fix: disallow () in entry points

compiler/noirc_frontend/src/hir_def/types.rs

@@ -1277,13 +1277,13 @@
             Type::FieldElement
             | Type::Integer(_, _)
             | Type::Bool
-            | Type::Unit
             | Type::Constant(_, _)
             | Type::TypeVariable(_)
             | Type::NamedGeneric(_)
             | Type::Error => true,
 
-            Type::FmtString(_, _)
+            Type::Unit
+            | Type::FmtString(_, _)
             | Type::Function(_, _, _, _)
             | Type::Reference(..)
             | Type::Forall(_, _)
@@ -2423,7 +2423,7 @@
         }
 
         let recur_on_binding = |id, replacement: &Type| {
             // Prevent recuring forever if there's a `T := T` binding
             if replacement.type_variable_id() == Some(id) {
                 replacement.clone()
             } else {
@@ -2509,7 +2509,7 @@
                 Type::Tuple(fields)
             }
             Type::Forall(typevars, typ) => {
                 // Trying to substitute_helper a variable de, substitute_bound_typevarsfined within a nested Forall
                 // is usually impossible and indicative of an error in the type checker somewhere.
                 for var in typevars {
                     assert!(!type_bindings.contains_key(&var.id()));
@@ -2676,7 +2676,7 @@
         }
     }
 
     /// Follow bindings if this is a type variable or generic to the first non-typevariable
     /// type. Unlike `follow_bindings`, this won't recursively follow any bindings on any
     /// fields or arguments of this type.
     pub fn follow_bindings_shallow(&self) -> Cow<Type> {
@@ -2701,7 +2701,7 @@
 
     /// Replace any `Type::NamedGeneric` in this type with a `Type::TypeVariable`
     /// using to the same inner `TypeVariable`. This is used during monomorphization
     /// to bind to named generics since they are unbindable during type checking.
     pub fn replace_named_generics_with_type_variables(&mut self) {
         match self {
             Type::FieldElement
@@ -3247,7 +3247,7 @@
                 len.hash(state);
                 env.hash(state);
             }
             Type::Tuple(elems) => elems.hash(state),
             Type::DataType(def, args) => {
                 def.hash(state);
                 args.hash(state);

test_programs/compile_failure/unit_in_main/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "unit_in_main"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.33.0"
+
+[dependencies]

test_programs/compile_failure/unit_in_main/src/main.nr

@@ -0,0 +1 @@
+fn main(_: ()) {}

test_programs/compile_failure/unit_in_main/stderr.txt

@@ -0,0 +1,8 @@
+error: Only sized types may be used in the entry point to a program
+  ┌─ src/main.nr:1:12
+  │
+1 │ fn main(_: ()) {}
+  │            -- Slices, references, or any type containing them may not be used in main, contract functions, or foldable functions
+  │
+
+Aborting due to 1 previous error