all memory behind a constant must be immutable

src/librustc_mir/interpret/intern.rs

@@ -304,9 +304,14 @@ pub fn intern_const_alloc_recursive(
 
     let mut todo: Vec<_> = leftover_relocations.iter().cloned().collect();
     while let Some(alloc_id) = todo.pop() {
-        if let Some((_, alloc)) = ecx.memory_mut().alloc_map.remove(&alloc_id) {
+        if let Some((_, mut alloc)) = ecx.memory_mut().alloc_map.remove(&alloc_id) {
             // We can't call the `intern_shallow` method here, as its logic is tailored to safe
             // references. So we hand-roll the interning logic here again.
+            if base_intern_mode != InternMode::Static {
+                // If it's not a static, it *must* be immutable.
+                // We cannot have mutable memory inside a constant.
+                alloc.mutability = Mutability::Immutable;
+            }
             let alloc = tcx.intern_const_alloc(alloc);
             tcx.alloc_map.lock().set_alloc_id_memory(alloc_id, alloc);
             for &(_, ((), reloc)) in alloc.relocations().iter() {

src/test/ui/consts/miri_unleashed/mutable_const.rs

@@ -0,0 +1,20 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+
+#![feature(const_raw_ptr_deref)]
+#![deny(const_err)]
+
+use std::cell::UnsafeCell;
+
+// make sure we do not just intern this as mutable
+const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
+
+const MUTATING_BEHIND_RAW: () = {
+    // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
+    unsafe {
+        *MUTABLE_BEHIND_RAW = 99 //~ WARN skipping const checks
+        //~^ ERROR any use of this value will cause an error
+        //~^^ tried to modify constant memory
+    }
+};
+
+fn main() {}

src/test/ui/consts/miri_unleashed/mutable_const.stderr

@@ -0,0 +1,27 @@
+warning: skipping const checks
+  --> $DIR/mutable_const.rs:14:9
+   |
+LL |         *MUTABLE_BEHIND_RAW = 99
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: any use of this value will cause an error
+  --> $DIR/mutable_const.rs:14:9
+   |
+LL | / const MUTATING_BEHIND_RAW: () = {
+LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
+LL | |     unsafe {
+LL | |         *MUTABLE_BEHIND_RAW = 99
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ tried to modify constant memory
+...  |
+LL | |     }
+LL | | };
+   | |__-
+   |
+note: lint level defined here
+  --> $DIR/mutable_const.rs:4:9
+   |
+LL | #![deny(const_err)]
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+