resolve_instance: ErrorReported for missing value

Author: lcnr

compiler/rustc_const_eval/src/interpret/util.rs

@@ -13,7 +13,7 @@ where
     T: TypeVisitable<'tcx>,
 {
     debug!("ensure_monomorphic_enough: ty={:?}", ty);
-    if !(ty.needs_subst() || ty.has_opaque_types())  {
+    if !(ty.needs_subst() || ty.has_opaque_types()) {
         return Ok(());
     }
 

compiler/rustc_ty_utils/src/instance.rs

@@ -279,7 +279,11 @@ fn resolve_associated_item<'tcx>(
 
             // If the item does not have a value, then we cannot return an instance.
             if !leaf_def.item.defaultness(tcx).has_value() {
-                return Ok(None);
+                let guard = tcx.sess.delay_span_bug(
+                    tcx.def_span(leaf_def.item.def_id),
+                    "missing value for assoc item in impl",
+                );
+                return Err(guard);
             }
 
             let substs = tcx.erase_regions(substs);

src/test/ui/const-generics/issues/issue-98629.rs

@@ -4,13 +4,12 @@ trait Trait {
     const N: usize;
 }
 
-// FIXME: We should mention that `N` is missing
 impl const Trait for i32 {}
+//~^ ERROR not all trait items implemented, missing: `N`
 
 fn f()
 where
     [(); <i32 as Trait>::N]:,
-    //~^ ERROR unable to use constant with a hidden value in the type system
 {}
 
 fn main() {}

src/test/ui/const-generics/issues/issue-98629.stderr

@@ -1,8 +1,12 @@
-error: unable to use constant with a hidden value in the type system
-  --> $DIR/issue-98629.rs:11:5
+error[E0046]: not all trait items implemented, missing: `N`
+  --> $DIR/issue-98629.rs:7:1
    |
-LL |     [(); <i32 as Trait>::N]:,
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
+LL |     const N: usize;
+   |     -------------- `N` from trait
+...
+LL | impl const Trait for i32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ missing `N` in implementation
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0046`.