address comments, add test for shadowed Box type

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2305,7 +2305,7 @@ pub fn recursive_type_with_infinite_size_error<'tcx>(
         path,
     );
     if spans.len() <= 4 {
-        // FIXME: This suggestion might be erroneous if Option or Box are shadowed
+        // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
         err.multipart_suggestion(
             &msg,
             spans
@@ -2356,14 +2356,10 @@ fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId
     }
 
     // Match a single generic arg in the 0th path segment
-    let generic_arg = path.segments.get(0)?.args?.args.get(0);
+    let generic_arg = path.segments.last()?.args?.args.get(0)?;
 
     // Take the span out of the type, if it's a type
-    if let Some(hir::GenericArg::Type(generic_ty)) = generic_arg {
-        Some(generic_ty.span)
-    } else {
-        None
-    }
+    if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
 }
 
 /// Summarizes information

src/test/ui/type/type-recursive-box-shadowed.rs

@@ -0,0 +1,12 @@
+//FIXME(compiler-errors): This fixup should suggest the full box path, not just `Box`
+
+struct Box<T> {
+    t: T,
+}
+
+struct Foo {
+    //~^ ERROR recursive type `Foo` has infinite size
+    inner: Foo,
+}
+
+fn main() {}

src/test/ui/type/type-recursive-box-shadowed.stderr

@@ -0,0 +1,17 @@
+error[E0072]: recursive type `Foo` has infinite size
+  --> $DIR/type-recursive-box-shadowed.rs:7:1
+   |
+LL | struct Foo {
+   | ^^^^^^^^^^ recursive type has infinite size
+LL |
+LL |     inner: Foo,
+   |            --- recursive without indirection
+   |
+help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `Foo` representable
+   |
+LL |     inner: Box<Foo>,
+   |            ++++   +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0072`.