feat: materialize tagged-union and fallible constants

source/ast/ast.hon

@@ -31,8 +31,14 @@ NodeId :: distinct u32
 @hide:file
 no_node :: maxval!(NodeId)
 
+#! void literals are generated constants only; there is no source void literal.
 LiteralKind :: enum {
-    int, float, string, bool, null
+    int
+    float
+    string
+    bool
+    null
+    void
 }
 
 ExprKind :: enum {
@@ -71,6 +77,7 @@ ExprKind :: enum {
     catch_expr
     typed_constant
     comptime_expr
+    literal_void
 }
 
 DeclKind :: enum {
@@ -176,6 +183,7 @@ NodeKind :: enum {
     expr_typed_constant
     expr_comptime
     literal_float_value
+    literal_void
 }
 
 @hide:file
@@ -490,6 +498,7 @@ add_literal proc(builder @mut Builder, kind LiteralKind, span Span) NodeId ! mem
         .string: .literal_string
         .bool: .literal_bool
         .null: .literal_null
+        .void: .literal_void
     }
     return try add_node(builder, Node{ kind = node_kind }, span)
 }
@@ -1408,6 +1417,7 @@ expression_kind proc(kind NodeKind) ExprKind {
         .expr_typed_constant: .typed_constant
         .expr_comptime: .comptime_expr
         .literal_float_value: .literal_float
+        .literal_void: .literal_void
         else: unreachable
     }
 }
@@ -1803,7 +1813,7 @@ node_layout proc(node Node) NodeLayout {
     layout := NodeLayout{}
     match node.kind {
         .literal_int, .literal_float, .literal_float_value, .literal_string, .literal_bool, .literal_null,
-        .expr_identifier, .invalid_expr: {}
+        .literal_void, .expr_identifier, .invalid_expr: {}
         .expr_unary: layout.direct_children = 2
         .expr_binary, .expr_range, .expr_index, .expr_selection, .expr_typed_constant,
         .expr_fallible_type, .expr_sum_type, .expr_array_type, .expr_slice_type: