Consider implementing "two-dimensional" index for ty::substs

[edwardw]

One core data structure of rustc is ty::substs:

pub struct substs {
    pub self_ty: Option<ty::t>,
    pub tps: Vec<t>,
    pub regions: RegionSubsts,
}

And, ty::ty_param and ty::ReEarlyBound variants of ty::t and ty::Region have indexes in them:

pub enum Region {
    ReEarlyBound(ast::NodeId, uint, ast::Name),
    ...
}
pub enum sty {
    ty_param(param_ty),
    ...
}
pub struct param_ty {
    pub idx: uint,
    pub def_id: DefId
}

These indexes are meant to distinguish type and lifetime parameters originated from trait, implementation and method bounds. In order to do so, they are monotonic so merging them from different bounds involves messy index manipulation, such as #13503.

@nikomatsakis in IRC:

the fix I had planned was basically to make the index "two-dimensional": that is, first a namespace and then an index
using def-ids would be another option, I tried it once, found it was kinda messy

[edwardw]

#14604 implements this. So this ticket can be closed as soon as #14604 lands.

[nikomatsakis]

@edwardw thanks, I couldn't find this issue when I searched for it. :)