Modify `regex::Captures::{at,name}` to return `Option`

Closes #14602.  As discussed in that issue, the existing `at` and `name`
functions represent two different results with the empty string:

1. Matched the empty string.
2. Did not match anything.

Consider the following example.  This regex has two named matched
groups, `key` and `value`. `value` is optional:

```rust
// Matches "foo", "foo;v=bar" and "foo;v=".
regex!(r"(?P<key>[a-z]+)(;v=(?P<value>[a-z]*))?");
```

We can access `value` using `caps.name("value")`, but there's no way for
us to distinguish between the `"foo"` and `"foo;v="` cases.

Early this year, @BurntSushi recommended modifying the existing `at` and
`name` functions to return `Option`, instead of adding new functions to
the API.

This is a [breaking-change], but the fix is easy:

- `refs.at(1)` becomes `refs.at(1).unwrap_or("")`.
- `refs.name(name)` becomes `refs.name(name).unwrap_or("")`.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/compiletest/errors.rs b/src/compiletest/errors.rs
index f15db7d9371..b7df43aabdd 100644
--- a/src/compiletest/errors.rs
+++ b/src/compiletest/errors.rs
@@ -66,10 +66,10 @@ fn parse_expected(last_nonfollow_error: Option<uint>,
                   line: &str,
                   re: &Regex) -> Option<(WhichLine, ExpectedError)> {
     re.captures(line).and_then(|caps| {
-        let adjusts = caps.name("adjusts").len();
-        let kind = caps.name("kind").to_ascii_lower();
-        let msg = caps.name("msg").trim().to_string();
-        let follow = caps.name("follow").len() > 0;
+        let adjusts = caps.name("adjusts").unwrap_or("").len();
+        let kind = caps.name("kind").unwrap_or("").to_ascii_lower();
+        let msg = caps.name("msg").unwrap_or("").trim().to_string();
+        let follow = caps.name("follow").unwrap_or("").len() > 0;
 
         let (which, line) = if follow {
             assert!(adjusts == 0, "use either //~| or //~^, not both.");