feat(hygiene): Print erased regions as underscore lifetimes

Author: zalanlevai

mutest-emit/src/analysis/ty.rs

@@ -435,7 +435,9 @@ pub mod print {
                     Ok(Some(ast::mk::lifetime(sp, ident)))
                 }
 
-                ty::RegionKind::ReVar(_) | ty::RegionKind::ReErased => Ok(None),
+                ty::RegionKind::ReVar(_) => Ok(None),
+
+                ty::RegionKind::ReErased => Ok(Some(ast::mk::lifetime(sp, Ident::new(kw::UnderscoreLifetime, sp)))),
 
                 ty::RegionKind::ReError(_) => Err("encountered region error".to_owned()),
             }

tests/ui/hygiene/ty/print_erased_regions_as_underscore_lifetimes.rs

@@ -0,0 +1,29 @@
+//@ build
+//@ stderr: empty
+
+#![feature(decl_macro)]
+
+#![allow(unused)]
+
+macro m() {
+    trait NestedTrait {
+        fn nested_assoc_fn() -> Self;
+    }
+
+    trait Trait {
+        type GenericAssocTy<'a>: NestedTrait;
+    }
+
+    // TEST: In the path referencing the assoc ty `Trait::GenericAssocTy`,
+    //       the region parameter is an erased region.
+    //       To create a valid path, this erased region must be represented
+    //       as the `'_` lifetime.
+    fn test_call_to_nested_assoc_fn_through_generic_assoc_ty<T>()
+    where
+        T: Trait
+    {
+        let _ = T::GenericAssocTy::nested_assoc_fn();
+    }
+}
+
+m!();