Suggest using let when a const uses outer function's generics

[eloff]

If you use:

const size = std::mem::size_of::<T>() instead of let size = ...

You get the misleading compiler error E401:

error[E0401]: can't use generic parameters from outer function

Which is total nonsense. You just need let instead of const here. This fooled me for a while, so I'm opening this issue so it will get addressed and not fool other developers.

For more info see: https://stackoverflow.com/questions/59808575/cant-create-a-constant-using-memsize-of-inside-a-function-on-a-sized-generic

[camelid]

Well, to my understanding, the error is correct, it's just a bit confusing. The compiler doesn't support using generic parameters from the function the const is defined in.

I'm labeling this as a diagnostics improvement issue.