feat: Allow numeric generics to non inlined ACIR functions (#4834)

# Description

## Problem\*

Resolves #4826. 

## Summary\*

When running `nargo info` with the following code:
```rust
use dep::std::hash::{pedersen_hash_with_separator, poseidon2::Poseidon2};

global NUM_HASHES = 2;
global HASH_LENGTH = 10;

#[fold]
pub fn poseidon_hash<N>(inputs: [Field; N]) -> Field {
    Poseidon2::hash(inputs, inputs.len())
}
fn main(
    to_hash: [[Field; HASH_LENGTH]; NUM_HASHES],
    enable: [bool; NUM_HASHES]
) -> pub [Field; NUM_HASHES] {
    let mut result = [0; NUM_HASHES];
    for i in 0..NUM_HASHES {
        let enable = enable[i];
        let to_hash = to_hash[i];
        if enable {
            result[i] = poseidon_hash(to_hash);
        }
    }
    result
}
```
<img width="739" alt="Screenshot 2024-04-17 at 3 46 54 PM"
src="https://github.com/noir-lang/noir/assets/43554004/fa31977d-96e9-45f7-8001-43865bc4a83c">

When using `pedersen_hash` we also get 10 opcodes for `main`. 

Once we get more granularity in our inline types (fold vs. inline for
proving) we can most likely fully resolve this issue
(#4688).

## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

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

@@ -929,6 +929,7 @@ impl<'a> Resolver<'a> {
         let name_ident = HirIdent::non_trait_method(id, location);
 
         let attributes = func.attributes().clone();
+        let should_fold = attributes.is_foldable();
 
         let mut generics = vecmap(&self.generics, |(_, typevar, _)| typevar.clone());
         let mut parameters = vec![];
@@ -1009,8 +1010,6 @@ impl<'a> Resolver<'a> {
             .map(|(name, typevar, _span)| (name.clone(), typevar.clone()))
             .collect();
 
-        let should_fold = attributes.is_foldable();
-
         FuncMeta {
             name: name_ident,
             kind: func.kind,
@@ -1039,7 +1038,7 @@ impl<'a> Resolver<'a> {
     /// 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) -> bool {
-        self.is_entry_point_function(func)
+        self.is_entry_point_function(func) || func.attributes().is_foldable()
     }
 
     fn is_entry_point_function(&self, func: &NoirFunction) -> bool {

compiler/noirc_frontend/src/hir/type_check/mod.rs

@@ -172,7 +172,9 @@ fn check_if_type_is_valid_for_program_input(
     errors: &mut Vec<TypeCheckError>,
 ) {
     let meta = type_checker.interner.function_meta(&func_id);
-    if (meta.is_entry_point || meta.should_fold) && !param.1.is_valid_for_program_input() {
+    if (meta.is_entry_point && !param.1.is_valid_for_program_input())
+        || (meta.should_fold && !param.1.is_valid_non_inlined_function_input())
+    {
         let span = param.0.span();
         errors.push(TypeCheckError::InvalidTypeForEntryPoint { span });
     }
@@ -632,12 +634,23 @@ pub mod test {
             #[fold]
             fn fold(x: &mut Field) -> Field {
                 *x
-            }
+        }
         "#;
 
         type_check_src_code_errors_expected(src, vec![String::from("fold")], 1);
     }
 
+    #[test]
+    fn fold_numeric_generic() {
+        let src = r#"
+        #[fold]
+            fn fold<T>(x: T) -> T {
+                x
+            }
+        "#;
+
+        type_check_src_code(src, vec![String::from("fold")]);
+    }
     // This is the same Stub that is in the resolver, maybe we can pull this out into a test module and re-use?
     struct TestPathResolver(HashMap<String, ModuleDefId>);
 

compiler/noirc_frontend/src/hir_def/types.rs

@@ -726,6 +726,54 @@ impl Type {
         }
     }
 
+    /// True if this type can be used as a parameter to an ACIR function that is not `main` or a contract function.
+    /// This encapsulates functions for which we may not want to inline during compilation.
+    ///
+    /// The inputs allowed for a function entry point differ from those allowed as input to a program as there are
+    /// certain types which through compilation we know what their size should be.
+    /// This includes types such as numeric generics.
+    pub(crate) fn is_valid_non_inlined_function_input(&self) -> bool {
+        match self {
+            // Type::Error is allowed as usual since it indicates an error was already issued and
+            // we don't need to issue further errors about this likely unresolved type
+            Type::FieldElement
+            | Type::Integer(_, _)
+            | Type::Bool
+            | Type::Unit
+            | Type::Constant(_)
+            | Type::TypeVariable(_, _)
+            | Type::NamedGeneric(_, _)
+            | Type::Error => true,
+
+            Type::FmtString(_, _)
+            // To enable this we would need to determine the size of the closure outputs at compile-time.
+            // This is possible as long as the output size is not dependent upon a witness condition.
+            | Type::Function(_, _, _)
+            | Type::Slice(_)
+            | Type::MutableReference(_)
+            | Type::Forall(_, _)
+            // TODO: probably can allow code as it is all compile time
+            | Type::Code
+            | Type::TraitAsType(..) => false,
+
+            Type::Alias(alias, generics) => {
+                let alias = alias.borrow();
+                alias.get_type(generics).is_valid_non_inlined_function_input()
+            }
+
+            Type::Array(length, element) => {
+                length.is_valid_non_inlined_function_input() && element.is_valid_non_inlined_function_input()
+            }
+            Type::String(length) => length.is_valid_non_inlined_function_input(),
+            Type::Tuple(elements) => elements.iter().all(|elem| elem.is_valid_non_inlined_function_input()),
+            Type::Struct(definition, generics) => definition
+                .borrow()
+                .get_fields(generics)
+                .into_iter()
+                .all(|(_, field)| field.is_valid_non_inlined_function_input()),
+        }
+    }
+
     /// Returns the number of `Forall`-quantified type variables on this type.
     /// Returns 0 if this is not a Type::Forall
     pub fn generic_count(&self) -> usize {

compiler/noirc_frontend/src/monomorphization/mod.rs

@@ -283,7 +283,13 @@ impl<'interner> Monomorphizer<'interner> {
         }
 
         let meta = self.interner.function_meta(&f).clone();
-        let func_sig = meta.function_signature();
+        let mut func_sig = meta.function_signature();
+        // Follow the bindings of the function signature for entry points
+        // which are not `main` such as foldable functions.
+        for param in func_sig.0.iter_mut() {
+            param.1 = param.1.follow_bindings();
+        }
+        func_sig.1 = func_sig.1.map(|return_type| return_type.follow_bindings());
 
         let modifiers = self.interner.function_modifiers(&f);
         let name = self.interner.function_name(&f).to_owned();

test_programs/execution_success/fold_numeric_generic_poseidon/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "fold_numeric_generic_poseidon"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.27.0"
+
+[dependencies]

test_programs/execution_success/fold_numeric_generic_poseidon/Prover.toml

@@ -0,0 +1,2 @@
+enable = [true, false]
+to_hash = [[0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]

test_programs/execution_success/fold_numeric_generic_poseidon/src/main.nr

@@ -0,0 +1,33 @@
+use dep::std::hash::{pedersen_hash_with_separator, poseidon2::Poseidon2};
+
+global NUM_HASHES = 2;
+global HASH_LENGTH = 10;
+
+#[fold]
+pub fn poseidon_hash<N>(inputs: [Field; N]) -> Field {
+    Poseidon2::hash(inputs, inputs.len())
+}
+
+fn main(
+    to_hash: [[Field; HASH_LENGTH]; NUM_HASHES],
+    enable: [bool; NUM_HASHES]
+) -> pub [Field; NUM_HASHES + 1] {
+    let mut result = [0; NUM_HASHES + 1];
+    for i in 0..NUM_HASHES {
+        let enable = enable[i];
+        let to_hash = to_hash[i];
+        if enable {
+            result[i] = poseidon_hash(to_hash);
+        }
+    }
+
+    // We want to make sure that the foldable function with a numeric generic
+    // is monomorphized correctly.
+    let mut double_preimage = [0; 20];
+    for i in 0..HASH_LENGTH * 2 {
+        double_preimage[i] = to_hash[0][i % HASH_LENGTH];
+    }
+    result[NUM_HASHES] = poseidon_hash(double_preimage);
+
+    result
+}

tooling/debugger/ignored-tests.txt

@@ -16,4 +16,5 @@ fold_basic
 fold_basic_nested_call
 fold_call_witness_condition
 fold_after_inlined_calls
+fold_numeric_generic_poseidon