argon2: const ParamsBuilder

[C0D3-M4513R]

Can the ParamsBuilder methods be made const?
Const fn's would help out, because then I could make the Params object a Compile time const.
I took a little look at the code and saw no obvious reason why it should not be possible.

const PARAMS:argon2::Params = match argon2::ParamsBuilder::new().build() {
    Ok(v) => v,
    Err(_) => panic!("Failed to create argon2 params"),
};

https://github.com/RustCrypto/password-hashes/blob/master/argon2/src/params.rs#L367

[tarcieri]

It seems like new and build are the only methods of ParamsBuilder that could be made const fn as the setters require &mut self. That seems like a circuitous way of returning the default Params.

If that's all you want, we could hoist out the Default impl into a proper constant: https://github.com/RustCrypto/password-hashes/blob/c29e6d7/argon2/src/params.rs#L189-L196

[C0D3-M4513R]

I‘d also be interested in having the settees const. I‘ll try to make a pr with your message in mind.
Edit: okay, I didn't know that &mut T was not allowed in const fn's.

[tarcieri]

Here's a simpler approach to a Params::DEFAULT constant: #439

[C0D3-M4513R]

"completed"...
I will see to redoing my pr at a later point in time.
I don't have the time rn.

[tarcieri]

The PR auto-closed it as completed when it was merged, then the tests failed on master