ICE when casting &mut Trait to *mut Trait, same for casting &Trait to *const Trait

[tbu-]

trait Trait { }

fn function(t: &mut Trait) {
    t as *mut Trait
}

fn main() { }

This ICEs with the following message:

$ RUST_BACKTRACE=1 rustc a.rs 
a.rs:4:5: 4:6 error: internal compiler error: expected object type
a.rs:4     t as *mut Trait
           ^
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 'Box<Any>', /home/hein/git/rust/src/libsyntax/diagnostic.rs:116

stack backtrace:
   1:     0x7fba24ef8d50 - rt::backtrace::imp::write::h62769dcaa7983ed8FNs
   2:     0x7fba24efbdd0 - <unknown>
   3:     0x7fba256a43e0 - unwind::begin_unwind_inner::h3fca55fd1ce1f347o9c
   4:     0x7fba23e4f460 - <unknown>
   5:     0x7fba23e4f3e0 - diagnostic::SpanHandler::span_bug::hea3a64b392293aeaDRF
   6:     0x7fba25c49ad0 - driver::session::Session::span_bug::hb3878c8c794bf1dcHNC
   7:     0x7fba25f4a260 - middle::typeck::check::vtable::check_object_cast::h7d7ee59c0ae09980HKN
   8:     0x7fba25fb8090 - <unknown>
   9:     0x7fba25fe62b0 - <unknown>
  10:     0x7fba25fab4c0 - <unknown>
  11:     0x7fba25fa7eb0 - <unknown>
  12:     0x7fba25fa7bf0 - <unknown>
  13:     0x7fba25fa3cc0 - middle::typeck::check::check_item::h22ee00fbae37d0570pW
  14:     0x7fba25fa79b0 - middle::typeck::check::check_item_types::h7de0aa0b52df316614V
  15:     0x7fba25a9c960 - <unknown>
  16:     0x7fba262c69f0 - middle::typeck::check_crate::hee34369af2a06be9Unp
  17:     0x7fba2632e290 - driver::driver::phase_3_run_analysis_passes::h44dc3d0e5ab4847dJ4B
  18:     0x7fba263292b0 - driver::driver::compile_input::hb7d473b561e2b247vLB
  19:     0x7fba263afb50 - <unknown>
  20:     0x7fba263afa40 - <unknown>
  21:     0x7fba25ab5f60 - <unknown>
  22:     0x7fba25ab5d50 - <unknown>
  23:     0x7fba26c0aa00 - <unknown>
  24:     0x7fba25706aa0 - <unknown>
  25:     0x7fba25706a90 - rust_try
  26:     0x7fba256a1d60 - unwind::try::h0709f530ff508047aYc
  27:     0x7fba256a1bf0 - task::Task::run::h3302eb43beb0760es4b
  28:     0x7fba26c0a740 - <unknown>
  29:     0x7fba256a3400 - <unknown>
  30:     0x7fba20872250 - start_thread
  31:     0x7fba2537d3b9 - clone
  32:                0x0 - <unknown>

[oli-obk]

duplicate of #18998

[tbu-]

@oli-obk
Is it? This one is a legitimate cast, the other issue is not.

[oli-obk]

i don't think the legitimacy has anything to do with the error.
I'm not sure what you need an unsafe trait pointer for (which actually are two pointers. one to the vtable, one to the struct). I'm sure there are better ways, but that's not the issue here.

these are already implicitly converted. so the explicit conversion is unnecessary anyway.
if the rust-gods deem it necessary to exist, i'd implement it and add a notice that it's not necessary + hint using implicit cast.
but the ICE should be fixed to an error message (note: this is an explicit panic, it is expected to be happening there)

trait Trait { }

fn function_trait(t: &mut Trait) {
    let x : *mut Trait = t;
}

fn main() {}

[tbu-]

That the implicit conversion works while the explicit one doesn't is an interesting observation…

Take this issue as "the explicit conversion does not work while it should" and the other one as "ICE when doing an incorrect type case to a raw trait pointer".

[oli-obk]

maybe the explicit conversion that are also implictly possible should be sugar for an implicit conversion instead of having its own code. Not sure how easy that is to detect.

[alexcrichton]

Fixed now, just needs a test!