rust - How to conditionally assign a type to a reference -

i have been playing rust afternoon , decided write simple hashing tool can major digesting algorithms.

i'm trying (intent should obvious):

let mut hasher; match alg {     "md5" => { hasher = md5::new() }     "sha1" => { hasher = sha1::new() }     _ => { println!("algorithm not implemented");            process::exit(1); } } hash_file(&file_name, &mut hasher).unwrap(); 

when compiling above, due first match, assumes hasher of type md5 , fails when in "sha1" match branch, tries assign sha1 type. of types intend use in match statement implementers of trait digest feel there should way this.

i tried:

let mut hasher: digest; 

but didn't work either.

it sounds want use trait objects. example this:

let mut hasher; match alg {     "md5" => { hasher = box::new(md5::new()) box<digest>}     "sha1" => { hasher = box::new(sha1::new()) box<digest>}     _ => { println!("algorithm not implemented");            process::exit(1); } } hash_file(&file_name, &mut hasher).unwrap(); 

this work under conditions (for example, trait has object safe), think it's way want.