Fix ICE monomorphize collection encountered polymorphic constant

compiler/rustc_monomorphize/messages.ftl

@@ -22,6 +22,8 @@ monomorphize_symbol_already_defined = symbol `{$symbol}` is already defined
 
 monomorphize_type_length_limit = reached the type-length limit while instantiating `{$shrunk}`
 
+monomorphize_unexpected_polymorphic_constant = unexpected polymorphic constant
+
 monomorphize_unknown_cgu_collection_mode =
     unknown codegen-item collection mode '{$mode}', falling back to 'lazy' mode
 

compiler/rustc_monomorphize/src/collector.rs

@@ -193,6 +193,7 @@ use std::path::PathBuf;
 
 use crate::errors::{
     EncounteredErrorWhileInstantiating, LargeAssignmentsLint, RecursionLimit, TypeLengthLimit,
+    UnexpectedPolymorphicConst,
 };
 
 #[derive(PartialEq)]
@@ -705,11 +706,11 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                         // The `monomorphize` call should have evaluated that constant already.
                         Ok(val) => val,
                         Err(ErrorHandled::Reported(_)) => return,
-                        Err(ErrorHandled::TooGeneric) => span_bug!(
-                            self.body.source_info(location).span,
-                            "collection encountered polymorphic constant: {:?}",
-                            literal
-                        ),
+                        Err(ErrorHandled::TooGeneric) => {
+                            self.tcx.sess.emit_fatal(UnexpectedPolymorphicConst {
+                                span: self.body.source_info(location).span,
+                            })
+                        }
                     }
                 }
                 _ => return,
@@ -720,11 +721,11 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                     // The `monomorphize` call should have evaluated that constant already.
                     Ok(val) => val,
                     Err(ErrorHandled::Reported(_)) => return,
-                    Err(ErrorHandled::TooGeneric) => span_bug!(
-                        self.body.source_info(location).span,
-                        "collection encountered polymorphic constant: {:?}",
-                        literal
-                    ),
+                    Err(ErrorHandled::TooGeneric) => {
+                        self.tcx.sess.emit_fatal(UnexpectedPolymorphicConst {
+                            span: self.body.source_info(location).span,
+                        })
+                    }
                 }
             }
         };

compiler/rustc_monomorphize/src/errors.rs

@@ -98,3 +98,10 @@ pub struct EncounteredErrorWhileInstantiating {
 pub struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(monomorphize_unexpected_polymorphic_constant)]
+pub struct UnexpectedPolymorphicConst {
+    #[primary_span]
+    pub span: Span,
+}

tests/ui/invalid/issue-114484-unexpected-polymorphic-const.rs

@@ -0,0 +1,67 @@
+// build-fail
+use std::marker::PhantomData;
+
+trait MyTrait {
+    fn virtualize(&self) -> &dyn MyTrait;
+}
+
+struct VirtualWrapper<T, const L: u8>(T);
+
+impl<T, const L: u8> VirtualWrapper<T, L> {
+    pub fn wrap(value: &T) -> &Self {
+        unsafe { &*(value as *const T as *const Self) }
+    }
+}
+
+impl<T: MyTrait + 'static, const L: u8> MyTrait for VirtualWrapper<T, L> {
+    fn virtualize(&self) -> &dyn MyTrait {
+        unsafe { virtualize_my_trait(L, self) }
+        // unsafe { virtualize_my_trait(L, &self.0) }  // <-- this code fixes the problem
+    }
+}
+
+const unsafe fn virtualize_my_trait<T>(level: u8, obj: &T) -> &dyn MyTrait
+where
+    T: MyTrait + 'static,
+{
+    const fn gen_vtable_ptr<T, const L: u8>() -> *const ()
+    where
+        T: MyTrait + 'static,
+    {
+        let [_, vtable] = unsafe {
+            std::mem::transmute::<*const dyn MyTrait, [*const (); 2]>(std::ptr::null::<
+                VirtualWrapper<T, L>,
+            >())
+        };
+        vtable
+    }
+
+    struct Vtable<T>(PhantomData<T>);
+
+    impl<T> Vtable<T>
+    where
+        T: MyTrait + 'static,
+    {
+        const LEVELS: [*const (); 2] = [gen_vtable_ptr::<T, 1>(), gen_vtable_ptr::<T, 2>()];
+    }
+
+    let vtable = Vtable::<T>::LEVELS[(level != 0) as usize]; //~ ERROR unexpected polymorphic constant
+
+    let data = obj as *const T as *const ();
+    let ptr: *const dyn MyTrait = std::mem::transmute([data, vtable]);
+
+    &*ptr
+}
+
+struct SomeData<const N: usize>([u8; N]);
+
+impl<const N: usize> MyTrait for SomeData<N> {
+    fn virtualize(&self) -> &dyn MyTrait {
+        VirtualWrapper::<Self, 0>::wrap(self)
+    }
+}
+
+fn main() {
+    let test = SomeData([0; 256]);
+    test.virtualize();
+}

tests/ui/invalid/issue-114484-unexpected-polymorphic-const.stderr

@@ -0,0 +1,16 @@
+error: reached the recursion limit finding the struct tail for `[u8; 256]`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]`
+
+error: reached the recursion limit finding the struct tail for `[u8; 256]`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]`
+
+error: unexpected polymorphic constant
+  --> $DIR/issue-114484-unexpected-polymorphic-const.rs:48:18
+   |
+LL |     let vtable = Vtable::<T>::LEVELS[(level != 0) as usize];
+   |                  ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+