feat: Allow secret functions to use public parameters (noir-lang#1051)

* Allow secret functions to use public parameters

* Add carve-out for the Unit type for pub return values

* Fix contract test

crates/nargo/tests/test_data/contracts/src/main.nr

@@ -3,6 +3,6 @@ fn main(x : Field, y : pub Field) {
 }
 
 contract Foo {
-    fn double(x: Field) -> Field { x * 2 }
-    fn triple(x: Field) -> Field { x * 3 }
+    fn double(x: Field) -> pub Field { x * 2 }
+    fn triple(x: Field) -> pub Field { x * 3 }
 }

crates/noirc_frontend/src/hir/resolution/errors.rs

@@ -166,7 +166,7 @@ impl From<ResolverError> for Diagnostic {
                     ident.0.span(),
                 );
 
-                diag.add_note("The `pub` keyword only has effects on arguments to the main function of a program. Thus, adding it to other function parameters can be deceiving and should be removed".to_owned());
+                diag.add_note("The `pub` keyword only has effects on arguments to the entry-point function of a program. Thus, adding it to other function parameters can be deceiving and should be removed".to_owned());
                 diag
             }
             ResolverError::NecessaryPub { ident } => {
@@ -178,7 +178,7 @@ impl From<ResolverError> for Diagnostic {
                     ident.0.span(),
                 );
 
-                diag.add_note("The `pub` keyword is mandatory for the main function return type because the verifier cannot retrieve private witness and thus the function will not be able to return a 'priv' value".to_owned());
+                diag.add_note("The `pub` keyword is mandatory for the entry-point function return type because the verifier cannot retrieve private witness and thus the function will not be able to return a 'priv' value".to_owned());
                 diag
             }
             ResolverError::ExpectedComptimeVariable { name, span } => Diagnostic::simple_error(

crates/noirc_frontend/src/hir/resolution/resolver.rs

@@ -601,7 +601,7 @@ impl<'a> Resolver<'a> {
         let mut parameter_types = vec![];
 
         for (pattern, typ, visibility) in func.parameters().iter().cloned() {
-            if func.name() != "main" && visibility == noirc_abi::AbiVisibility::Public {
+            if visibility == noirc_abi::AbiVisibility::Public && !self.pub_allowed(func) {
                 self.push_err(ResolverError::UnnecessaryPub { ident: func.name_ident().clone() })
             }
 
@@ -615,8 +615,9 @@ impl<'a> Resolver<'a> {
 
         self.declare_numeric_generics(&parameter_types, &return_type);
 
-        if func.name() == "main"
-            && *return_type != Type::Unit
+        // 'pub_allowed' also implies 'pub' is required on return types
+        if self.pub_allowed(func)
+            && return_type.as_ref() != &Type::Unit
             && func.def.return_visibility != noirc_abi::AbiVisibility::Public
         {
             self.push_err(ResolverError::NecessaryPub { ident: func.name_ident().clone() })
@@ -650,6 +651,15 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    /// True if the 'pub' keyword is allowed on parameters in this function
+    fn pub_allowed(&self, func: &NoirFunction) -> bool {
+        if self.in_contract() {
+            !func.def.is_unconstrained && !func.def.is_open
+        } else {
+            func.name() == "main"
+        }
+    }
+
     fn handle_function_type(&mut self, func: &NoirFunction) -> Option<ContractFunctionType> {
         if func.def.is_open {
             if self.in_contract() {