Implement pipeline constructor from exec iterator

[AlexanderThaller]

Reimplentation of #34 which allows to pass a iterator instead of a vector.

[hniksic]

Pipeline::from_iter() should check that the iterator produced at least two Execs. For example, Pipeline::popen() expects it, and asserts that self.cmds.len() >= 2.

Note that even without this PR it's possible to create pipelines with a dynamic number of components, it's just less convenient:

let cmds: Vec<Exec> = ...;
cmds.reverse();  // easier to pop from the back
let mut pipeline = cmds.pop().unwrap() | cmds.pop.unwrap();
while let Some(cmd) = cmds.pop() {
    pipeline = pipeline | cmd;
}

[AlexanderThaller]

Pipeline::from_iter() should check that the iterator produced at least two Execs. For example, Pipeline::popen() expects it, and asserts that self.cmds.len() >= 2.

Note that even without this PR it's possible to create pipelines with a dynamic number of components, it's just less convenient:

let cmds: Vec<Exec> = ...;
cmds.reverse();  // easier to pop from the back
let mut pipeline = cmds.pop().unwrap() | cmds.pop.unwrap();
while let Some(cmd) = cmds.pop() {
    pipeline = pipeline | cmd;
}

I added an assert like the one in Pipeline::popen(). Maybe it would be nicer to introduce a Result instead of panicing but that is probably outside of the scope of this pr.

[hniksic]

Thanks. If you don't mind, let's make it an explicit panic!() rather than assert(), which should be reserved for "impossible" conditions which indicate a bug in the crate. For example, the assert() in popen() should never happen simply because there is (prior to the introduction of from_iter()) no public API to create a Pipeline with less than two commands.

[AlexanderThaller]

I changed the assert into a panic and also added a documentation example that panics.

[AlexanderThaller]

Probably should also use a different name for the function as clippy complains: https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait

Not sure if from_iter trait would be the right choice here as we have the length limitation.

The clippy lint:

warning: method `from_iter` can be confused for the standard trait method `std::iter::FromIterator::from_iter`
   --> src/builder.rs:834:9
    |
834 | /         pub fn from_iter<I>(iterator: I) -> Pipeline
835 | |         where
836 | |             I: Iterator<Item = Exec>,
837 | |         {
...   |
850 | |             }
851 | |         }
    | |_________^
    |
    = note: `#[warn(clippy::should_implement_trait)]` on by default
    = help: consider implementing the trait `std::iter::FromIterator` or choosing a less ambiguous method name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait

Maybe new_from_iter would be a better name.

[hniksic]

from_exec_iter might be a good choice for the name.

Also, it could take IntoIterator<Item = Exec>, so it can be called with actual Vec (and also with iterators, etc.).

[AlexanderThaller]

from_exec_iter might be a good choice for the name.

Also, it could take IntoIterator<Item = Exec>, so it can be called with actual Vec (and also with iterators, etc.).

Renamed it to from_exec_iter and switchted to the IntoIterator trait.

[hniksic]

Looks great, thanks for the patience. I've additionally renamed the iterator parameter to iterable. If there're no further changes, I'll merge soon.

At some point it's worth removing the panic for less than two Execs - at least one should be reasonable, and possibly even zero. But that might come in a separate PR.

[AlexanderThaller]

Thanks for helping with the pull request! I guess its ready to be merged :D