fix: error on returning slice from main

compiler/noirc_frontend/src/elaborator/mod.rs

@@ -16,6 +16,7 @@ use crate::{
     node_interner::{GlobalValue, QuotedTypeId},
     token::SecondaryAttributeKind,
     usage_tracker::UsageTracker,
+    validity::length_is_zero,
 };
 use crate::{
     EnumVariant, Shared, Type, TypeVariable,
@@ -1021,6 +1022,16 @@ impl<'context> Elaborator<'context> {
 
         let return_type = Box::new(self.use_type(func.return_type(), wildcard_allowed));
 
+        // Temporary allow slices for contract functions, until contracts are re-factored.
+        if !func.attributes().has_contract_library_method() {
+            self.check_if_type_is_valid_for_program_output(
+                &return_type,
+                is_entry_point || is_test_or_fuzz,
+                has_inline_attribute,
+                location,
+            );
+        }
+
         let mut typ = Type::Function(
             parameter_types,
             return_type,
@@ -1181,6 +1192,32 @@ impl<'context> Elaborator<'context> {
         }
     }
 
+    fn check_if_type_is_valid_for_program_output(
+        &mut self,
+        typ: &Type,
+        is_entry_point: bool,
+        has_inline_attribute: bool,
+        location: Location,
+    ) {
+        match typ {
+            Type::Unit => return,
+            Type::Array(length, _) | Type::String(length) => {
+                if length_is_zero(length) {
+                    //returning zero length arrays is allowed
+                    return;
+                }
+            }
+            _ => (),
+        }
+
+        self.check_if_type_is_valid_for_program_input(
+            typ,
+            is_entry_point,
+            has_inline_attribute,
+            location,
+        );
+    }
+
     /// True if the `pub` keyword is allowed on parameters in this function
     /// `pub` on function parameters is only allowed for entry point functions
     fn pub_allowed(&self, func: &NoirFunction, in_contract: bool) -> bool {

compiler/noirc_frontend/src/hir_def/types/validity.rs

@@ -247,6 +247,6 @@ impl Type {
     }
 }
 
-fn length_is_zero(length: &Type) -> bool {
+pub(crate) fn length_is_zero(length: &Type) -> bool {
     length.evaluate_to_u32(Location::dummy()).unwrap_or(0) == 0
 }

compiler/noirc_frontend/src/tests.rs

@@ -4514,6 +4514,19 @@ fn cannot_assign_to_nested_struct() {
     check_errors!(src);
 }
 
+#[test]
+fn cannot_return_slice_from_main() {
+    let src = r#"
+    fn main() -> pub [Field]{
+       ^^^^ Invalid type found in the entry point to a program
+       ~~~~ Slice is not a valid entry point type. Found: [Field]
+        &[1,2]
+
+    }
+        "#;
+    check_errors!(src);
+}
+
 #[test]
 fn disallows_references_in_globals() {
     let src = r#"

test_programs/compile_failure/noirc_frontend_tests_cannot_return_slice_from_main/Nargo.toml

@@ -0,0 +1,7 @@
+
+            [package]
+            name = "noirc_frontend_tests_cannot_return_slice_from_main"
+            type = "bin"
+            authors = [""]
+
+            [dependencies]

test_programs/compile_failure/noirc_frontend_tests_cannot_return_slice_from_main/src/main.nr

@@ -0,0 +1,6 @@
+
+    fn main() -> pub [Field]{
+        &[1,2]
+
+    }
+

test_programs/compile_failure/noirc_frontend_tests_cannot_return_slice_from_main/src_hash.txt

@@ -0,0 +1 @@
+1631102025577823237