Uplift clippy::cast_ref_to_mut lint

compiler/rustc_lint/messages.ftl

@@ -155,6 +155,8 @@ lint_builtin_unused_doc_comment = unused doc comment
 lint_builtin_while_true = denote infinite loops with `loop {"{"} ... {"}"}`
     .suggestion = use `loop`
 
+lint_cast_ref_to_mut = casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
+
 lint_check_name_deprecated = lint name `{$lint_name}` is deprecated and does not have an effect anymore. Use: {$new_name}
 
 lint_check_name_unknown = unknown lint: `{$lint_name}`

compiler/rustc_lint/src/cast_ref_to_mut.rs

@@ -0,0 +1,72 @@
+use rustc_ast::Mutability;
+use rustc_hir::{Expr, ExprKind, MutTy, TyKind, UnOp};
+use rustc_middle::ty;
+use rustc_span::sym;
+
+use crate::{lints::CastRefToMutDiag, LateContext, LateLintPass, LintContext};
+
+declare_lint! {
+    /// The `cast_ref_to_mut` lint checks for casts of `&T` to `&mut T`
+    /// without using interior mutability.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// fn x(r: &i32) {
+    ///     unsafe {
+    ///         *(r as *const i32 as *mut i32) += 1;
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Casting `&T` to `&mut T` without using interior mutability is undefined behavior,
+    /// as it's a violation of Rust reference aliasing requirements.
+    ///
+    /// `UnsafeCell` is the only way to obtain aliasable data that is considered
+    /// mutable.
+    CAST_REF_TO_MUT,
+    Deny,
+    "casts of `&T` to `&mut T` without interior mutability"
+}
+
+declare_lint_pass!(CastRefToMut => [CAST_REF_TO_MUT]);
+
+impl<'tcx> LateLintPass<'tcx> for CastRefToMut {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        let ExprKind::Unary(UnOp::Deref, e) = &expr.kind else { return; };
+
+        let e = e.peel_blocks();
+        let e = if let ExprKind::Cast(e, t) = e.kind
+            && let TyKind::Ptr(MutTy { mutbl: Mutability::Mut, .. }) = t.kind {
+            e
+        } else if let ExprKind::MethodCall(_, expr, [], _) = e.kind
+            && let Some(def_id) = cx.typeck_results().type_dependent_def_id(e.hir_id)
+            && cx.tcx.is_diagnostic_item(sym::ptr_cast_mut, def_id) {
+            expr
+        } else {
+            return;
+        };
+
+        let e = e.peel_blocks();
+        let e = if let ExprKind::Cast(e, t) = e.kind
+            && let TyKind::Ptr(MutTy { mutbl: Mutability::Not, .. }) = t.kind {
+            e
+        } else if let ExprKind::Call(path, [arg]) = e.kind
+            && let ExprKind::Path(ref qpath) = path.kind
+            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
+            && cx.tcx.is_diagnostic_item(sym::ptr_from_ref, def_id) {
+            arg
+        } else {
+            return;
+        };
+
+        let e = e.peel_blocks();
+        if let ty::Ref(..) = cx.typeck_results().node_type(e.hir_id).kind() {
+            cx.emit_spanned_lint(CAST_REF_TO_MUT, expr.span, CastRefToMutDiag);
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -50,6 +50,7 @@ extern crate tracing;
 
 mod array_into_iter;
 pub mod builtin;
+mod cast_ref_to_mut;
 mod context;
 mod deref_into_dyn_supertrait;
 mod drop_forget_useless;
@@ -97,6 +98,7 @@ use rustc_span::Span;
 
 use array_into_iter::ArrayIntoIter;
 use builtin::*;
+use cast_ref_to_mut::*;
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
@@ -214,6 +216,7 @@ late_lint_methods!(
             BoxPointers: BoxPointers,
             PathStatements: PathStatements,
             LetUnderscore: LetUnderscore,
+            CastRefToMut: CastRefToMut,
             // Depends on referenced function signatures in expressions
             UnusedResults: UnusedResults,
             NonUpperCaseGlobals: NonUpperCaseGlobals,

compiler/rustc_lint/src/lints.rs

@@ -718,6 +718,11 @@ pub enum InvalidFromUtf8Diag {
     },
 }
 
+// cast_ref_to_mut.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_cast_ref_to_mut)]
+pub struct CastRefToMutDiag;
+
 // hidden_unicode_codepoints.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_hidden_unicode_codepoints)]

compiler/rustc_span/src/symbol.rs

@@ -1146,6 +1146,8 @@ symbols! {
         profiler_builtins,
         profiler_runtime,
         ptr,
+        ptr_cast_mut,
+        ptr_from_ref,
         ptr_guaranteed_cmp,
         ptr_mask,
         ptr_null,

library/core/src/ptr/const_ptr.rs

@@ -104,6 +104,7 @@ impl<T: ?Sized> *const T {
     /// refactored.
     #[stable(feature = "ptr_const_cast", since = "1.65.0")]
     #[rustc_const_stable(feature = "ptr_const_cast", since = "1.65.0")]
+    #[rustc_diagnostic_item = "ptr_cast_mut"]
     #[inline(always)]
     pub const fn cast_mut(self) -> *mut T {
         self as _

library/core/src/ptr/mod.rs

@@ -698,6 +698,7 @@ where
 #[inline(always)]
 #[must_use]
 #[unstable(feature = "ptr_from_ref", issue = "106116")]
+#[rustc_diagnostic_item = "ptr_from_ref"]
 pub const fn from_ref<T: ?Sized>(r: &T) -> *const T {
     r
 }

src/tools/clippy/clippy_lints/src/casts/mod.rs

@@ -9,7 +9,6 @@ mod cast_possible_truncation;
 mod cast_possible_wrap;
 mod cast_precision_loss;
 mod cast_ptr_alignment;
-mod cast_ref_to_mut;
 mod cast_sign_loss;
 mod cast_slice_different_sizes;
 mod cast_slice_from_raw_parts;
@@ -330,41 +329,6 @@ declare_clippy_lint! {
     "casting a function pointer to any integer type"
 }
 
-declare_clippy_lint! {
-    /// ### What it does
-    /// Checks for casts of `&T` to `&mut T` anywhere in the code.
-    ///
-    /// ### Why is this bad?
-    /// It’s basically guaranteed to be undefined behavior.
-    /// `UnsafeCell` is the only way to obtain aliasable data that is considered
-    /// mutable.
-    ///
-    /// ### Example
-    /// ```rust,ignore
-    /// fn x(r: &i32) {
-    ///     unsafe {
-    ///         *(r as *const _ as *mut _) += 1;
-    ///     }
-    /// }
-    /// ```
-    ///
-    /// Instead consider using interior mutability types.
-    ///
-    /// ```rust
-    /// use std::cell::UnsafeCell;
-    ///
-    /// fn x(r: &UnsafeCell<i32>) {
-    ///     unsafe {
-    ///         *r.get() += 1;
-    ///     }
-    /// }
-    /// ```
-    #[clippy::version = "1.33.0"]
-    pub CAST_REF_TO_MUT,
-    correctness,
-    "a cast of reference to a mutable pointer"
-}
-
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for expressions where a character literal is cast
@@ -680,7 +644,6 @@ impl_lint_pass!(Casts => [
     CAST_POSSIBLE_TRUNCATION,
     CAST_POSSIBLE_WRAP,
     CAST_LOSSLESS,
-    CAST_REF_TO_MUT,
     CAST_PTR_ALIGNMENT,
     CAST_SLICE_DIFFERENT_SIZES,
     UNNECESSARY_CAST,
@@ -747,7 +710,6 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
             }
         }
 
-        cast_ref_to_mut::check(cx, expr);
         cast_ptr_alignment::check(cx, expr);
         char_lit_as_u8::check(cx, expr);
         ptr_as_ptr::check(cx, expr, &self.msrv);

src/tools/clippy/clippy_lints/src/declared_lints.rs

@@ -81,7 +81,6 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::casts::CAST_POSSIBLE_WRAP_INFO,
     crate::casts::CAST_PRECISION_LOSS_INFO,
     crate::casts::CAST_PTR_ALIGNMENT_INFO,
-    crate::casts::CAST_REF_TO_MUT_INFO,
     crate::casts::CAST_SIGN_LOSS_INFO,
     crate::casts::CAST_SLICE_DIFFERENT_SIZES_INFO,
     crate::casts::CAST_SLICE_FROM_RAW_PARTS_INFO,

src/tools/clippy/clippy_lints/src/renamed_lints.rs

@@ -31,6 +31,7 @@ pub static RENAMED_LINTS: &[(&str, &str)] = &[
     ("clippy::stutter", "clippy::module_name_repetitions"),
     ("clippy::to_string_in_display", "clippy::recursive_format_impl"),
     ("clippy::zero_width_space", "clippy::invisible_characters"),
+    ("clippy::cast_ref_to_mut", "cast_ref_to_mut"),
     ("clippy::clone_double_ref", "suspicious_double_ref_op"),
     ("clippy::drop_bounds", "drop_bounds"),
     ("clippy::drop_copy", "dropping_copy_types"),

src/tools/clippy/tests/ui/rename.fixed

@@ -29,6 +29,7 @@
 #![allow(clippy::recursive_format_impl)]
 #![allow(clippy::invisible_characters)]
 #![allow(suspicious_double_ref_op)]
+#![allow(cast_ref_to_mut)]
 #![allow(drop_bounds)]
 #![allow(dropping_copy_types)]
 #![allow(dropping_references)]
@@ -76,6 +77,7 @@
 #![warn(clippy::module_name_repetitions)]
 #![warn(clippy::recursive_format_impl)]
 #![warn(clippy::invisible_characters)]
+#![warn(cast_ref_to_mut)]
 #![warn(suspicious_double_ref_op)]
 #![warn(drop_bounds)]
 #![warn(dropping_copy_types)]

src/tools/clippy/tests/ui/rename.rs

@@ -29,6 +29,7 @@
 #![allow(clippy::recursive_format_impl)]
 #![allow(clippy::invisible_characters)]
 #![allow(suspicious_double_ref_op)]
+#![allow(cast_ref_to_mut)]
 #![allow(drop_bounds)]
 #![allow(dropping_copy_types)]
 #![allow(dropping_references)]
@@ -76,6 +77,7 @@
 #![warn(clippy::stutter)]
 #![warn(clippy::to_string_in_display)]
 #![warn(clippy::zero_width_space)]
+#![warn(clippy::cast_ref_to_mut)]
 #![warn(clippy::clone_double_ref)]
 #![warn(clippy::drop_bounds)]
 #![warn(clippy::drop_copy)]