Clarify `invalid_reference_casting` lint around interior mutable types

5 files changed, 68 insertions, 32 deletions

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 7377c6e2f35..494fb15793a 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -319,6 +319,8 @@ lint_invalid_reference_casting_borrow_as_mut = casting `&T` to `&mut T` is undef
 
 lint_invalid_reference_casting_note_book = for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
+lint_invalid_reference_casting_note_ty_has_interior_mutability = even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
+
 lint_lintpass_by_hand = implementing `LintPass` by hand
     .help = try using `declare_lint_pass!` or `impl_lint_pass!` instead
 
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index c091c260a47..b8310133ed9 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -771,12 +771,16 @@ pub enum InvalidReferenceCastingDiag {
     BorrowAsMut {
         #[label]
         orig_cast: Option<Span>,
+        #[note(lint_invalid_reference_casting_note_ty_has_interior_mutability)]
+        ty_has_interior_mutability: Option<()>,
     },
     #[diag(lint_invalid_reference_casting_assign_to_ref)]
     #[note(lint_invalid_reference_casting_note_book)]
     AssignToRef {
         #[label]
         orig_cast: Option<Span>,
+        #[note(lint_invalid_reference_casting_note_ty_has_interior_mutability)]
+        ty_has_interior_mutability: Option<()>,
     },
 }
 
diff --git a/compiler/rustc_lint/src/reference_casting.rs b/compiler/rustc_lint/src/reference_casting.rs
index 39def599be8..0c52fbaf78c 100644
--- a/compiler/rustc_lint/src/reference_casting.rs
+++ b/compiler/rustc_lint/src/reference_casting.rs
@@ -43,19 +43,19 @@ impl<'tcx> LateLintPass<'tcx> for InvalidReferenceCasting {
 
         let init = cx.expr_or_init(e);
 
-        let orig_cast = if is_cast_from_const_to_mut(cx, init) {
-            if init.span != e.span { Some(init.span) } else { None }
-        } else {
+        let Some(ty_has_interior_mutability) = is_cast_from_const_to_mut(cx, init) else {
             return;
         };
+        let orig_cast = if init.span != e.span { Some(init.span) } else { None };
+        let ty_has_interior_mutability = ty_has_interior_mutability.then_some(());
 
         cx.emit_spanned_lint(
             INVALID_REFERENCE_CASTING,
             expr.span,
             if is_assignment {
-                InvalidReferenceCastingDiag::AssignToRef { orig_cast }
+                InvalidReferenceCastingDiag::AssignToRef { orig_cast, ty_has_interior_mutability }
             } else {
-                InvalidReferenceCastingDiag::BorrowAsMut { orig_cast }
+                InvalidReferenceCastingDiag::BorrowAsMut { orig_cast, ty_has_interior_mutability }
             },
         );
     }
@@ -104,7 +104,10 @@ fn is_operation_we_care_about<'tcx>(
     deref_assign_or_addr_of(e).or_else(|| ptr_write(cx, e))
 }
 
-fn is_cast_from_const_to_mut<'tcx>(cx: &LateContext<'tcx>, orig_expr: &'tcx Expr<'tcx>) -> bool {
+fn is_cast_from_const_to_mut<'tcx>(
+    cx: &LateContext<'tcx>,
+    orig_expr: &'tcx Expr<'tcx>,
+) -> Option<bool> {
     let mut need_check_freeze = false;
     let mut e = orig_expr;
 
@@ -112,7 +115,7 @@ fn is_cast_from_const_to_mut<'tcx>(cx: &LateContext<'tcx>, orig_expr: &'tcx Expr
 
     // Bail out early if the end type is **not** a mutable pointer.
     if !matches!(end_ty.kind(), ty::RawPtr(TypeAndMut { ty: _, mutbl: Mutability::Mut })) {
-        return false;
+        return None;
     }
 
     loop {
@@ -155,10 +158,11 @@ fn is_cast_from_const_to_mut<'tcx>(cx: &LateContext<'tcx>, orig_expr: &'tcx Expr
         //
         // We also consider non concrete skeleton types (ie generics)
         // to be an issue since there is no way to make it safe for abitrary types.
-        !need_check_freeze
-            || inner_ty.is_freeze(cx.tcx, cx.param_env)
-            || !inner_ty.has_concrete_skeleton()
+        let inner_ty_has_interior_mutability =
+            !inner_ty.is_freeze(cx.tcx, cx.param_env) && inner_ty.has_concrete_skeleton();
+        (!need_check_freeze || !inner_ty_has_interior_mutability)
+            .then_some(inner_ty_has_interior_mutability)
     } else {
-        false
+        None
     }
 }
diff --git a/tests/ui/lint/reference_casting.rs b/tests/ui/lint/reference_casting.rs
index fba8789e999..25e0c75f70d 100644
--- a/tests/ui/lint/reference_casting.rs
+++ b/tests/ui/lint/reference_casting.rs
@@ -64,6 +64,10 @@ unsafe fn ref_to_mut() {
     let _num = &mut *num;
     //~^ ERROR casting `&T` to `&mut T` is undefined behavior
 
+    let cell = &std::cell::UnsafeCell::new(0);
+    let _num = &mut *(cell as *const _ as *mut i32);
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+
     unsafe fn generic_ref_cast_mut<T>(this: &T) -> &mut T {
         &mut *((this as *const _) as *mut _)
         //~^ ERROR casting `&T` to `&mut T` is undefined behavior
@@ -106,6 +110,8 @@ unsafe fn assign_to_ref() {
         std::mem::transmute::<*const i32, *mut i32>(num),
         -1i32,
     );
+    *((&std::cell::UnsafeCell::new(0)) as *const _ as *mut i32) = 5;
+    //~^ ERROR assigning to `&T` is undefined behavior
 
     let value = num as *const i32 as *mut i32;
     *value = 1;
@@ -148,6 +154,8 @@ unsafe fn no_warn() {
     *RAW_PTR = 42; // RAW_PTR is defined outside the function body,
                    // make sure we don't ICE on it when trying to
                    // determine if we should lint on it or not.
+    let cell = &std::cell::UnsafeCell::new(0);
+    let _num = &mut *(cell.get() as *mut i32);
 
     fn safe_as_mut<T>(x: &std::cell::UnsafeCell<T>) -> &mut T {
         unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *const _) }
diff --git a/tests/ui/lint/reference_casting.stderr b/tests/ui/lint/reference_casting.stderr
index 8f89cf9805b..8d5f8da6852 100644
--- a/tests/ui/lint/reference_casting.stderr
+++ b/tests/ui/lint/reference_casting.stderr
@@ -158,7 +158,16 @@ LL |     let _num = &mut *num;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:68:9
+  --> $DIR/reference_casting.rs:68:16
+   |
+LL |     let _num = &mut *(cell as *const _ as *mut i32);
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
+   = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
+
+error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:72:9
    |
 LL |         &mut *((this as *const _) as *mut _)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -166,7 +175,7 @@ LL |         &mut *((this as *const _) as *mut _)
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:73:18
+  --> $DIR/reference_casting.rs:77:18
    |
 LL |         unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *const _) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -174,7 +183,7 @@ LL |         unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *con
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:78:18
+  --> $DIR/reference_casting.rs:82:18
    |
 LL |         unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *const _) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -182,7 +191,7 @@ LL |         unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *con
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:88:5
+  --> $DIR/reference_casting.rs:92:5
    |
 LL |     *(a as *const _ as *mut _) = String::from("Replaced");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -190,7 +199,7 @@ LL |     *(a as *const _ as *mut _) = String::from("Replaced");
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:90:5
+  --> $DIR/reference_casting.rs:94:5
    |
 LL |     *(a as *const _ as *mut String) += " world";
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -198,7 +207,7 @@ LL |     *(a as *const _ as *mut String) += " world";
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:92:5
+  --> $DIR/reference_casting.rs:96:5
    |
 LL |     *std::ptr::from_ref(num).cast_mut() += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -206,7 +215,7 @@ LL |     *std::ptr::from_ref(num).cast_mut() += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:94:5
+  --> $DIR/reference_casting.rs:98:5
    |
 LL |     *std::ptr::from_ref({ num }).cast_mut() += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -214,7 +223,7 @@ LL |     *std::ptr::from_ref({ num }).cast_mut() += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:96:5
+  --> $DIR/reference_casting.rs:100:5
    |
 LL |     *{ std::ptr::from_ref(num) }.cast_mut() += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -222,7 +231,7 @@ LL |     *{ std::ptr::from_ref(num) }.cast_mut() += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:98:5
+  --> $DIR/reference_casting.rs:102:5
    |
 LL |     *(std::ptr::from_ref({ num }) as *mut i32) += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -230,7 +239,7 @@ LL |     *(std::ptr::from_ref({ num }) as *mut i32) += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:100:5
+  --> $DIR/reference_casting.rs:104:5
    |
 LL |     *std::mem::transmute::<_, *mut i32>(num) += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -238,7 +247,7 @@ LL |     *std::mem::transmute::<_, *mut i32>(num) += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:102:5
+  --> $DIR/reference_casting.rs:106:5
    |
 LL |     *(std::mem::transmute::<_, *mut i32>(num) as *mut i32) += 1;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -246,7 +255,7 @@ LL |     *(std::mem::transmute::<_, *mut i32>(num) as *mut i32) += 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:104:5
+  --> $DIR/reference_casting.rs:108:5
    |
 LL | /     std::ptr::write(
 LL | |
@@ -258,7 +267,16 @@ LL | |     );
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:111:5
+  --> $DIR/reference_casting.rs:113:5
+   |
+LL |     *((&std::cell::UnsafeCell::new(0)) as *const _ as *mut i32) = 5;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
+   = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
+
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:117:5
    |
 LL |     let value = num as *const i32 as *mut i32;
    |                 ----------------------------- casting happend here
@@ -268,7 +286,7 @@ LL |     *value = 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:114:5
+  --> $DIR/reference_casting.rs:120:5
    |
 LL |     let value = num as *const i32 as *mut i32;
    |                 ----------------------------- casting happend here
@@ -279,7 +297,7 @@ LL |     *value_rebind = 1;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:116:5
+  --> $DIR/reference_casting.rs:122:5
    |
 LL |     *(num as *const i32).cast::<i32>().cast_mut() = 2;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -287,7 +305,7 @@ LL |     *(num as *const i32).cast::<i32>().cast_mut() = 2;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:118:5
+  --> $DIR/reference_casting.rs:124:5
    |
 LL |     *(num as *const _ as usize as *mut i32) = 2;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -295,7 +313,7 @@ LL |     *(num as *const _ as usize as *mut i32) = 2;
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:120:5
+  --> $DIR/reference_casting.rs:126:5
    |
 LL |     let value = num as *const i32 as *mut i32;
    |                 ----------------------------- casting happend here
@@ -306,7 +324,7 @@ LL |     std::ptr::write(value, 2);
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:122:5
+  --> $DIR/reference_casting.rs:128:5
    |
 LL |     let value = num as *const i32 as *mut i32;
    |                 ----------------------------- casting happend here
@@ -317,7 +335,7 @@ LL |     std::ptr::write_unaligned(value, 2);
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:124:5
+  --> $DIR/reference_casting.rs:130:5
    |
 LL |     let value = num as *const i32 as *mut i32;
    |                 ----------------------------- casting happend here
@@ -328,12 +346,12 @@ LL |     std::ptr::write_volatile(value, 2);
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
 error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:128:9
+  --> $DIR/reference_casting.rs:134:9
    |
 LL |         *(this as *const _ as *mut _) = a;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
 
-error: aborting due to 38 previous errors
+error: aborting due to 40 previous errors