fix the doc-comment-decoration-trimming edge-case rustdoc ICE

[zackmdavis]

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.

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[zackmdavis]

r? @QuietMisdreavus

[QuietMisdreavus]

Can you add a test for this? Something like this:

src/test/rustdoc/issue-47197.rs:

// copyright header

// libsyntax ICE when trimming the blank line in the middle of that comment

// @has issue_47197/fn.x.html
/**
* a

* b
*/
pub fn x() { }

[zackmdavis]

(updated)

[QuietMisdreavus]

@bors r+ rollup

Thanks!

[bors]

📌 Commit 3cfea33 has been approved by QuietMisdreavus