fix: prevent declarations of blackbox functions outside of the stdlib (#4177)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

I found this [semaphore lib which is redeclaring pedersen for some
reason](https://github.com/siosw/semaphore-noir/blob/main/circuits/src/identity.nr).
Let's ~break it!~ give a nice error message to tell users that they're
doing something wrong here.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

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

Author: TomAFrench

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

@@ -84,6 +84,8 @@ pub enum ResolverError {
     InvalidTypeForEntryPoint { span: Span },
     #[error("Nested slices are not supported")]
     NestedSlices { span: Span },
+    #[error("Usage of the `#[foreign]` or `#[builtin]` function attributes are not allowed outside of the Noir standard library")]
+    LowLevelFunctionOutsideOfStdlib { ident: Ident },
 }
 
 impl ResolverError {
@@ -311,6 +313,11 @@ impl From<ResolverError> for Diagnostic {
                 "Try to use a constant sized array instead".into(),
                 span,
             ),
+            ResolverError::LowLevelFunctionOutsideOfStdlib { ident } => Diagnostic::simple_error(
+                "Definition of low-level function outside of standard library".into(),
+                "Usage of the `#[foreign]` or `#[builtin]` function attributes are not allowed outside of the Noir standard library".into(),
+                ident.span(),
+            ),
         }
     }
 }

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

@@ -908,6 +908,13 @@ impl<'a> Resolver<'a> {
                 position: PubPosition::ReturnType,
             });
         }
+        let is_low_level_function =
+            func.attributes().function.as_ref().map_or(false, |func| func.is_low_level());
+        if !self.path_resolver.module_id().krate.is_stdlib() && is_low_level_function {
+            let error =
+                ResolverError::LowLevelFunctionOutsideOfStdlib { ident: func.name_ident().clone() };
+            self.push_err(error);
+        }
 
         // 'pub' is required on return types for entry point functions
         if self.is_entry_point_function(func)

compiler/noirc_frontend/src/tests.rs

@@ -52,10 +52,12 @@ mod test {
     ) -> (ParsedModule, Context, Vec<(CompilationError, FileId)>) {
         let root = std::path::Path::new("/");
         let fm = FileManager::new(root);
+
         let mut context = Context::new(fm, Default::default());
         context.def_interner.populate_dummy_operator_traits();
         let root_file_id = FileId::dummy();
         let root_crate_id = context.crate_graph.add_crate_root(root_file_id);
+
         let (program, parser_errors) = parse_program(src);
         let mut errors = vecmap(parser_errors, |e| (e.into(), root_file_id));
         remove_experimental_warnings(&mut errors);

test_programs/compile_failure/builtin_function_declaration/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "builtin_function_declaration"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.23.0"
+
+[dependencies]

test_programs/compile_failure/builtin_function_declaration/src/main.nr

@@ -0,0 +1,10 @@
+// This test prevents users from trying to create their own builtin functions as these should only exist in the stdlib.
+
+// This would otherwise be a perfectly valid declaration of the `to_le_bits` builtin function
+#[builtin(to_le_bits)]
+fn to_le_bits(_x: Field, _bit_size: u32) -> [u1] {}
+
+fn main(x: Field) -> pub u1 {
+    let bits = to_le_bits(x, 100);
+    bits[0]
+}

test_programs/compile_failure/foreign_function_declaration/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "foreign_function_declaration"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.23.0"
+
+[dependencies]

test_programs/compile_failure/foreign_function_declaration/src/main.nr

@@ -0,0 +1,10 @@
+// This test prevents users from trying to create their own blackbox functions as these should only exist in the stdlib.
+
+// This would otherwise be a perfectly valid definition of the `pedersen_hash` black box function,
+// however executing the circuit results in an unhelpful ICE.
+#[foreign(pedersen_hash)]
+fn my_pedersen_hash<N>(_input: [Field; N]) -> Field {}
+
+fn main() -> pub Field {
+    my_pedersen_hash([1])
+}