This is part of the Rust tutorial session. The whole purpose of the exercise was to find a mistake in the block expression.

See Exercise 2.1
http://pnkfelix.github.io/cyot/tutorial/exercises/ex_part_2

With rust compile this:

fn foo() -> i32 {
   42
}

pub fn main() {

    let the_sum :i32 = {
        foo();
    };

    let the_other_sum = {
        foo();
    };

}

It outputs this error:

bug_block.rs:7:24: 9:6 error: mismatched types:
 expected `i32`,
    found `()`
(expected i32,
    found ()) [E0308]
bug_block.rs:7     let the_sum :i32 = {
bug_block.rs:8         foo();
bug_block.rs:9     };
error: aborting due to previous error

It should also put a warning for line 12: "possibly missing return value". This would catch the bug.