To be included in: http://saml.rilspace.org/moar-languagez-gc-content-in-python-d-fpc-c-and-c

gc_count.rs

use std::io::BufferedReader;
use std::io::File;
use std::str::StrSlice;

fn main() {
    let path = Path::new("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa");
    let mut file = BufferedReader::new(File::open(&path));
    let mut gc = 0i; 
    let mut at = 0i; 
    for line in file.lines() {
        for c in line.unwrap().as_slice().chars() {
            match c { 
                'G' => gc += 1,
                'C' => gc += 1,
                'A' => at += 1,
                'T' => at += 1,
                _   => {}  
            }
        }
    }   
    let gc_frac: f64 = (gc as f64) / ((at as f64) + (gc as f64));
    println!("GC fraction: {}", gc_frac)
}

Did you catch the memory mapped version at https://gist.github.com/Blei/32d22fb92a3365da86b6?

@huonw: I think you're right about some unfortunate differences in the code examples. I really should compile an updated benchmark soon.