feat(achilles): Implement a Unit type r/2356

Add support for a zero-sized Unit type. This requires some special at
the codegen level because LLVM (unsurprisingly) only allows Void types
in function return position - to make that a little easier to handle
there's a new pass that strips any unit-only expressions and pulls
unit-only function arguments up to new `let` bindings, so we never have
to actually pass around unit values.

Change-Id: I0fc18a516821f2d69172c42a6a5d246b23471e38
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2695
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 1 insertions, 0 deletions

diff --git a/users/glittershark/achilles/src/interpreter/mod.rs b/users/glittershark/achilles/src/interpreter/mod.rs
index bcd474b3ab..a8ba2dd3ac 100644
--- a/users/glittershark/achilles/src/interpreter/mod.rs
+++ b/users/glittershark/achilles/src/interpreter/mod.rs
@@ -30,6 +30,7 @@ impl<'a> Interpreter<'a> {
             Expr::Literal(Literal::Int(i), _) => Ok((*i).into()),
             Expr::Literal(Literal::Bool(b), _) => Ok((*b).into()),
             Expr::Literal(Literal::String(s), _) => Ok(s.clone().into()),
+            Expr::Literal(Literal::Unit, _) => unreachable!(),
             Expr::UnaryOp { op, rhs, .. } => {
                 let rhs = self.eval(rhs)?;
                 match op {