Confusing error message with Fn(&T) and unsatisfied trait bounds.

[orodley]

The following (invalid) code triggers an ICE:

trait Foo { }

fn foo<T: Foo>(x: T) { }

fn main() {
    // '&mut int' also triggers an ICE, as does replacing 'int' with any other type.
    // Just 'int' produces a very similar compile error, but with no ICE.
    let x: |&int| = foo;
}

Compiler output:

$ RUST_BACKTRACE=1 rustc ice.rs 
ice.rs:8:21: 8:24 error: the trait `Foo` is not implemented for the type `&int`
ice.rs:8     let x: |&int| = foo;
                             ^~~
ice.rs:8:21: 8:24 note: the trait `Foo` must be implemented because it is required by `foo`
ice.rs:8     let x: |&int| = foo;
                             ^~~
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' panicked at 'assertion failed: !ty::type_needs_infer(ty)', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/librustc/middle/typeck/mod.rs:287

stack backtrace:
   1: 0xb6240e70 - rt::backtrace::imp::write::h7a758165f6ba41d0scq
   2: 0xb6243cb0 - failure::on_fail::h9832b9ccb4fdbc0e3xq
   3: 0xb65bd590 - unwind::begin_unwind_inner::h34603f1d03c7301c7Rd
   4: 0xb674db90 - unwind::begin_unwind::h16442260839102883864
   5: 0xb6c3b800 - middle::typeck::write_ty_to_tcx::h8af730520c710be1amp
   6: 0xb6c37e10 - middle::typeck::check::writeback::WritebackCx<'cx, 'tcx>::visit_node_id::hc06bb3e173d3905ecZO
   7: 0xb6c363f0 - middle::typeck::check::writeback::WritebackCx<'cx, 'tcx>.Visitor<'v>::visit_expr::h17ddfb3651503235cOO
   8: 0xb6c39800 - middle::typeck::check::writeback::WritebackCx<'cx, 'tcx>.Visitor<'v>::visit_local::h4608353f0c9b5790JSO
   9: 0xb6c3aca0 - visit::walk_block::h11861875672251434685
  10: 0xb6c37b90 - middle::typeck::check::writeback::resolve_type_vars_in_fn::h6349b48253cd1f93iJO
  11: 0xb6c91630 - middle::typeck::check::check_bare_fn::he6d7c7b3bc920a95UhW
  12: 0xb6c8d650 - middle::typeck::check::check_item::h60fc147f59e915423BW
  13: 0xb6c913e0 - middle::typeck::check::check_item_types::he4f43b9788b1001f4gW
  14: 0xb6f9d480 - middle::typeck::check_crate::closure.140551
  15: 0xb676adc0 - util::common::time::h5171196413795613556
  16: 0xb6f9c590 - middle::typeck::check_crate::hfe6d18bc6c2f50a0JEp
  17: 0xb7008cf0 - driver::driver::phase_3_run_analysis_passes::h9d97fef3fdd62215JoC
  18: 0xb7003970 - driver::driver::compile_input::h3c2343e4d8471330u5B
  19: 0xb7089540 - driver::run_compiler::haa3ef0d50c3e4454LVF
  20: 0xb7089420 - driver::run::closure.146534
  21: 0xb67835b0 - task::TaskBuilder<S>::try_future::closure.104847
  22: 0xb67833a0 - task::TaskBuilder<S>::spawn_internal::closure.104818
  23: 0xb7707d60 - task::NativeSpawner.Spawner::spawn::closure.8448
  24: 0xb65bd010 - unwind::try::try_fn::h8e5dedf36bdf5fe00Id
  25: 0xb66197d0 - rust_try_inner
  26: 0xb6619790 - rust_try
  27: 0xb65bae40 - unwind::try::h4eb595f617e1a97fPGd
  28: 0xb65bacb0 - task::Task::run::h7184ce8d54e7ef9aFMc
  29: 0xb7707ac0 - task::NativeSpawner.Spawner::spawn::closure.8386
  30: 0xb65bc540 - thread::thread_start::hbe8cf1b92d073927U7c
  31: 0xb611ce30 - start_thread
  32: 0xb64b4de2 - clone
  33:        0x0 - <unknown>

This occurs under:

$ uname -a
Linux <hostname> 3.16-3-686-pae #1 SMP Debian 3.16.5-1 (2014-10-10) i686 GNU/Linux
$ rustc --version=verbose
rustc 0.13.0-nightly (45cbdec41 2014-11-07 00:02:18 +0000)
binary: rustc
commit-hash: 45cbdec4174778bf915f17561ef971c068a7fcbc
commit-date: 2014-11-07 00:02:18 +0000
host: i686-unknown-linux-gnu
release: 0.13.0-nightly

[steveklabnik]

This syntax is now gone, so I'm not sure this issue is relevant. Does an updated version still ICE for you?

[orodley]

An updated version on rustc 1.0.0-nightly (8903c21d6 2015-01-15 22:42:58 +0000) doesn't ICE, but I do get a very unhelpful error message in the &i32 case compared to the i32 case.

Compiling:

trait Foo { }

fn foo<T: Foo>(x: T) { }

fn main() {
    let x: &Fn(i32) = &foo;
}

Produces:

$ rustc ice.rs
ice.rs:6:24: 6:27 error: the trait `Foo` is not implemented for the type `i32`
ice.rs:6     let x: &Fn(i32) = &foo;
                                ^~~
error: aborting due to previous error

Which is exactly what I would expect.

Compiling:

trait Foo { }

fn foo<T: Foo>(x: T) { }

fn main() {
    let x: &Fn(&i32) = &foo;
}

Produces:

$ rustc ice.rs
ice.rs:6:24: 6:28 error: type mismatch: the type `fn(_) {foo}` implements the trait `core::ops::Fn(_)`, but the trait `for<'r> core::ops::Fn(&'r i32)` is required (expected concrete lifetime, found bound lifetime parameter )
ice.rs:6     let x: &Fn(&i32) = &foo;
                                ^~~~
ice.rs:6:24: 6:28 note: required for the cast to the object type `for<'r> core::ops::Fn(&'r i32)`
ice.rs:6     let x: &Fn(&i32) = &foo;
                                ^~~~
error: aborting due to previous error

Which is basically incomprehensible to me. I feel like these two cases should probably be producing the same error message, as the problem in both cases is the same - the trait isn't implemented for the specified type.

[orodley]

An update on this:

I tested it again under the latest nightly (rustc 1.0.0-beta (9854143cb 2015-04-02) (built 2015-04-02)), and the error message for the &i32 case is even more confusing:

Compiling:

trait Foo {
    fn foo(s: Self);
}

fn foo<T: Foo>(x: T) { }

fn main() {
    let x: &Fn(&i32) = &foo;
}

produces:

$ rustc test.rs
test.rs:8:24: 8:28 error: type mismatch: the type `fn(_) {foo}` implements the trait `core::ops::Fn<(_,)>`, but the trait `for<'r> core::ops::Fn<(&'r i32,)>` is required (expected concrete lifetime, found bound lifetime parameter ) [E0281]
test.rs:8     let x: &Fn(&i32) = &foo;
                                 ^~~~
test.rs:8:24: 8:28 note: required for the cast to the object type `for<'r> core::ops::Fn(&'r i32)`
test.rs:8     let x: &Fn(&i32) = &foo;
                                 ^~~~
test.rs:8:24: 8:28 error: type mismatch resolving `for<'r> <fn(_) {foo} as core::ops::FnOnce<(&'r i32,)>>::Output == ()`:
 expected bound lifetime parameter ,
    found concrete lifetime [E0271]
test.rs:8     let x: &Fn(&i32) = &foo;
                                 ^~~~
test.rs:8:24: 8:28 note: required for the cast to the object type `for<'r> core::ops::Fn(&'r i32)`
test.rs:8     let x: &Fn(&i32) = &foo;
                                 ^~~~
error: aborting due to 2 previous errors

[steveklabnik]

Triage: no change

[estebank]

Current output, no change:

error[E0281]: type mismatch: `fn(_) {foo::<_>}` implements the trait `std::ops::Fn<(_,)>`, but the trait `for<'r> std::ops::Fn<(&'r i32,)>` is required
 --> src/main.rs:6:24
  |
6 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^ expected concrete lifetime, found bound lifetime parameter
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)`

error[E0271]: type mismatch resolving `for<'r> <fn(_) {foo::<_>} as std::ops::FnOnce<(&'r i32,)>>::Output == ()`
 --> src/main.rs:6:24
  |
6 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^ expected bound lifetime parameter, found concrete lifetime
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)`

[estebank]

Current output:

error[E0631]: type mismatch in function arguments
 --> src/main.rs:8:24
  |
8 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^
  |                        |
  |                        expected signature of `for<'r> fn(&'r i32) -> _`
  |                        found signature of `fn(_) -> _`
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)`

error[E0271]: type mismatch resolving `for<'r> <fn(_) {foo::<_>} as std::ops::FnOnce<(&'r i32,)>>::Output == ()`
 --> src/main.rs:8:24
  |
8 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^ expected bound lifetime parameter, found concrete lifetime
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)

---

Updated output:

error[E0631]: type mismatch in function arguments
 --> src/main.rs:8:24
  |
5 | fn foo<T: Foo>(x: T) { }
  | -------------------- found signature of `fn(_) -> _`
...
8 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^ expected signature of `for<'r> fn(&'r i32) -> _`
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)`

error[E0271]: type mismatch resolving `for<'r> <fn(_) {foo::<_>} as std::ops::FnOnce<(&'r i32,)>>::Output == ()`
 --> src/main.rs:8:24
  |
8 |     let x: &Fn(&i32) = &foo;
  |                        ^^^^ expected bound lifetime parameter, found concrete lifetime
  |
  = note: required for the cast to the object type `for<'r> std::ops::Fn(&'r i32)`

error: aborting due to 2 previous errors