This is a really simple implementation of what I'd like a real interactive mode for rust to look like. You can enter statements (right now, use, import, fn declarations, and any expression) and it will evaluate the output. The way it is actually implemented is a hack - it stores view items (use and import), declarations (let and fn) and a statement (which is updated as you go). It then writes them to a file, compiles that, and runs it. If it encounters an error compiling, it shows just that error. It actually works surprisingly well.
rust> 1+1
2
rust> 1+'a'
error: internal compiler error: char type passed to convert_integral_ty_to_int_ty_set()
rust> 1+~"a"
error: mismatched types: expected `<VI0>` but found `~str` (integral variable vs ~str)
rust> 1+a
error: unresolved name: a
rust> fn hello() { ~"hello world" }
error: mismatched types: expected `()` but found `~str` (() vs ~str)
rust> fn hello() -> ~str { ~"hello world" }
rust> hello()
~"hello world"
rust> hello() + ~"...goodbye"
~"hello world...goodbye"
rust> let a = 23
rust> a/2
11
rust> (a as float)/2.0
11.5
It is really convenient to be able to move around the line (ie, back arrow to correct, move to the beginning / end) to edit it. This can be accomplished with the readline library, which, if this were anything but a hack, would eventually be integrated. But, in the spirit of this being a hack, you can install a wrapper (ie, rlwrap), and then run the repl like: rlwrap rustrepl and get readline support.
The way forward for this is here: https://github.com/z0w0/rust-repl. This is a JIT based approach that should (eventually) function better than this does, and more importantly, be much faster (and eventually provide more features, like type inspection, etc). Right now, it is pretty rough, I believe, but it is improving rapidly!