Auto merge of #112431 - Urgau:cast_ref_to_mut_improvments, r=Nilstrieb

Improve `invalid_reference_casting` lint

This PR is a follow-up to https://github.com/rust-lang/rust/pull/111567 and https://github.com/rust-lang/rust/pull/113422.

This PR does multiple things:
 - First it adds support for deferred de-reference, the goal is to support code like this, where the casting and de-reference are not done on the same expression
    ```rust
    let myself = self as *const Self as *mut Self;
    *myself = Self::Ready(value);
    ```
 - Second it does not lint anymore on SB/TB UB code by only checking assignments (`=`, `+=`, ...) and creation of mutable references `&mut *`
 - Thirdly it greatly improves the diagnostics in particular for cast from `&mut` to `&mut` or assignments
 - ~~And lastly it renames the lint from `cast_ref_to_mut` to `invalid_reference_casting` which is more consistent with the ["rules"](https://github.com/rust-lang/rust-clippy/issues/2845) and also more consistent with what the lint checks~~ *https://github.com/rust-lang/rust/pull/113422*

This PR is best reviewed commit by commit.

r? compiler

11 files changed, 240 insertions, 111 deletions

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 16e17fc9d6a..f482e3d7c12 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -318,7 +318,11 @@ lint_invalid_nan_comparisons_eq_ne = incorrect NaN comparison, NaN cannot be dir
 
 lint_invalid_nan_comparisons_lt_le_gt_ge = incorrect NaN comparison, NaN is not orderable
 
-lint_invalid_reference_casting = casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
+lint_invalid_reference_casting_assign_to_ref = assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+    .label = casting happend here
+
+lint_invalid_reference_casting_borrow_as_mut = casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
+    .label = casting happend here
 
 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/lib.rs b/compiler/rustc_lint/src/lib.rs
index 53089294fe2..96fd3ccf774 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -218,7 +218,7 @@ late_lint_methods!(
             BoxPointers: BoxPointers,
             PathStatements: PathStatements,
             LetUnderscore: LetUnderscore,
-            InvalidReferenceCasting: InvalidReferenceCasting,
+            InvalidReferenceCasting: InvalidReferenceCasting::default(),
             // Depends on referenced function signatures in expressions
             UnusedResults: UnusedResults,
             NonUpperCaseGlobals: NonUpperCaseGlobals,
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 968172693a9..a6a48bf4ffa 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -745,8 +745,18 @@ pub enum InvalidFromUtf8Diag {
 
 // reference_casting.rs
 #[derive(LintDiagnostic)]
-#[diag(lint_invalid_reference_casting)]
-pub struct InvalidReferenceCastingDiag;
+pub enum InvalidReferenceCastingDiag {
+    #[diag(lint_invalid_reference_casting_borrow_as_mut)]
+    BorrowAsMut {
+        #[label]
+        orig_cast: Option<Span>,
+    },
+    #[diag(lint_invalid_reference_casting_assign_to_ref)]
+    AssignToRef {
+        #[label]
+        orig_cast: Option<Span>,
+    },
+}
 
 // hidden_unicode_codepoints.rs
 #[derive(LintDiagnostic)]
diff --git a/compiler/rustc_lint/src/reference_casting.rs b/compiler/rustc_lint/src/reference_casting.rs
index d343aaf35d5..ed3d4721049 100644
--- a/compiler/rustc_lint/src/reference_casting.rs
+++ b/compiler/rustc_lint/src/reference_casting.rs
@@ -1,7 +1,8 @@
 use rustc_ast::Mutability;
-use rustc_hir::{Expr, ExprKind, MutTy, TyKind, UnOp};
-use rustc_middle::ty;
-use rustc_span::sym;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_hir::{def::Res, Expr, ExprKind, HirId, Local, QPath, StmtKind, UnOp};
+use rustc_middle::ty::{self, TypeAndMut};
+use rustc_span::{sym, Span};
 
 use crate::{lints::InvalidReferenceCastingDiag, LateContext, LateLintPass, LintContext};
 
@@ -12,7 +13,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail
-    /// # #![deny(invalid_reference_casting)]
     /// fn x(r: &i32) {
     ///     unsafe {
     ///         *(r as *const i32 as *mut i32) += 1;
@@ -30,46 +30,103 @@ declare_lint! {
     /// `UnsafeCell` is the only way to obtain aliasable data that is considered
     /// mutable.
     INVALID_REFERENCE_CASTING,
-    Allow,
+    Deny,
     "casts of `&T` to `&mut T` without interior mutability"
 }
 
-declare_lint_pass!(InvalidReferenceCasting => [INVALID_REFERENCE_CASTING]);
+#[derive(Default)]
+pub struct InvalidReferenceCasting {
+    casted: FxHashMap<HirId, Span>,
+}
+
+impl_lint_pass!(InvalidReferenceCasting => [INVALID_REFERENCE_CASTING]);
 
 impl<'tcx> LateLintPass<'tcx> for InvalidReferenceCasting {
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
-        let ExprKind::Unary(UnOp::Deref, e) = &expr.kind else {
+    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx rustc_hir::Stmt<'tcx>) {
+        let StmtKind::Local(local) = stmt.kind else {
+            return;
+        };
+        let Local { init: Some(init), els: None, .. } = local 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) {
+        if is_cast_from_const_to_mut(cx, init) {
+            self.casted.insert(local.pat.hir_id, init.span);
+        }
+    }
+
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        // &mut <expr>
+        let inner = if let ExprKind::AddrOf(_, Mutability::Mut, expr) = expr.kind {
+            expr
+        // <expr> = ...
+        } else if let ExprKind::Assign(expr, _, _) = expr.kind {
+            expr
+        // <expr> += ...
+        } else if let ExprKind::AssignOp(_, expr, _) = expr.kind {
             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
+        let ExprKind::Unary(UnOp::Deref, e) = &inner.kind else {
+            return;
+        };
+
+        let orig_cast = if is_cast_from_const_to_mut(cx, e) {
+            None
+        } else if let ExprKind::Path(QPath::Resolved(_, path)) = e.kind
+            && let Res::Local(hir_id) = &path.res
+            && let Some(orig_cast) = self.casted.get(hir_id) {
+            Some(*orig_cast)
         } else {
             return;
         };
 
-        let e = e.peel_blocks();
-        if let ty::Ref(..) = cx.typeck_results().node_type(e.hir_id).kind() {
-            cx.emit_spanned_lint(INVALID_REFERENCE_CASTING, expr.span, InvalidReferenceCastingDiag);
-        }
+        cx.emit_spanned_lint(
+            INVALID_REFERENCE_CASTING,
+            expr.span,
+            if matches!(expr.kind, ExprKind::AddrOf(..)) {
+                InvalidReferenceCastingDiag::BorrowAsMut { orig_cast }
+            } else {
+                InvalidReferenceCastingDiag::AssignToRef { orig_cast }
+            },
+        );
     }
 }
+
+fn is_cast_from_const_to_mut<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'tcx>) -> bool {
+    let e = e.peel_blocks();
+
+    // <expr> as *mut ...
+    let e = if let ExprKind::Cast(e, t) = e.kind
+        && let ty::RawPtr(TypeAndMut { mutbl: Mutability::Mut, .. }) = cx.typeck_results().node_type(t.hir_id).kind() {
+        e
+    // <expr>.cast_mut()
+    } 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 false;
+    };
+
+    let e = e.peel_blocks();
+
+    // <expr> as *const ...
+    let e = if let ExprKind::Cast(e, t) = e.kind
+        && let ty::RawPtr(TypeAndMut { mutbl: Mutability::Not, .. }) = cx.typeck_results().node_type(t.hir_id).kind() {
+        e
+    // ptr::from_ref(<expr>)
+    } 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 false;
+    };
+
+    let e = e.peel_blocks();
+    matches!(cx.typeck_results().node_type(e.hir_id).kind(), ty::Ref(_, _, Mutability::Not))
+}
diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs
index 909b32547e7..bf4c682d33e 100644
--- a/library/core/src/cell.rs
+++ b/library/core/src/cell.rs
@@ -1893,7 +1893,8 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 /// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
 /// same memory layout, the following is not allowed and undefined behavior:
 ///
-/// ```rust,no_run
+#[cfg_attr(bootstrap, doc = "```rust,no_run")]
+#[cfg_attr(not(bootstrap), doc = "```rust,compile_fail")]
 /// # use std::cell::UnsafeCell;
 /// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
 ///   let t = ptr as *const UnsafeCell<T> as *mut T;
diff --git a/src/tools/miri/tests/fail/both_borrows/illegal_write1.rs b/src/tools/miri/tests/fail/both_borrows/illegal_write1.rs
index d3991949cc3..92f273f67bb 100644
--- a/src/tools/miri/tests/fail/both_borrows/illegal_write1.rs
+++ b/src/tools/miri/tests/fail/both_borrows/illegal_write1.rs
@@ -1,6 +1,8 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 
+#![allow(invalid_reference_casting)]
+
 fn main() {
     let target = Box::new(42); // has an implicit raw
     let xref = &*target;
diff --git a/src/tools/miri/tests/fail/stacked_borrows/illegal_write3.rs b/src/tools/miri/tests/fail/stacked_borrows/illegal_write3.rs
index 6f55b63cb5c..f79f7b561b8 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/illegal_write3.rs
+++ b/src/tools/miri/tests/fail/stacked_borrows/illegal_write3.rs
@@ -1,3 +1,5 @@
+#![allow(invalid_reference_casting)]
+
 fn main() {
     let target = 42;
     // Make sure raw ptr with raw tag cannot mutate frozen location without breaking the shared ref.
diff --git a/tests/ui/const-generics/issues/issue-100313.rs b/tests/ui/const-generics/issues/issue-100313.rs
index 4e9d3626aa8..9af9b5ca458 100644
--- a/tests/ui/const-generics/issues/issue-100313.rs
+++ b/tests/ui/const-generics/issues/issue-100313.rs
@@ -9,6 +9,7 @@ impl <const B: &'static bool> T<B> {
         unsafe {
             *(B as *const bool as *mut bool) = false;
             //~^ ERROR evaluation of constant value failed [E0080]
+            //~| ERROR assigning to `&T` is undefined behavior
         }
     }
 }
diff --git a/tests/ui/const-generics/issues/issue-100313.stderr b/tests/ui/const-generics/issues/issue-100313.stderr
index d4b486376ca..42ad4d61c8e 100644
--- a/tests/ui/const-generics/issues/issue-100313.stderr
+++ b/tests/ui/const-generics/issues/issue-100313.stderr
@@ -1,3 +1,11 @@
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/issue-100313.rs:10:13
+   |
+LL |             *(B as *const bool as *mut bool) = false;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(invalid_reference_casting)]` on by default
+
 error[E0080]: evaluation of constant value failed
   --> $DIR/issue-100313.rs:10:13
    |
@@ -10,11 +18,11 @@ note: inside `T::<&true>::set_false`
 LL |             *(B as *const bool as *mut bool) = false;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: inside `_`
-  --> $DIR/issue-100313.rs:18:5
+  --> $DIR/issue-100313.rs:19:5
    |
 LL |     x.set_false();
    |     ^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/lint/reference_casting.rs b/tests/ui/lint/reference_casting.rs
index 9963820499e..6e70626ef99 100644
--- a/tests/ui/lint/reference_casting.rs
+++ b/tests/ui/lint/reference_casting.rs
@@ -1,7 +1,6 @@
 // check-fail
 
 #![feature(ptr_from_ref)]
-#![deny(invalid_reference_casting)]
 
 extern "C" {
     // N.B., mutability can be easily incorrect in FFI calls -- as
@@ -10,42 +9,63 @@ extern "C" {
     fn int_ffi(c: *mut i32);
 }
 
-fn main() {
+unsafe fn ref_to_mut() {
+    let num = &3i32;
+
+    let _num = &mut *(num as *const i32 as *mut i32);
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+    let _num = &mut *(num as *const i32).cast_mut();
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+    let _num = &mut *std::ptr::from_ref(num).cast_mut();
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+    let _num = &mut *std::ptr::from_ref({ num }).cast_mut();
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+    let _num = &mut *{ std::ptr::from_ref(num) }.cast_mut();
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+    let _num = &mut *(std::ptr::from_ref({ num }) as *mut i32);
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+
+    let deferred = num as *const i32 as *mut i32;
+    let _num = &mut *deferred;
+    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
+}
+
+unsafe fn assign_to_ref() {
     let s = String::from("Hello");
     let a = &s;
-    unsafe {
-        let num = &3i32;
-        let mut_num = &mut 3i32;
-
-        (*(a as *const _ as *mut String)).push_str(" world");
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *(a as *const _ as *mut _) = String::from("Replaced");
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *(a as *const _ as *mut String) += " world";
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        let _num = &mut *(num as *const i32 as *mut i32);
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        let _num = &mut *(num as *const i32).cast_mut();
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        let _num = *{ num as *const i32 }.cast_mut();
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *std::ptr::from_ref(num).cast_mut() += 1;
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *std::ptr::from_ref({ num }).cast_mut() += 1;
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *{ std::ptr::from_ref(num) }.cast_mut() += 1;
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-        *(std::ptr::from_ref({ num }) as *mut i32) += 1;
-        //~^ ERROR casting `&T` to `&mut T` is undefined behavior
-
-        // Shouldn't be warned against
-        println!("{}", *(num as *const _ as *const i16));
-        println!("{}", *(mut_num as *mut _ as *mut i16));
-        ffi(a.as_ptr() as *mut _);
-        int_ffi(num as *const _ as *mut _);
-        int_ffi(&3 as *const _ as *mut _);
-        let mut value = 3;
-        let value: *const i32 = &mut value;
-        *(value as *const i16 as *mut i16) = 42;
-    }
+    let num = &3i32;
+
+    *(a as *const _ as *mut _) = String::from("Replaced");
+    //~^ ERROR assigning to `&T` is undefined behavior
+    *(a as *const _ as *mut String) += " world";
+    //~^ ERROR assigning to `&T` is undefined behavior
+    *std::ptr::from_ref(num).cast_mut() += 1;
+    //~^ ERROR assigning to `&T` is undefined behavior
+    *std::ptr::from_ref({ num }).cast_mut() += 1;
+    //~^ ERROR assigning to `&T` is undefined behavior
+    *{ std::ptr::from_ref(num) }.cast_mut() += 1;
+    //~^ ERROR assigning to `&T` is undefined behavior
+    *(std::ptr::from_ref({ num }) as *mut i32) += 1;
+    //~^ ERROR assigning to `&T` is undefined behavior
+    let value = num as *const i32 as *mut i32;
+    *value = 1;
+    //~^ ERROR assigning to `&T` is undefined behavior
 }
+
+unsafe fn no_warn() {
+    let num = &3i32;
+    let mut_num = &mut 3i32;
+    let a = &String::from("ffi");
+
+    *(num as *const i32 as *mut i32);
+    println!("{}", *(num as *const _ as *const i16));
+    println!("{}", *(mut_num as *mut _ as *mut i16));
+    ffi(a.as_ptr() as *mut _);
+    int_ffi(num as *const _ as *mut _);
+    int_ffi(&3 as *const _ as *mut _);
+    let mut value = 3;
+    let value: *const i32 = &mut value;
+    *(value as *const i16 as *mut i16) = 42;
+}
+
+fn main() {}
diff --git a/tests/ui/lint/reference_casting.stderr b/tests/ui/lint/reference_casting.stderr
index d5b9bbef643..02b23600557 100644
--- a/tests/ui/lint/reference_casting.stderr
+++ b/tests/ui/lint/reference_casting.stderr
@@ -1,68 +1,92 @@
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:20:9
+  --> $DIR/reference_casting.rs:15:16
    |
-LL |         (*(a as *const _ as *mut String)).push_str(" world");
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *(num as *const i32 as *mut i32);
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: the lint level is defined here
-  --> $DIR/reference_casting.rs:4:9
-   |
-LL | #![deny(invalid_reference_casting)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `#[deny(invalid_reference_casting)]` on by default
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:22:9
+  --> $DIR/reference_casting.rs:17:16
    |
-LL |         *(a as *const _ as *mut _) = String::from("Replaced");
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *(num as *const i32).cast_mut();
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:24:9
+  --> $DIR/reference_casting.rs:19:16
    |
-LL |         *(a as *const _ as *mut String) += " world";
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *std::ptr::from_ref(num).cast_mut();
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:26:25
+  --> $DIR/reference_casting.rs:21:16
    |
-LL |         let _num = &mut *(num as *const i32 as *mut i32);
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *std::ptr::from_ref({ num }).cast_mut();
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:28:25
+  --> $DIR/reference_casting.rs:23:16
    |
-LL |         let _num = &mut *(num as *const i32).cast_mut();
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *{ std::ptr::from_ref(num) }.cast_mut();
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:30:20
+  --> $DIR/reference_casting.rs:25:16
    |
-LL |         let _num = *{ num as *const i32 }.cast_mut();
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _num = &mut *(std::ptr::from_ref({ num }) as *mut i32);
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:32:9
+  --> $DIR/reference_casting.rs:29:16
    |
-LL |         *std::ptr::from_ref(num).cast_mut() += 1;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let deferred = num as *const i32 as *mut i32;
+   |                    ----------------------------- casting happend here
+LL |     let _num = &mut *deferred;
+   |                ^^^^^^^^^^^^^^
 
-error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:34:9
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:38:5
    |
-LL |         *std::ptr::from_ref({ num }).cast_mut() += 1;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     *(a as *const _ as *mut _) = String::from("Replaced");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:36:9
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:40:5
    |
-LL |         *{ std::ptr::from_ref(num) }.cast_mut() += 1;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     *(a as *const _ as *mut String) += " world";
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
-  --> $DIR/reference_casting.rs:38:9
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:42:5
+   |
+LL |     *std::ptr::from_ref(num).cast_mut() += 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:44:5
+   |
+LL |     *std::ptr::from_ref({ num }).cast_mut() += 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:46:5
+   |
+LL |     *{ std::ptr::from_ref(num) }.cast_mut() += 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:48:5
+   |
+LL |     *(std::ptr::from_ref({ num }) as *mut i32) += 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
+  --> $DIR/reference_casting.rs:51:5
    |
-LL |         *(std::ptr::from_ref({ num }) as *mut i32) += 1;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let value = num as *const i32 as *mut i32;
+   |                 ----------------------------- casting happend here
+LL |     *value = 1;
+   |     ^^^^^^^^^^
 
-error: aborting due to 10 previous errors
+error: aborting due to 14 previous errors