const fn has no result expression

[nagisa]

#![feature(const_fn)]
pub const fn test() {}
const X: () = test();
fn main() {}

ICEs with

error: internal compiler error: unexpected panic
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 'const fn has no result expression', src/libcore/option.rs:700
stack backtrace:
   1:     0x7fc09db94d37 - sys::backtrace::tracing::imp::write::h8f11a6299019b37aN7u
   2:     0x7fc09dba498b - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.46063
   3:     0x7fc09dba4505 - panicking::default_hook::he79d5441312473c9vjA
   4:     0x7fc09db62cef - sys_common::unwind::begin_unwind_inner::h850ac67cf2e6431duZt
   5:     0x7fc09db659f8 - sys_common::unwind::begin_unwind_fmt::h7f9772c177da56ceAYt
   6:     0x7fc09db925e1 - rust_begin_unwind
   7:     0x7fc09dbba94f - panicking::panic_fmt::h7b8691efcf0ee41102L
   8:     0x7fc09dbe0c64 - option::expect_failed::h2f7b609e15d08c701RO
   9:     0x7fc098f2d6f2 - eval::eval_const_expr_partial::hd768fd0c71d5d67ewOa
  10:     0x7fc09cf75fd7 - consts::CheckCrateVisitor<'a, 'tcx>::global_expr::h3fa9a0801caa760cgsa
  11:     0x7fc09cf7dd76 - consts::CheckCrateVisitor<'a, 'tcx>.Visitor<'v>::visit_item::h4e6b6664f41ad6ddvDa
  12:     0x7fc09cf824d7 - consts::check_crate::hc2396fa9b810de44Hlb
  13:     0x7fc09e0ad934 - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.30847
  14:     0x7fc09e0ab601 - ty::context::TyCtxt<'tcx>::create_and_enter::h12752561671132360966
  15:     0x7fc09e0a6852 - driver::phase_3_run_analysis_passes::h7514814909072594316
  16:     0x7fc09e0776d1 - driver::compile_input::h7e38a782c304f126Uca
  17:     0x7fc09e063a80 - run_compiler::ha6ad3fc70e3bb8f8IKc
  18:     0x7fc09e060d3b - sys_common::unwind::try::try_fn::h9022798910832087097
  19:     0x7fc09db9256b - __rust_try
  20:     0x7fc09db924fd - sys_common::unwind::inner_try::h42271e72fc2229cdwWt
  21:     0x7fc09e0615ff - boxed::F.FnBox<A>::call_box::h6627780591262321388
  22:     0x7fc09dba2214 - sys::thread::Thread::new::thread_start::h23b56e13da45f381dhz
  23:     0x7fc09692b423 - start_thread
  24:     0x7fc09d80ccbc - clone
  25:                0x0 - <unknown>

The result expression should probably default to unit?

[Aatch]

Default to unit, but generate a warning. A const fn with no result is probably a mistake.

[arielb1]

@Aatch

It's not a particularly easy mistake to make, given that your function would have to be empty and return unit. More than that, in the future we may have asserts in const fns, so unit-returning const fns would be useful.

[oli-obk]

We can't fix this yet, because we can't create struct and tuple ConstVals, we can only use existing constructor expressions. I had a fix for it once, but it was a breaking change, I have an idea how to make sure it isn't a breaking change.

[jagourq]

I get the same error if I run:

#![feature(const_fn)]
const fn foo(x: i32) -> i32 {
    return x;
}
fn main() {
    let x = foo(2);
    println!("{}", x);
}

Although, if I omit the semicolon in return x; I get:

error: internal compiler error: ../src/librustc_trans/consts.rs:1006: bad constant expression type in consts::const_expr

[Mark-Simulacrum]

This no longer ICEs.