remove use of "either" in rt.rs and followingly mod.rs

[wakandan]

regarding #9157 (#9157) to remove "either", this is my first commit for rt.rs and mod.rs

Great help from https://github.com/huonw

[alexcrichton]

Sadly I think that you picked one of the tricky cases to tackle first! There are actually no written-down constructors of these variants, but rather they're generated by the format_args! macro. If you use a compiler built with this code, it should fail to compile:

fn main() {
  println!("{0, plural, =1{#} other{no match}}", 1);
}

In addition to updating these modules, I think you'll also need to remove the usage of Left and Right from the syntax expander in src/libsyntax/ext/format.rs.

If you need any help though, feel free to post here or ping me on IRC (I'm acrichto)

[wakandan]

It's quite a surprise to me, thanks for letting me know this. How could I detect such a case? After modifying these module I was using "make check-stage1-std" to run the tests. Should I re-compile the whole project?

[alexcrichton]

If you run make check-stage1-rpass TESTNAME=ifmt it should fail unless you've updated it correctly.

[pnkfelix]

I'm closing this pull request to get it off the bors queue. (Feel free to open another PR if you get this working.)