Hi everyone,

In Rust the language has the notion of the unit type '()', so for example:

 fn foo ->i32 { ... }
 fn bar() { ... }

Foo has the return type i32, and bar has no return type, which means it
is unit-type so that it can be a value assignable just like any other
function call. You can also declare unit-structs or unit as a type on
variables:

 struct foobar; // empty unit struct
 let a:() = (); // unit type

I thought I could use GCC's void_type_node to represent the unit type
and void_node for the value when assigning or using it, but this causes
the ICE:

```
In function ‘test’:
rust1: internal compiler error: in create_tmp_var, at gimple-expr.c:482
0x13fd3bf create_tmp_var(tree_node*, char const*)
        ../../gccrs/gcc/gimple-expr.c:482
0xe5d195 Gcc_backend::temporary_variable(Bfunction*, Bblock*, Btype*,
Bexpression*, bool, Location, Bstatement**)
        ../../gccrs/gcc/rust/rust-gcc.cc:2889
0xfe3479 Rust::HIR::Function::accept_vis(Rust::HIR::HIRVisitor&)
        ../../gccrs/gcc/rust/hir/tree/rust-hir-full-test.cc:4414
0xf95cdb Rust::Compile::CompileCrate::go()
        ../../gccrs/gcc/rust/backend/rust-compile.cc:49
0xf95b8b Rust::Compile::CompileCrate::Compile(Rust::HIR::Crate&,
Rust::Compile::Context*)
        ../../gccrs/gcc/rust/backend/rust-compile.cc:39
0xee92e7 Rust::Session::parse_file(char const*)
        ../../gccrs/gcc/rust/rust-session-manager.cc:596
0xee8d76 Rust::Session::parse_files(int, char const**)
        ../../gccrs/gcc/rust/rust-session-manager.cc:459
0xe45264 grs_langhook_parse_file
        ../../gccrs/gcc/rust/rust-lang.cc:171

```

I think because void_node is likely not COMPLETE_TYPE_P which means it
hits the assertion when you need temporary's.


Then Tom Tromey suggested I try a zero precision integer so I called:
make_unsigned_type (0) for the type and then use integer_zero_node for
the value, and this solves the problem; however, if I use this zero
precision integer type for the return type on functions and turn
optimizations on I get the ICE:

   ```
   test.rs: In function ‘main’:
   test.rs:16:1: internal compiler error: in min_value, at wide-int.cc:346
      16 | fn main() {
         | ^
   0x1d551d5 wi::min_value(unsigned int, signop)
           ../../gccrs/gcc/wide-int.cc:346
   0x1146ca5 irange::set_varying(tree_node*)
           ../../gccrs/gcc/value-range.h:476
   0x1ce5970 value_range_equiv::set_varying(tree_node*)
           ../../gccrs/gcc/value-range-equiv.cc:71
   0x1d3da07 vr_values::set_def_to_varying(tree_node const*)
           ../../gccrs/gcc/vr-values.c:230
   0x1d3da70 vr_values::set_defs_to_varying(gimple*)
           ../../gccrs/gcc/vr-values.c:241
   0x1c78b2f vrp_prop::visit_stmt(gimple*, edge_def**, tree_node**)
           ../../gccrs/gcc/tree-vrp.c:4001
   0x1ad8519 ssa_propagation_engine::simulate_stmt(gimple*)
           ../../gccrs/gcc/tree-ssa-propagate.c:230
   0x1ad8a0e ssa_propagation_engine::simulate_block(basic_block_def*)
           ../../gccrs/gcc/tree-ssa-propagate.c:337
   0x1ad9f2e ssa_propagation_engine::ssa_propagate()
           ../../gccrs/gcc/tree-ssa-propagate.c:800
   0x1c7a0b0 execute_vrp
           ../../gccrs/gcc/tree-vrp.c:4512
   0x1c7a3e4 execute
           ../../gccrs/gcc/tree-vrp.c:4620
   Please submit a full bug report,
   ```

The backtrace looks as though the optimizer is looking for min value for
a default for the return value, but it's a zero precision integer that
hits the assertion. Note running with -O0, the assertion does not get hit.


At the moment, I have left functions with return type unit to keep using
void_type_node and everywhere else, use this new zero precision integer.
I am not sure what the best approach is here; I was hoping to solicit
feedback on what I am doing with the folks here on the mailing list.

Thanks

--Phil