Turn ICE for dangling pointers into error

src/librustc_mir/interpret/memory.rs

@@ -732,6 +732,11 @@ where
             if self.alloc_map.contains_key(&alloc) {
                 // Not yet interned, so proceed recursively
                 self.intern_static(alloc, mutability)?;
+            } else if self.dead_alloc_map.contains_key(&alloc) {
+                // danging pointer
+                return err!(ValidationFailure(
+                    "encountered dangling pointer in final constant".into(),
+                ))
             }
         }
         Ok(())

src/test/ui/consts/dangling-alloc-id-ice.rs

@@ -0,0 +1,15 @@
+// https://github.com/rust-lang/rust/issues/55223
+
+#![feature(const_let)]
+
+union Foo<'a> {
+    y: &'a (),
+    long_live_the_unit: &'static (),
+}
+
+const FOO: &() = { //~ ERROR this constant cannot be used
+    let y = ();
+    unsafe { Foo { y: &y }.long_live_the_unit }
+};
+
+fn main() {}

src/test/ui/consts/dangling-alloc-id-ice.stderr

@@ -0,0 +1,13 @@
+error: this constant cannot be used
+  --> $DIR/dangling-alloc-id-ice.rs:10:1
+   |
+LL | / const FOO: &() = { //~ ERROR this constant cannot be used
+LL | |     let y = ();
+LL | |     unsafe { Foo { y: &y }.long_live_the_unit }
+LL | | };
+   | |__^ type validation failed: encountered dangling pointer in final constant
+   |
+   = note: #[deny(const_err)] on by default
+
+error: aborting due to previous error
+