Remove invalid help diagnostics for const pointer

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1198,18 +1198,21 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, '_, 'infcx, 'tcx> {
                     sugg.push(s);
                 }
 
-                err.multipart_suggestion_verbose(
-                    format!(
-                        "consider changing this to be a mutable {pointer_desc}{}",
-                        if is_trait_sig {
-                            " in the `impl` method and the `trait` definition"
-                        } else {
-                            ""
-                        }
-                    ),
-                    sugg,
-                    Applicability::MachineApplicable,
-                );
+                if sugg.iter().all(|(span, _)| !self.infcx.tcx.sess.source_map().is_imported(*span))
+                {
+                    err.multipart_suggestion_verbose(
+                        format!(
+                            "consider changing this to be a mutable {pointer_desc}{}",
+                            if is_trait_sig {
+                                " in the `impl` method and the `trait` definition"
+                            } else {
+                                ""
+                            }
+                        ),
+                        sugg,
+                        Applicability::MachineApplicable,
+                    );
+                }
             }
             Some((false, err_label_span, message, _)) => {
                 let def_id = self.body.source.def_id();

tests/ui/suggestions/dont_suggest_raw_pointer_syntax-issue-127562.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let val = 2;
+    let ptr = std::ptr::addr_of!(val);
+    unsafe {
+        *ptr = 3; //~ ERROR cannot assign to `*ptr`, which is behind a `*const` pointer
+    }
+}

tests/ui/suggestions/dont_suggest_raw_pointer_syntax-issue-127562.stderr

@@ -0,0 +1,14 @@
+error[E0594]: cannot assign to `*ptr`, which is behind a `*const` pointer
+  --> $DIR/dont_suggest_raw_pointer_syntax-issue-127562.rs:5:9
+   |
+LL |         *ptr = 3;
+   |         ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written
+   |
+help: consider specifying this binding's type
+   |
+LL |     let ptr: *mut i32 = std::ptr::addr_of!(val);
+   |            ++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0594`.