`cannot relate bound region` when using `impl Trait`

[lawliet89]

When I changed a signature of a function from returning a struct that implementing some trait Responder to impl Responder, the compiler panicked. The panic stack trace is at the end of the post.

Code that works:

[package]
name = "rustc-crash"
version = "0.1.0"

[dependencies]
rocket = "0.3"
rocket_codegen = "0.3"

#![feature(plugin, custom_derive, conservative_impl_trait)]
#![plugin(rocket_codegen)]
extern crate rocket;

use rocket::State;
use rocket::response::Responder;

struct SomeState;

/// This is OK
#[get("/one")]
fn test_one() -> &'static str {
    "Hello world"
}

/// This panics
#[get("/")]
fn test_two(state: State<SomeState>) -> &'static str{
    "Hello world"
}

fn main() {
    rocket::ignite()
        .mount(
            "/",
            routes![test_one, test_two],
        )
        .manage(SomeState)
        .launch();
}

Code that panics:

#![feature(plugin, custom_derive, conservative_impl_trait)]
#![plugin(rocket_codegen)]
extern crate rocket;

use rocket::State;
use rocket::response::Responder;

struct SomeState;

/// This is OK
#[get("/one")]
fn test_one<'r>() -> impl Responder<'r> {
    "Hello world"
}

/// This panics
#[get("/")]
fn test_two(state: State<SomeState>) -> impl Responder {
    "Hello world"
}

fn main() {
    rocket::ignite()
        .mount(
            "/",
            routes![test_one, test_two],
        )
        .manage(SomeState)
        .launch();
}

More information

Rocket.rs routes are any functions that returns something that implements the Responder trait. The codegen plugin will wire the routes up via wrapper functions.

Consider the snippet of cargo expand for the code above:

#[allow(unreachable_code)]
fn rocket_route_fn_test_one<'_b>(
    __req: &'_b ::rocket::Request,
    __data: ::rocket::Data,
) -> ::rocket::handler::Outcome<'_b> {
    let responder = test_one();
    ::rocket::handler::Outcome::from(__req, responder)
}
/// Rocket code generated static route information structure.
#[allow(non_upper_case_globals)]
#[rocket_route_info]
pub static static_rocket_route_info_for_test_one: ::rocket::StaticRouteInfo =
    ::rocket::StaticRouteInfo {
        method: ::rocket::http::Method::Get,
        path: "/one",
        handler: rocket_route_fn_test_one,
        format: None,
        rank: None,
    };
/// This is OK
#[rocket_route(static_rocket_route_info_for_test_one)]
fn test_one<'r>() -> impl Responder<'r> {
    "Hello world"
}

I suspect this might be because of the generated line ::rocket::handler::Outcome::from(__req, responder), which has this signature.

Apologies that I don't know any more of rustc's internals to offer any more information.

EDIT:
If I change the signature to fn test_two<'r>(state: State<'r, SomeState>) -> impl Responder<'r>, the code will compile.

This might be a duplicate of #39872. Please feel free to close this if they are the same.

Meta

rustc --version --verbose:
rustc 1.20.0-nightly (ae98ebf 2017-07-20)
binary: rustc
commit-hash: ae98ebf
commit-date: 2017-07-20
host: x86_64-unknown-linux-gnu
release: 1.20.0-nightly
LLVM version: 4.0

Backtrace:

$ RUST_BACKTRACE=1 cargo test
   Compiling rustc-crash v0.1.0 (file:///home/yongwen/work/scratch/rustc-crash)
error: internal compiler error: /checkout/src/librustc/infer/region_inference/mod.rs:706: cannot relate bound region: ReLateBound(DebruijnIndex { depth: 2 }, BrAnon(0)) <= '_#26r
  --> src/main.rs:17:1
   |
17 | #[get("/")]
   | ^^^^^^^^^^^

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 1.20.0-nightly (ae98ebfcb 2017-07-20) running on x86_64-unknown-linux-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:437:8
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:390
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:611
   5: std::panicking::begin_panic_new
   6: rustc_errors::Handler::span_bug
   7: rustc::session::opt_span_bug_fmt::{{closure}}
   8: rustc::session::opt_span_bug_fmt
   9: rustc::session::span_bug_fmt
  10: rustc::infer::region_inference::RegionVarBindings::make_subregion
  11: rustc::infer::region_inference::RegionVarBindings::make_eqregion
  12: rustc::ty::relate::relate_substs::{{closure}}
  13: <<core::result::Result<V, E> as core::iter::traits::FromIterator<core::result::Result<A, E>>>::from_iter::Adapter<Iter, E> as core::iter::iterator::Iterator>::next
  14: <core::result::Result<V, E> as core::iter::traits::FromIterator<core::result::Result<A, E>>>::from_iter
  15: <core::result::Result<T, E> as rustc::ty::context::InternIteratorElement<T, R>>::intern_with
  16: rustc::infer::at::At::sub_exp
  17: rustc::traits::select::SelectionContext::match_projection
  18: rustc::traits::select::SelectionContext::match_projection_obligation_against_definition_bounds
  19: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  20: rustc::dep_graph::graph::DepGraph::with_anon_task
  21: rustc::traits::select::SelectionContext::candidate_from_obligation
  22: rustc::traits::select::SelectionContext::select
  23: <rustc::traits::fulfill::FulfillProcessor<'a, 'b, 'gcx, 'tcx> as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation
  24: rustc::traits::fulfill::FulfillmentContext::select
  25: rustc::traits::fulfill::FulfillmentContext::select_where_possible
  26: rustc_typeck::check::FnCtxt::select_obligations_where_possible
  27: rustc_typeck::check::FnCtxt::check_argument_types
  28: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::confirm_builtin_call
  29: rustc_typeck::check::FnCtxt::check_expr_kind
  30: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref
  31: rustc_typeck::check::FnCtxt::check_block_with_expected::{{closure}}
  32: rustc_typeck::check::FnCtxt::check_block_with_expected
  33: rustc_typeck::check::FnCtxt::check_expr_kind
  34: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref
  35: rustc_typeck::check::FnCtxt::check_return_expr
  36: rustc_typeck::check::check_fn
  37: rustc_typeck::check::typeck_tables_of::{{closure}}
  38: rustc_typeck::check::typeck_tables_of
  39: rustc::dep_graph::graph::DepGraph::with_task
  40: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::try_get
  41: rustc::ty::maps::TyCtxtAt::typeck_tables_of
  42: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_tables_of
  43: rustc_typeck::check::typeck_item_bodies
  44: rustc::dep_graph::graph::DepGraph::with_task
  45: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::try_get
  46: rustc::ty::maps::TyCtxtAt::typeck_item_bodies
  47: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_item_bodies
  48: rustc_typeck::check_crate
  49: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  50: rustc_driver::driver::phase_3_run_analysis_passes
  51: rustc_driver::driver::compile_input
  52: rustc_driver::run_compiler

error: Could not compile `rustc-crash`.

To learn more, run the command again with --verbose.

[cramertj]

@lawliet89 I believe this should have been fixed. Can you try on the latest nightly and confirm?

[lawliet89]

Yep I can confirm it's fixed. Thanks!