ptr_arg warning should take user-defined types into account

[nirvdrum]

I have a user-defined type to make intent and usage clearer:

type BasicBlock = Vec<Statement>;

I do need/want the type to be a Vec. Unfortunately, the ptr_arg warning is issued on the resolved type, rather than taking the user-defined type into account. E.g., I have a function signature like the following:

fn enter_block(
    table: &SymbolTable<BasicBlock>,
    block: &BasicBlock
) -> SymbolTable<BasicBlock> {

Since this method doesn't really require a Vec and could get away using a slice, clippy issues the following warning:

warning: writing `&Vec<_>` instead of `&[_]` involves one more reference and cannot be used with non-Vec-based slices.
   --> src/canon.rs:123:20
    |
123 |             block: &BasicBlock,
    |                    ^^^^^^^^^^^
    |
    = note: #[warn(clippy::ptr_arg)] on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

In this case, however, I really do want to specify the argument type as a BasicBlock, which means it is essentially a Vec. I could define it as &[Statement] to pacify the warning, but I would lose some of the clarity that the user-defined type provides. Perhaps the real answer is to rework the type entirely, but there are other contexts where I need it to work like a Vec so I'd be trading off something else.

Generally, the warning is quite is useful so I don't want to disable it as it may apply to other arguments in the function. If possible, though, I think it should silence itself when encountering a user-defined type.

[flip1995]

Duplicate of #3381

We haven't found a nice solution for this problem yet, but at least there should be a help message noting, that the defined type is just a Vec.