Don't mark replace_box suggestion machine-applicable for Box<&mut _>

clippy_lints/src/replace_box.rs

@@ -12,7 +12,7 @@ use rustc_hir_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, Pl
 use rustc_lint::{LateContext, LateLintPass, impl_lint_pass};
 use rustc_middle::hir::place::ProjectionKind;
 use rustc_middle::mir::FakeReadCause;
-use rustc_middle::ty;
+use rustc_middle::ty::{self, TypeVisitableExt as _};
 use rustc_span::{Symbol, sym};
 
 declare_clippy_lint! {
@@ -121,7 +121,15 @@ impl LateLintPass<'_> for ReplaceBox {
                 && let Some(rhs_inner) = get_box_new_payload(cx, rhs)
             {
                 span_lint_and_then(cx, REPLACE_BOX, expr.span, "creating a new box", |diag| {
-                    let mut app = Applicability::MachineApplicable;
+                    // Replacing the box in place (`*b = ..`) keeps any borrow already stored in it
+                    // alive across the assignment, while `b = Box::new(..)` drops the old box first.
+                    // When the boxed type carries a lifetime, that difference can change
+                    // borrow-check results, so don't offer this as a machine-applicable rewrite.
+                    let mut app = if inner_ty.has_type_flags(ty::TypeFlags::HAS_RE_ERASED) {
+                        Applicability::MaybeIncorrect
+                    } else {
+                        Applicability::MachineApplicable
+                    };
                     let suggestion = format!(
                         "{} = {}",
                         Sugg::hir_with_applicability(cx, lhs, "_", &mut app).deref(),

tests/ui/replace_box_unfixable.rs

@@ -0,0 +1,37 @@
+//@no-rustfix: the suggestion can change borrow-check results for boxed types carrying a lifetime
+#![warn(clippy::replace_box)]
+
+// Replacing the box in place keeps any borrow already stored in it alive across the assignment, so
+// for a boxed type that carries a lifetime the suggestion is only `MaybeIncorrect`, not applied.
+
+fn mut_ref() {
+    let mut a = 300;
+    let mut b = Box::new(&mut a);
+    **b += 1;
+    let mut c = 900;
+    b = Box::new(&mut c);
+    //~^ replace_box
+    **b += 1;
+}
+
+fn shared_ref() {
+    let x = 1;
+    let mut b = Box::new(&x);
+    let y = 2;
+    b = Box::new(&y);
+    //~^ replace_box
+    let _ = **b;
+}
+
+struct Wrapper<'a>(&'a u32);
+
+fn lifetime_struct() {
+    let x = 1;
+    let mut b = Box::new(Wrapper(&x));
+    let y = 2;
+    b = Box::new(Wrapper(&y));
+    //~^ replace_box
+    let _ = *b.0;
+}
+
+fn main() {}

tests/ui/replace_box_unfixable.stderr

@@ -0,0 +1,28 @@
+error: creating a new box
+  --> tests/ui/replace_box_unfixable.rs:12:5
+   |
+LL |     b = Box::new(&mut c);
+   |     ^^^^^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*b = &mut c`
+   |
+   = note: this creates a needless allocation
+   = note: `-D clippy::replace-box` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::replace_box)]`
+
+error: creating a new box
+  --> tests/ui/replace_box_unfixable.rs:21:5
+   |
+LL |     b = Box::new(&y);
+   |     ^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*b = &y`
+   |
+   = note: this creates a needless allocation
+
+error: creating a new box
+  --> tests/ui/replace_box_unfixable.rs:32:5
+   |
+LL |     b = Box::new(Wrapper(&y));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*b = Wrapper(&y)`
+   |
+   = note: this creates a needless allocation
+
+error: aborting due to 3 previous errors
+