wherein careful doc-decoration arithmetic proves quite the ICE-breaker

This `horizontal_trim` function strips the leading whitespace from
doc-comments that have a left-asterisk-margin:

  /**
   * You know what I mean—
   *
   * comments like this!
   */

The index of the column of asterisks is `i`, and if trimming is deemed
possible, we slice each line from `i+1` to the end of the line. But if, in
particular, `i` was 0 _and_ there was an empty line (as in the example
given in the reporting issue), we ended up panicking trying to slice an
empty string from 0+1 (== 1).

Let's tighten our check to say that we can't trim when `i` is even the same
as the length of the line, not just when it's greater. (Any such cases
would panic trying to slice `line` from `line.len()+1`.)

Resolves #47197.

src/libsyntax/parse/lexer/comments.rs

@@ -101,7 +101,7 @@ pub fn strip_doc_comment_decoration(comment: &str) -> String {
                     break;
                 }
             }
-            if i > line.len() {
+            if i >= line.len() {
                 can_trim = false;
             }
             if !can_trim {

src/test/rustdoc/issue-47197-blank-line-in-doc-block.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 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.
+
+// @has issue_47197_blank_line_in_doc_block/fn.whose_woods_these_are_i_think_i_know.html
+
+/**
+* snow
+
+* ice
+*/
+pub fn whose_woods_these_are_i_think_i_know() {}