ICE triggered by Encode, "coherence failed to report ambiguity"

[SSheldon]

Reported by @bvssvni at rust-windowing/glutin#359, compiling on the 2015-04-03 nightly causes an ICE:

src/declare.rs:138:21: 138:67 error: internal compiler error: coherence failed to report ambiguity: cannot locate the impl of the trait `core::marker::MarkerTrait` for the type `<F as declare::IntoMethodImp>::Ret`
src/declare.rs:138     let mut types = <<F as IntoMethodImp>::Ret as Encode>::encode().as_str().to_string();
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/src/libsyntax/diagnostic.rs:130

stack backtrace:
   1:        0x111246987 - sys::backtrace::write::h8872f4f658a62d46iJC
   2:        0x1112748a3 - panicking::on_panic::h845201cbea1c5a54uxI
   3:        0x111198cee - rt::unwind::begin_unwind_inner::hfc7848f7429ac5d45eI
   4:        0x1109711ae - rt::unwind::begin_unwind::h396107918539692450
   5:        0x11097115b - diagnostic::SpanHandler::span_bug::hc2e54b04fea5078eYmB
   6:        0x10e42d836 - middle::traits::error_reporting::report_fulfillment_error::hded3e0d571682b65FQN
   7:        0x10e299c01 - middle::traits::error_reporting::report_fulfillment_errors::hd4a5143b2dcf3b1bVPN
   8:        0x10d7dc432 - check::vtable::select_all_fcx_obligations_or_error::h68378d314e6e162eX1b
   9:        0x10d89b010 - check::check_bare_fn::hcbfa2ceb51ac809fvPn
  10:        0x10d894942 - check::check_item::hcd4af901dd683671g8n
  11:        0x10d8992f2 - visit::walk_item::h14171734647403984035
  12:        0x10d970751 - check_crate::closure.36252
  13:        0x10d96ad17 - check_crate::h34c23998ac3af75d1oC
  14:        0x10d68a2e5 - driver::phase_3_run_analysis_passes::h1e90d1b6a6af8247gGa
  15:        0x10d66ea15 - driver::compile_input::h43156fddb0e7acceQba
  16:        0x10d72c985 - run_compiler::hf5bcc2a2a9004c41S4b
  17:        0x10d72a152 - boxed::F.FnBox<A>::call_box::h9818158584254676665
  18:        0x10d729647 - rt::unwind::try::try_fn::h13069897533439949939
  19:        0x1112febe8 - rust_try_inner
  20:        0x1112febd5 - rust_try
  21:        0x10d72993a - boxed::F.FnBox<A>::call_box::h16467879214397932316
  22:        0x11125dd8d - sys::thread::create::thread_start::h81aa05d82aaf9eebbeH
  23:     0x7fff865d8267 - _pthread_body
  24:     0x7fff865d81e4 - _pthread_start

Looks to be this ICE: rust-lang/rust#23853