It is very easy for the body of bh.iter { ... } to be turned into a noop by DCE. e.g.

extern mod extra;
use extra::test::BenchHarness;

#[bench]
fn match_option_some(bh: &mut BenchHarness) {
    let x = Some(10);
    do bh.iter {
        let _q = match x {
            Some(y) => y,
            None => 11
        };
    }
}

Compiling with rustc -O --emit-llvm -S, the relevant part of the IR is:

define internal void @_ZN17match_option_some4anon12expr_fn_4234E({ i64, %tydesc*, i8*, i8*, i8 }* nocapture) #2 {
"function top level":
  ret void
}

This likely means that quite a few of the benchmarks currently in the tree are pointless.