VIM: What wizardry is this?

[Jace Bennett]

I'm new and trying to feel my way around here. I have MacVim + 
Janus on OSX 10.8. I pulled d.vim into the janus folder, and 
started messing around. On write, I get really cool and helpful 
errors along the lines of

"writeln is not defined, did you mean to import std.stdio?"

Why, yes, Vim, I did. Thanks.

What I don't understand is how it knows. Is there some linting 
script? Is it compiling in the background? I don't see either of 
those happening in d.vim.

Am I getting coolness from janus or from the dmd installer? In 
short, it's awesome, but wtf?