don't leave assoc const unnormalized due to unconstrained params

compiler/rustc_middle/src/traits/mod.rs

@@ -12,7 +12,7 @@ use std::borrow::Cow;
 use std::hash::{Hash, Hasher};
 use std::sync::Arc;
 
-use rustc_errors::{Applicability, Diag, EmissionGuarantee};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::HirId;
 use rustc_hir::def_id::DefId;
@@ -996,4 +996,7 @@ pub enum CodegenObligationError {
     /// but was included during typeck due to the trivial_bounds feature.
     Unimplemented,
     FulfillmentError,
+    /// The selected impl has unconstrained generic parameters. This will emit an error
+    /// during impl WF checking.
+    UnconstrainedParam(ErrorGuaranteed),
 }

compiler/rustc_traits/src/codegen.rs

@@ -80,16 +80,14 @@ pub(crate) fn codegen_select_candidate<'tcx>(
         // but never resolved, causing the return value of a query to contain inference
         // vars. We do not have a concept for this and will in fact ICE in stable hashing
         // of the return value. So bail out instead.
-        match impl_source {
-            ImplSource::UserDefined(impl_) => {
-                tcx.dcx().span_delayed_bug(
-                    tcx.def_span(impl_.impl_def_id),
-                    "this impl has unconstrained generic parameters",
-                );
-            }
+        let guar = match impl_source {
+            ImplSource::UserDefined(impl_) => tcx.dcx().span_delayed_bug(
+                tcx.def_span(impl_.impl_def_id),
+                "this impl has unconstrained generic parameters",
+            ),
             _ => unreachable!(),
-        }
-        return Err(CodegenObligationError::FulfillmentError);
+        };
+        return Err(CodegenObligationError::UnconstrainedParam(guar));
     }
 
     Ok(&*tcx.arena.alloc(impl_source))

compiler/rustc_ty_utils/src/instance.rs

@@ -112,6 +112,7 @@ fn resolve_associated_item<'tcx>(
             | CodegenObligationError::Unimplemented
             | CodegenObligationError::FulfillmentError,
         ) => return Ok(None),
+        Err(CodegenObligationError::UnconstrainedParam(guar)) => return Err(guar),
     };
 
     // Now that we know which impl is being used, we can dispatch to

tests/ui/layout/unconstrained-param-ice-137308.rs

@@ -15,4 +15,4 @@ impl<C: ?Sized> A for u8 { //~ ERROR: the type parameter `C` is not constrained
 }
 
 #[rustc_layout(debug)]
-struct S([u8; <u8 as A>::B]); //~ ERROR: the type `[u8; <u8 as A>::B]` has an unknown layout
+struct S([u8; <u8 as A>::B]); //~ ERROR: the type has an unknown layout

tests/ui/layout/unconstrained-param-ice-137308.stderr

@@ -4,7 +4,7 @@ error[E0207]: the type parameter `C` is not constrained by the impl trait, self
 LL | impl<C: ?Sized> A for u8 {
    |      ^ unconstrained type parameter
 
-error: the type `[u8; <u8 as A>::B]` has an unknown layout
+error: the type has an unknown layout
   --> $DIR/unconstrained-param-ice-137308.rs:18:1
    |
 LL | struct S([u8; <u8 as A>::B]);