[naga wgsl-in] Panic when converting negative f16 to u32 during const evaluation

[jamienicol]

Description
We panic here as f16::to_u32() returns None. f16::to_u32() under the hood calls f16::to_f32() which is infallible, and then calls f32::to_u32() (from num_traits::ToPrimitive) which if I'm following the code correctly is implemented here.

Repro steps

enable f16;
fn test() {
  var a = u32(-1h);
}

Expected vs observed behavior

I would expect this to evaluate to var a = 0u as it does for f32 or AbstractFloat.

Instead we get this panic:

thread 'main' panicked at naga/src/proc/constant_evaluator.rs:1719:60:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:692:5
   1: core::panicking::panic_fmt
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/panicking.rs:75:14
   2: core::panicking::panic
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/panicking.rs:145:5
   3: core::option::unwrap_failed
             at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/option.rs:2015:5
   4: core::option::Option<T>::unwrap
             at /home/jamie/.rustup/toolchains/1.85-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs:978:21
   5: naga::proc::constant_evaluator::ConstantEvaluator::cast
             at ./naga/src/proc/constant_evaluator.rs:1719:44
   6: naga::proc::constant_evaluator::ConstantEvaluator::try_eval_and_append_impl
             at ./naga/src/proc/constant_evaluator.rs:942:36
   7: naga::proc::constant_evaluator::ConstantEvaluator::try_eval_and_append
             at ./naga/src/proc/constant_evaluator.rs:802:35
   8: naga::front::wgsl::lower::ExpressionContext::append_expression
             at ./naga/src/front/wgsl/lower/mod.rs:500:9
   9: naga::front::wgsl::lower::construction::<impl naga::front::wgsl::lower::Lowerer>::construct
             at ./naga/src/front/wgsl/lower/construction.rs:561:20
  10: naga::front::wgsl::lower::Lowerer::expression_for_reference
             at ./naga/src/front/wgsl/lower/mod.rs:2068:30
  11: naga::front::wgsl::lower::Lowerer::expression_for_abstract
             at ./naga/src/front/wgsl/lower/mod.rs:2001:20
  12: naga::front::wgsl::lower::Lowerer::type_and_init
             at ./naga/src/front/wgsl/lower/mod.rs:1292:32
  13: naga::front::wgsl::lower::Lowerer::statement
             at ./naga/src/front/wgsl/lower/mod.rs:1542:45
  14: naga::front::wgsl::lower::Lowerer::block
             at ./naga/src/front/wgsl/lower/mod.rs:1470:13
  15: naga::front::wgsl::lower::Lowerer::function
             at ./naga/src/front/wgsl/lower/mod.rs:1377:24
  16: naga::front::wgsl::lower::Lowerer::lower
             at ./naga/src/front/wgsl/lower/mod.rs:1105:40
  17: naga::front::wgsl::Frontend::inner
             at ./naga/src/front/wgsl/mod.rs:50:22
  18: naga::front::wgsl::Frontend::parse
             at ./naga/src/front/wgsl/mod.rs:44:9
  19: naga::front::wgsl::parse_str
             at ./naga/src/front/wgsl/mod.rs:68:5
  20: naga::parse_input
             at ./naga-cli/src/bin/naga.rs:623:26
  21: naga::run
             at ./naga-cli/src/bin/naga.rs:478:9
  22: naga::main
             at ./naga-cli/src/bin/naga.rs:361:21
  23: core::ops::function::FnOnce::call_once
             at /home/jamie/.rustup/toolchains/1.85-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5