Fix ICE with use self;

src/librustc/hir/lowering.rs

@@ -2046,7 +2046,8 @@ impl<'a> LoweringContext<'a> {
                 };
 
                 // Correctly resolve `self` imports
-                if path.segments.last().unwrap().identifier.name == keywords::SelfValue.name() {
+                if path.segments.len() > 1 &&
+                   path.segments.last().unwrap().identifier.name == keywords::SelfValue.name() {
                     let _ = path.segments.pop();
                     if ident.name == keywords::SelfValue.name() {
                         *name = path.segments.last().unwrap().identifier.name;

src/test/ui/issue-47623.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use self; //~ERROR `self` imports are only allowed within a { } list
+
+fn main() {}

src/test/ui/issue-47623.stderr

@@ -0,0 +1,8 @@
+error[E0429]: `self` imports are only allowed within a { } list
+  --> $DIR/issue-47623.rs:11:5
+   |
+11 | use self; //~ERROR `self` imports are only allowed within a { } list
+   |     ^^^^
+
+error: aborting due to previous error
+