I think this should include the compiler version, it can be retrieved by `option_env!("CFG_VERSION")` which gives `Option<&'static str>` that is Some when the CFG_VERSION env var exists, and None if it doesn't.

I actually think adding a `static VERSION: Option<&'static str> = option_env!("CFG_VERSION");` to the `librustc/lib.rs` file, and using that in the 2 places that CFG_VERSION is currently used, and here would be best (those two places are `version` in the `lib.rs` file, and `compile_unit_metadata` in `librustc/middle/trans/debuginfo.rs`; the latter would change to `::VERSION.unwrap("unknown")`).

Something like
```rust
json.insert(~"version", json::String(::VERSION.unwrap("unknown").to_owned()))
```

(FWIW, the `::...` syntax means "from the crate root".)