Change syntax of auto traits

[dtolnay]

The syntax has changed from:

trait Send {}
impl Send for .. {}

to:

auto trait Send {}

[mystor]

Do we still want to parse the old-style auto trait syntax, or should we only support parsing the new style syntax?

EDIT: It looks like rustc still supports parsing the old syntax for now, with a deny-by-default lint, so we should probably leave it in.

[dtolnay]

From the discussion in rust-lang/rust#45247 it sounds like they intend to remove usage of the old form in rust-lang/rust as soon as stage0 is able to parse the new form, which may or may not happen before syn 0.12 is done. I would definitely prefer to remove the old form from the AST. The round trip test still needs to deal with the old form, so I leave it up to you whether we blacklist all the files containing impl for .. or parse it into some sentinel Item::Macro which is recognized and treated specially by ToTokens for ItemMacro.

// input tokens
impl Send for .. {}

// parsed into Item::Macro as if they had been this
syn::verbatim! {
    impl Send for .. {}
}

// printed by ToTokens
impl Send for .. {}