turn panic into span_delayed_bug

compiler/rustc_passes/src/eii.rs

@@ -116,7 +116,8 @@ pub(crate) fn check_externally_implementable_items<'tcx>(tcx: TyCtxt<'tcx>, ():
         }
 
         if default_impls.len() > 1 {
-            panic!("multiple not supported right now");
+            let decl_span = tcx.def_ident_span(decl_did).unwrap();
+            tcx.dcx().span_delayed_bug(decl_span, "multiple not supported right now");
         }
 
         let (local_impl, is_default) =

tests/ui/eii/default/multiple-default-impls.rs

@@ -1,8 +1,18 @@
 #![crate_type = "lib"]
 #![feature(extern_item_impls)]
+// `eii` expands to, among other things, `macro eii() {}`.
+// If we have two eiis named the same thing, we have a duplicate definition
+// for that macro. The compiler happily continues compiling on duplicate
+// definitions though, to emit as many diagnostics as possible.
+// However, in the case of eiis, this can break the assumption that every
+// eii has only one default implementation, since the default for both eiis will
+// name resolve to the same eii definiton (since the other definition was duplicate)
+// This test tests for the previously-ICE that occurred when this assumption
+// (of 1 default) was broken which was reported in #149982.
 
 #[eii(eii1)]
 fn a() {}
 
 #[eii(eii1)]
+//~^ ERROR the name `eii1` is defined multiple times
 fn b() {}

tests/ui/eii/default/multiple-default-impls.stderr

@@ -0,0 +1,14 @@
+error[E0428]: the name `eii1` is defined multiple times
+  --> $DIR/multiple-default-impls.rs:16:1
+   |
+LL | #[eii(eii1)]
+   | ------------ previous definition of the macro `eii1` here
+...
+LL | #[eii(eii1)]
+   | ^^^^^^^^^^^^ `eii1` redefined here
+   |
+   = note: `eii1` must be defined only once in the macro namespace of this module
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0428`.