Iterable args: a way to expand a given argument of `test_matrix`

[fenollp]

I'd like to be able to

fn things() -> Vec<T> { .. }

#[test_matrix([false,true], things())]
fn my_func(a: bool, b: T) { .. } 

(meaning: expand things() into its values, like the literal list to the left).
Today this doesn't compile, saying that b's type doesn't match things() (and that's expected).

How do you think this can be solved?

• should we allow this IFF things() returns impl IntoIterator<Item = T>?
• should we allow a modifier on arguments to mark these to interpret as iterators?

#[test_matrix([false,true], from: things())]
fn my_func(a: bool, b: T) { .. } 

// How about a prelude providing say `iter(..)`
#[test_matrix([false,true], iter(things()))]
fn my_func(a: bool, b: T) { .. } 

• is there a const restriction on things()?

BTW could you merge test_matrix's wiki this repo's? There's a new page as well as other modifications
#128 (comment)

Thanks!