beta ICE: new iron: &mut iron::Request<'_, '_> was a subtype of &mut _ but now is not?

[FauxFaux]

Upgrading iron in my project causes an ICE. This is possibly the same as #36701, but this must be new as I only see the ICE on beta/nightly, not on the current stable channel, which is way newer than the bug report.

Test repo: https://github.com/FauxFaux/quad-image/blob/ice/src/main.rs
git clone -b ice https://github.com/FauxFaux/quad-image.git

Relevant code:

fn handle(req: &mut Request) -> IronResult<Response> {
    req.get_ref::<Params>();
    unimplemented!()
}

error: internal compiler error: /checkout/src/librustc_typeck/check/method/confirm.rs:330: &mut iron::Request<'_, '_> was a subtype of &mut _ but now is not?
 --> src/main.rs:8:9
  |
8 |     req.get_ref::<Params>();
  |         ^^^^^^^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.23.0-nightly (3b82e4c74 2017-11-05) running on x86_64-unknown-linux-gnu

rustc 1.23.0-nightly (3b82e4c74 2017-11-05)
binary: rustc
commit-hash: 3b82e4c74d43fc1273244532c3a90bf9912061cf
commit-date: 2017-11-05
host: x86_64-unknown-linux-gnu
release: 1.23.0-nightly
LLVM version: 4.0

On stable:

error[E0277]: the trait bound `params::Params: plugin::Plugin<iron::Request<'_, '_>>` is not satisfied
 --> src/main.rs:8:9
  |
8 |     req.get_ref::<Params>();
  |         ^^^^^^^ the trait `plugin::Plugin<iron::Request<'_, '_>>` is not implemented for `params::Params`
  |
  = help: the following implementations were found:
            <params::Params as plugin::Plugin<iron::request::Request<'a, 'b>>>

error: aborting due to previous error

I note that (on all releases), cargo decides to download two different versions of iron? This doesn't seem right.

   Compiling iron v0.5.1
   Compiling iron v0.6.0

Cargo.toml:

[dependencies]
iron = "0.6.0"
params = "0.7.1"

% cargo tree  | fgrep -B2 iron
boom v0.1.0 (file:///home/faux/code/quad-image)
├── iron v0.6.0
--
└── params v0.7.1
    ├── bodyparser v0.6.0
    │   ├── iron v0.5.1
...

[arielb1]

Minified:

struct Params;

pub trait Plugin<E: ?Sized> {
    type Error;
}

pub trait Pluggable {
    fn get_ref<P: Plugin<Self>>(&mut self) -> Option<P::Error> {
        None
    }
}

impl Plugin<()> for Params {
    type Error = ();
}

impl Pluggable for i32 {}

fn handle(req: &mut i32) {
    req.get_ref::<Params>();
}

fn main() {}

[arielb1]

So the main question here is how did this ever work, but given #36701 I suppose the answer is "entirely by accident".