Compiler panics when trying to calculate layout of const generic based type

[maplant]

I have the following code:

pub type TruncatedVector<T, const N: usize> = Vector<T, {N - 1}>;

impl<T, const N: usize> Vector<T, {N}> {
    /// Drop the last component and return the vector with one fewer dimension.
    pub fn trunc(self) -> (TruncatedVector<T, {N}>, T) {
         // ....
    }
}

Attempting to use this function, such as in the following manner:

let (xyz, w): (TruncatedVector<u32, 4>, u32) = vec4(0u32, 1, 2, 3).trunc();

causes the compiler to crash with the following backtrace:

error: internal compiler error: src/librustc_codegen_llvm/context.rs:856: failed to get layout for `Vector<u32, {N - 1}>`: the type `[u32; _]` has an unknown layout

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:650:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:212
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:479
   8: std::panicking::begin_panic
   9: rustc_errors::Handler::bug
  10: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  11: rustc::ty::context::tls::with_opt::{{closure}}
  12: rustc::ty::context::tls::with_context_opt
  13: rustc::ty::context::tls::with_opt
  14: rustc::util::bug::opt_span_bug_fmt
  15: rustc::util::bug::bug_fmt
  16: <rustc_codegen_llvm::context::CodegenCx as rustc_target::abi::LayoutOf>::layout_of::{{closure}}
  17: <rustc_codegen_llvm::context::CodegenCx as rustc_target::abi::LayoutOf>::layout_of
  18: <rustc_target::abi::call::FnType<&rustc::ty::TyS> as rustc::ty::layout::FnTypeExt<C>>::new_internal::{{closure}}
  19: <rustc_target::abi::call::FnType<&rustc::ty::TyS> as rustc::ty::layout::FnTypeExt<C>>::new
  20: rustc_codegen_llvm::declare::<impl rustc_codegen_ssa::traits::declare::DeclareMethods for rustc_codegen_llvm::context::CodegenCx>::declare_fn
  21: rustc_codegen_llvm::mono_item::<impl rustc_codegen_ssa::traits::declare::PreDefineMethods for rustc_codegen_llvm::context::CodegenCx>::predefine_fn
  22: <rustc::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::predefine
  23: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  24: rustc::dep_graph::graph::DepGraph::with_task
  25: rustc_codegen_llvm::base::compile_codegen_unit
  26: rustc_codegen_ssa::base::codegen_crate
  27: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  28: rustc::util::common::time
  29: rustc_interface::passes::start_codegen
  30: rustc::ty::context::tls::enter_global
  31: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  32: rustc_interface::passes::create_global_ctxt::{{closure}}
  33: rustc_interface::passes::BoxedGlobalCtxt::enter
  34: rustc_interface::queries::Query<T>::compute
  35: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  36: rustc_interface::interface::run_compiler_in_existing_thread_pool
  37: std::thread::local::LocalKey<T>::with
  38: scoped_tls::ScopedKey<T>::set
  39: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack
error: aborting due to previous error


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 1.37.0-nightly (8ebd67e4e 2019-06-27) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental

This issue has been assigned to @DutchGhost via this comment.

[daboross]

Looks like this was fixed at some point. The following minimal example ICEs on nightly-2019-06-26, but doesn't on nightly-2020-03-11!

#![feature(const_generics)]
pub struct Vector<T, const N: usize>([T; N]);

pub type TruncatedVector<T, const N: usize> = Vector<T, { N - 1 }>;

impl<T, const N: usize> Vector<T, { N }> {
    /// Drop the last component and return the vector with one fewer dimension.
    pub fn trunc(self) -> (TruncatedVector<T, { N }>, T) {
        unimplemented!()
    }
}

fn vec4<T>(a: T, b: T, c: T, d: T) -> Vector<T, 4> {
    Vector([a, b, c, d])
}

fn main() {
    let (_xyz, _w): (TruncatedVector<u32, 4>, u32) = vec4(0u32, 1, 2, 3).trunc();
}

[DutchGhost]

I'll look into adding a test for this tomorrow! :)

[Dylan-DPC-zz]

@rustbot assign @DutchGhost