Expand variadic generics design notes

[Jules-Bertholet]

Adds:

• Links to helpful background reading
• List of use cases
• Prior art from other languages
• Overview of my own draft proposal

[Tamschi]

I'm not sure where to put this, but rather than

for<T, U in Ts, Us>

I would find either of

for<(T, U) in Ts * Us> // Borrowed from scalar vector multiplication, definitely strange too though.

or

for<(T, U) in Zipped<Ts, Us>> // where `Zipped<...>` is a built-in variadic utility type accepting
                              // only tuples of tuples with the same arity as its parameter list.

more intuitively readable, as

for<T, U>

already stands for the Cartesian product of the comma-separated types.

Either would also match the syntax in for-loop expressions well in terms of symbol precedence.

[Jules-Bertholet]

@Tamschi

I would appreciate further discussion of my specific proposal on the Internals thread, or directly on the HackMD for smaller comments.

There are two big reasons why the design of for<... in ...> is the way it is:

• I would rather keep magic to a minimum; e.g., Zipped<Ts, Us> should be implementable in user code.
• I want to support mixed use of type, lifetime, and const generics, e.g., for<'a, T, const N: usize in 'as, Ts, Ns>.

[scottmcm]

Is there a summary of what's different from the previous note? With the file rename it's unclear to me what the additions are here.

[Jules-Bertholet]

The rename is its own commit, so if you review commit-by-commit you should be able to see the diff.

[programmerjake]

syntax bikeshedding: rather than static for use type for since it's kinda a type-level for loop since it iterates over the field types.

[Jules-Bertholet]

@programmerjake Please direct comments/bikeshed on my proposal specifically to the Internals thread, or directly on the HackMD for minor suggestions, so I can have it all in one place to review. (Addressing your type for suggestion: while I am not overly fond of static for, I would note that it designed to allow looping over values, lifetimes, types, and consts. type for type T in Ts would seem to me to be particularly awkward).