Fix ICE with feature self_struct_ctor

Author: estebank

src/librustc/middle/reachable.rs

@@ -116,6 +116,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ReachableContext<'a, 'tcx> {
             Some(Def::Local(node_id)) | Some(Def::Upvar(node_id, ..)) => {
                 self.reachable_symbols.insert(node_id);
             }
+            Some(Def::Err) => {}  // #56202: calling `def.def_id()` would be an error
             Some(def) => {
                 let def_id = def.def_id();
                 if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {

src/test/ui/issues/issue-56202.rs

@@ -0,0 +1,15 @@
+#![feature(self_struct_ctor)]
+
+trait FooTrait {}
+
+trait BarTrait {
+    fn foo<T: FooTrait>(_: T) -> Self;
+}
+
+struct FooStruct(u32);
+
+impl BarTrait for FooStruct {
+    fn foo<T: FooTrait>(_: T) -> Self {
+        Self(u32::default())
+    }
+}

src/test/ui/issues/issue-56202.stderr

@@ -0,0 +1,7 @@
+error[E0601]: `main` function not found in crate `issue_56202`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-56202.rs`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0601`.