Auto merge of #22561 - richo:as_slice-as_str, r=Manishearth

This may not be quite ready to go out, I fixed some docs but suspect I missed a bunch.

I also wound up fixing a bunch of redundant `[]` suffixes, but on closer inspection I don't believe that can land until after a snapshot.

src/test/run-make/unicode-input/span_length.rs

@@ -80,7 +80,7 @@ fn main() {
                          .arg(format!("{} {}",
                                       rustc,
                                       main_file.as_str()
-                                               .unwrap()).as_slice())
+                                               .unwrap()))
                          .output().unwrap();
 
     let err = String::from_utf8_lossy(result.error.as_slice());