GENERIC_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Generic(std::ptr::null()); //~ ERROR interior mutable
+    const FROZEN_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Frozen(5);
+
+    fn function() {
+        let _ = &Self::UNFROZEN_VARIANT; //~ ERROR interior mutability
+        let _ = &Self::GENERIC_VARIANT; //~ ERROR interior mutability
+        let _ = &Self::FROZEN_VARIANT;
+    }
+}
+
+fn main() {
+    // constants defined in foreign crates
+    let _ = &helper::WRAPPED_PRIVATE_UNFROZEN_VARIANT; //~ ERROR interior mutability
+    let _ = &helper::WRAPPED_PRIVATE_FROZEN_VARIANT;
+}
diff --git a/tests/ui/borrow_interior_mutable_const/enums.stderr b/tests/ui/borrow_interior_mutable_const/enums.stderr
new file mode 100644
index 00000000000..654a1ee7df6
--- /dev/null
+++ b/tests/ui/borrow_interior_mutable_const/enums.stderr
@@ -0,0 +1,75 @@
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:22:14
+   |
+LL |     let _ = &UNFROZEN_VARIANT; //~ ERROR interior mutability
+   |              ^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::borrow-interior-mutable-const` implied by `-D warnings`
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:37:18
+   |
+LL |         let _ = &Self::TO_BE_FROZEN_VARIANT; //~ ERROR interior mutable
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:41:18
+   |
+LL |         let _ = &Self::DEFAULTED_ON_FROZEN_VARIANT; //~ ERROR interior mutable
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:50:18
+   |
+LL |         let _ = &<Self as AssocConsts>::TO_BE_UNFROZEN_VARIANT; //~ ERROR interior mutable
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:52:18
+   |
+LL |         let _ = &Self::DEFAULTED_ON_UNFROZEN_VARIANT; //~ ERROR interior mutable
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:74:18
+   |
+LL |         let _ = &<Self as AssocTypes>::TO_BE_UNFROZEN_VARIANT; //~ ERROR interior mutable
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:91:18
+   |
+LL |         let _ = &Self::UNFROZEN_VARIANT; //~ ERROR interior mutability
+   |                  ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:92:18
+   |
+LL |         let _ = &Self::GENERIC_VARIANT; //~ ERROR interior mutability
+   |                  ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: a `const` item with interior mutability should not be borrowed
+  --> $DIR/enums.rs:99:14
+   |
+LL |     let _ = &helper::WRAPPED_PRIVATE_UNFROZEN_VARIANT; //~ ERROR interior mutability
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: aborting due to 9 previous errors
+
diff --git a/tests/ui/declare_interior_mutable_const/enums.rs b/tests/ui/declare_interior_mutable_const/enums.rs
new file mode 100644
index 00000000000..f44518694b8
--- /dev/null
+++ b/tests/ui/declare_interior_mutable_const/enums.rs
@@ -0,0 +1,123 @@
+#![warn(clippy::declare_interior_mutable_const)]
+
+use std::cell::Cell;
+use std::sync::atomic::AtomicUsize;
+
+enum OptionalCell {
+    Unfrozen(Cell<bool>),
+    Frozen,
+}
+
+// a constant with enums should be linted only when the used variant is unfrozen (#3962).
+const UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(true)); //~ ERROR interior mutable
+const FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+
+const fn unfrozen_variant() -> OptionalCell {
+    OptionalCell::Unfrozen(Cell::new(false))
+}
+
+const fn frozen_variant() -> OptionalCell {
+    OptionalCell::Frozen
+}
+
+const UNFROZEN_VARIANT_FROM_FN: OptionalCell = unfrozen_variant(); //~ ERROR interior mutable
+const FROZEN_VARIANT_FROM_FN: OptionalCell = frozen_variant();
+
+enum NestedInnermost {
+    Unfrozen(AtomicUsize),
+    Frozen,
+}
+
+struct NestedInner {
+    inner: NestedInnermost,
+}
+
+enum NestedOuter {
+    NestedInner(NestedInner),
+    NotNested(usize),
+}
+
+struct NestedOutermost {
+    outer: NestedOuter,
+}
+
+// a constant with enums should be linted according to its value, no matter how structs involve.
+const NESTED_UNFROZEN_VARIANT: NestedOutermost = NestedOutermost {
+    outer: NestedOuter::NestedInner(NestedInner {
+        inner: NestedInnermost::Unfrozen(AtomicUsize::new(2)),
+    }),
+}; //~ ERROR interior mutable
+const NESTED_FROZEN_VARIANT: NestedOutermost = NestedOutermost {
+    outer: NestedOuter::NestedInner(NestedInner {
+        inner: NestedInnermost::Frozen,
+    }),
+};
+
+trait AssocConsts {
+    // When there's no default value, lint it only according to its type.
+    // Further details are on the corresponding code (`NonCopyConst::check_trait_item`).
+    const TO_BE_UNFROZEN_VARIANT: OptionalCell; //~ ERROR interior mutable
+    const TO_BE_FROZEN_VARIANT: OptionalCell; //~ ERROR interior mutable
+
+    // Lint default values accordingly.
+    const DEFAULTED_ON_UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false)); //~ ERROR interior mutable
+    const DEFAULTED_ON_FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+}
+
+// The lint doesn't trigger for an assoc constant in a trait impl with an unfrozen type even if it
+// has enums. Further details are on the corresponding code in 'NonCopyConst::check_impl_item'.
+impl AssocConsts for u64 {
+    const TO_BE_UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false));
+    const TO_BE_FROZEN_VARIANT: OptionalCell = OptionalCell::Frozen;
+
+    // even if this sets an unfrozen variant, the lint ignores it.
+    const DEFAULTED_ON_FROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false));
+}
+
+// At first, I thought I'd need to check every patterns in `trait.rs`; but, what matters
+// here are values; and I think substituted generics at definitions won't appear in MIR.
+trait AssocTypes {
+    type ToBeUnfrozen;
+
+    const TO_BE_UNFROZEN_VARIANT: Option<Self::ToBeUnfrozen>;
+    const TO_BE_FROZEN_VARIANT: Option<Self::ToBeUnfrozen>;
+}
+
+impl AssocTypes for u64 {
+    type ToBeUnfrozen = AtomicUsize;
+
+    const TO_BE_UNFROZEN_VARIANT: Option<Self::ToBeUnfrozen> = Some(Self::ToBeUnfrozen::new(4)); //~ ERROR interior mutable
+    const TO_BE_FROZEN_VARIANT: Option<Self::ToBeUnfrozen> = None;
+}
+
+// Use raw pointers since direct generics have a false negative at the type level.
+enum BothOfCellAndGeneric<T> {
+    Unfrozen(Cell<*const T>),
+    Generic(*const T),
+    Frozen(usize),
+}
+
+impl<T> BothOfCellAndGeneric<T> {
+    const UNFROZEN_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Unfrozen(Cell::new(std::ptr::null())); //~ ERROR interior mutable
+
+    // This is a false positive. The argument about this is on `is_value_unfrozen_raw`
+    const GENERIC_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Generic(std::ptr::null()); //~ ERROR interior mutable
+
+    const FROZEN_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Frozen(5);
+
+    // This is what is likely to be a false negative when one tries to fix
+    // the `GENERIC_VARIANT` false positive.
+    const NO_ENUM: Cell<*const T> = Cell::new(std::ptr::null()); //~ ERROR interior mutable
+}
+
+// associated types here is basically the same as the one above.
+trait BothOfCellAndGenericWithAssocType {
+    type AssocType;
+
+    const UNFROZEN_VARIANT: BothOfCellAndGeneric<Self::AssocType> =
+        BothOfCellAndGeneric::Unfrozen(Cell::new(std::ptr::null())); //~ ERROR interior mutable
+    const GENERIC_VARIANT: BothOfCellAndGeneric<Self::AssocType> = BothOfCellAndGeneric::Generic(std::ptr::null()); //~ ERROR interior mutable
+    const FROZEN_VARIANT: BothOfCellAndGeneric<Self::AssocType> = BothOfCellAndGeneric::Frozen(5);
+}
+
+fn main() {}
diff --git a/tests/ui/declare_interior_mutable_const/enums.stderr b/tests/ui/declare_interior_mutable_const/enums.stderr
new file mode 100644
index 00000000000..84198d54615
--- /dev/null
+++ b/tests/ui/declare_interior_mutable_const/enums.stderr
@@ -0,0 +1,89 @@
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:12:1
+   |
+LL | const UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(true)); //~ ERROR interior mutable
+   | -----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | make this a static item (maybe with lazy_static)
+   |
+   = note: `-D clippy::declare-interior-mutable-const` implied by `-D warnings`
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:23:1
+   |
+LL | const UNFROZEN_VARIANT_FROM_FN: OptionalCell = unfrozen_variant(); //~ ERROR interior mutable
+   | -----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | make this a static item (maybe with lazy_static)
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:45:1
+   |
+LL |   const NESTED_UNFROZEN_VARIANT: NestedOutermost = NestedOutermost {
+   |   ^----
+   |   |
+   |  _make this a static item (maybe with lazy_static)
+   | |
+LL | |     outer: NestedOuter::NestedInner(NestedInner {
+LL | |         inner: NestedInnermost::Unfrozen(AtomicUsize::new(2)),
+LL | |     }),
+LL | | }; //~ ERROR interior mutable
+   | |__^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:59:5
+   |
+LL |     const TO_BE_UNFROZEN_VARIANT: OptionalCell; //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:60:5
+   |
+LL |     const TO_BE_FROZEN_VARIANT: OptionalCell; //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:63:5
+   |
+LL |     const DEFAULTED_ON_UNFROZEN_VARIANT: OptionalCell = OptionalCell::Unfrozen(Cell::new(false)); //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:89:5
+   |
+LL |     const TO_BE_UNFROZEN_VARIANT: Option<Self::ToBeUnfrozen> = Some(Self::ToBeUnfrozen::new(4)); //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:101:5
+   |
+LL |     const UNFROZEN_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Unfrozen(Cell::new(std::ptr::null())); //~ ERROR interior mut...
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:104:5
+   |
+LL |     const GENERIC_VARIANT: BothOfCellAndGeneric<T> = BothOfCellAndGeneric::Generic(std::ptr::null()); //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:110:5
+   |
+LL |     const NO_ENUM: Cell<*const T> = Cell::new(std::ptr::null()); //~ ERROR interior mutable
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:117:5
+   |
+LL | /     const UNFROZEN_VARIANT: BothOfCellAndGeneric<Self::AssocType> =
+LL | |         BothOfCellAndGeneric::Unfrozen(Cell::new(std::ptr::null())); //~ ERROR interior mutable
+   | |____________________________________________________________________^
+
+error: a `const` item should never be interior mutable
+  --> $DIR/enums.rs:119:5
+   |
+LL |     const GENERIC_VARIANT: BothOfCellAndGeneric<Self::AssocType> = BothOfCellAndGeneric::Generic(std::ptr::null()); //~ ERROR interior mu...
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 12 previous errors
+