My code has lots of type parameters and I'm running out of letters that make sense. Is there some established convention for naming type parameters with more than a single letter? Which do any idiomatic codebases use such a convention?
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libstd has gems like this:
impl<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11> Debug for (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11)
but I think the convention is to limit use of generic parameters to no more than 2-4 
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That's not practical in my case, because I have a facade struct that's composed of many different types (4-6), and that struct has a facade trait implementation to avoid having loads of where predicates on every item that uses this facade. It would be nice to find a way to break it down, though.
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There's no real reason why type parameters need to be one character. It's mainly because single-letter type parameters are shorter and their meaning is evident from context. If you've got so many type parameters that you can't tell what they mean at a glance, maybe it's time to give them descriptive names like any other variable?
For example, in my gcode crate there's a type called Comment<'input>. Sure, it could have been Comment<'a> (the usual convention for lifetime parameters), but I chose to use 'input because it's more descriptive.
If you're creating a facade type, why not use boxed trait objects? The purpose of a facade is to wrap a complex system and provide a simpler interface. If users are needing to make all their code generic and write half a dozen type parameters I'd argue that the facade is leaking implementation details and isn't doing its job.
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That is the purpose of the facade. Indeed, the parameters are only necessary in its implementation, the user code only needs one parameter.
Per https://rust-lang-nursery.github.io/api-guidelines/naming.html:
Type parameters: concise
UpperCamelCase, usually single uppercase letter:T
The more you'd have the more I'd say understandability trumps concision.
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