Rollup merge of #120885 - RalfJung:normal-visitor, r=compiler-errors

interpret/visitor: ensure we only see normalized types

[Prior discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/Normalization.20after.20field.20projection)
r? `@compiler-errors`

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -149,6 +149,8 @@ pub fn project_field<P: Projectable<'tcx, M::Provenance>>(
             "`field` projection called on a slice -- call `index` projection instead"
         );
         let offset = base.layout().fields.offset(field);
+        // Computing the layout does normalization, so we get a normalized type out of this
+        // even if the field type is non-normalized (possible e.g. via associated types).
         let field_layout = base.layout().field(self, field);
 
         // Offset may need adjustment for unsized fields.

compiler/rustc_const_eval/src/interpret/visitor.rs

@@ -153,6 +153,16 @@ fn walk_value(&mut self, v: &Self::V) -> InterpResult<'tcx> {
                 // We visited all parts of this one.
                 return Ok(());
             }
+
+            // Non-normalized types should never show up here.
+            ty::Param(..)
+            | ty::Alias(..)
+            | ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Infer(..)
+            | ty::Error(..) => throw_inval!(TooGeneric),
+
+            // The rest is handled below.
             _ => {}
         };