feat: implement Null -> Unit conversion

Author: indietyp

libs/@local/hashql/mir/src/reify/atom.rs

@@ -1,6 +1,6 @@
 use core::alloc::Allocator;
 
-use hashql_core::{id::Id as _, r#type::kind::TypeKind};
+use hashql_core::{id::Id as _, r#type::kind::TypeKind, value::Primitive};
 use hashql_hir::node::{
     Node,
     access::{Access, FieldAccess, IndexAccess},
@@ -165,6 +165,7 @@ impl<'heap, A: Allocator, S: Allocator> Reifier<'_, '_, '_, '_, 'heap, A, S> {
                 // In the future this would be a simple FnPtr
                 Operand::Constant(Constant::Unit)
             }
+            NodeKind::Data(Data::Primitive(Primitive::Null)) => Operand::Constant(Constant::Unit),
             NodeKind::Data(Data::Primitive(primitive)) => {
                 // First try if we can promote the primitive to a non-opaque constant:
                 let constant = match Int::try_from(primitive) {

libs/@local/hashql/mir/src/reify/rvalue.rs

@@ -4,6 +4,7 @@ use hashql_core::{
     id::{Id as _, IdVec},
     symbol::sym,
     r#type::{TypeBuilder, Typed, builder},
+    value::Primitive,
 };
 use hashql_hir::node::{
     HirPtr, Node,
@@ -37,6 +38,7 @@ use crate::{
 impl<'mir, 'heap, A: Allocator, S: Allocator> Reifier<'_, 'mir, '_, '_, 'heap, A, S> {
     fn rvalue_data(&mut self, data: Data<'heap>) -> RValue<'heap> {
         match data {
+            Data::Primitive(Primitive::Null) => RValue::Load(Operand::Constant(Constant::Unit)),
             Data::Primitive(primitive) => {
                 // First try if we can promote the primitive to a non-opaque constant:
                 let constant = match Int::try_from(primitive) {
@@ -76,6 +78,9 @@ impl<'mir, 'heap, A: Allocator, S: Allocator> Reifier<'_, 'mir, '_, '_, 'heap, A
                     operands,
                 })
             }
+            Data::Tuple(Tuple { fields }) if fields.is_empty() => {
+                RValue::Load(Operand::Constant(Constant::Unit))
+            }
             Data::Tuple(Tuple { fields }) => {
                 let mut operands = IdVec::with_capacity_in(fields.len(), self.context.mir.heap);
                 for &field in fields {

libs/@local/hashql/mir/tests/ui/pass/inline/filter-aggressive.stdout

@@ -0,0 +1,3 @@
+//@ run: pass
+//@ description: empty tuple should be reified as a unit constant
+{ "#tuple": [] }

libs/@local/hashql/mir/tests/ui/reify/empty-tuple.jsonc

@@ -0,0 +1,12 @@
+//@ run: pass
+//@ description: null should be promoted to a unit constant
+[
+  "if",
+  { "#literal": true },
+  [
+    "let",
+    "foo",
+    { "#literal": null },
+    ["let", "bar", { "#literal": null }, ["==", "foo", "bar"]]
+  ]
+]

libs/@local/hashql/mir/tests/ui/reify/empty-tuple.stdout

@@ -0,0 +1,3 @@
+//@ run: pass
+//@ description: null should be promoted to a unit constant
+["let", "foo", { "#literal": null }, "foo"]