fix: unconstrained fn mismatch is now a warning (#5764)

# Description

## Problem

Resolves #5763

## Summary

This code is handled differently than the other `unsafe` checks, and I
forgot about that.

## Additional Context

This issue is preventing noir syncs on Aztec Packages.

## Documentation

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** 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.

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -506,7 +506,7 @@ impl<'context> Elaborator<'context> {
                 unseen_fields.remove(&field_name);
                 seen_fields.insert(field_name.clone());
 
-                self.unify_with_coercions(&field_type, expected_type, resolved, || {
+                self.unify_with_coercions(&field_type, expected_type, resolved, field_span, || {
                     TypeCheckError::TypeMismatch {
                         expected_typ: expected_type.to_string(),
                         expr_typ: field_type.to_string(),

compiler/noirc_frontend/src/elaborator/statements.rs

@@ -78,7 +78,7 @@ impl<'context> Elaborator<'context> {
         let r#type = if annotated_type != Type::Error {
             // Now check if LHS is the same type as the RHS
             // Importantly, we do not coerce any types implicitly
-            self.unify_with_coercions(&expr_type, &annotated_type, expression, || {
+            self.unify_with_coercions(&expr_type, &annotated_type, expression, expr_span, || {
                 TypeCheckError::TypeMismatch {
                     expected_typ: annotated_type.to_string(),
                     expr_typ: expr_type.to_string(),
@@ -136,7 +136,7 @@ impl<'context> Elaborator<'context> {
             self.push_err(TypeCheckError::VariableMustBeMutable { name, span });
         }
 
-        self.unify_with_coercions(&expr_type, &lvalue_type, expression, || {
+        self.unify_with_coercions(&expr_type, &lvalue_type, expression, span, || {
             TypeCheckError::TypeMismatchWithSource {
                 actual: expr_type.clone(),
                 expected: lvalue_type.clone(),

compiler/noirc_frontend/src/elaborator/types.rs

@@ -641,10 +641,18 @@ impl<'context> Elaborator<'context> {
         actual: &Type,
         expected: &Type,
         expression: ExprId,
+        span: Span,
         make_error: impl FnOnce() -> TypeCheckError,
     ) {
         let mut errors = Vec::new();
-        actual.unify_with_coercions(expected, expression, self.interner, &mut errors, make_error);
+        actual.unify_with_coercions(
+            expected,
+            expression,
+            span,
+            self.interner,
+            &mut errors,
+            make_error,
+        );
         self.errors.extend(errors.into_iter().map(|error| (error.into(), self.file)));
     }
 
@@ -736,10 +744,12 @@ impl<'context> Elaborator<'context> {
         }
 
         for (param, (arg, arg_expr_id, arg_span)) in fn_params.iter().zip(callsite_args) {
-            self.unify_with_coercions(arg, param, *arg_expr_id, || TypeCheckError::TypeMismatch {
-                expected_typ: param.to_string(),
-                expr_typ: arg.to_string(),
-                expr_span: *arg_span,
+            self.unify_with_coercions(arg, param, *arg_expr_id, *arg_span, || {
+                TypeCheckError::TypeMismatch {
+                    expected_typ: param.to_string(),
+                    expr_typ: arg.to_string(),
+                    expr_span: *arg_span,
+                }
             });
         }
 
@@ -1449,7 +1459,7 @@ impl<'context> Elaborator<'context> {
                 });
             }
         } else {
-            self.unify_with_coercions(&body_type, declared_return_type, body_id, || {
+            self.unify_with_coercions(&body_type, declared_return_type, body_id, func_span, || {
                 let mut error = TypeCheckError::TypeMismatchWithSource {
                     expected: declared_return_type.clone(),
                     actual: body_type.clone(),

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

@@ -138,6 +138,8 @@ pub enum TypeCheckError {
     UnconstrainedSliceReturnToConstrained { span: Span },
     #[error("Call to unconstrained function is unsafe and must be in an unconstrained function or unsafe block")]
     Unsafe { span: Span },
+    #[error("Converting an unconstrained fn to a non-unconstrained fn is unsafe")]
+    UnsafeFn { span: Span },
     #[error("Slices must have constant length")]
     NonConstantSliceLength { span: Span },
     #[error("Only sized types may be used in the entry point to a program")]
@@ -361,6 +363,9 @@ impl<'a> From<&'a TypeCheckError> for Diagnostic {
             TypeCheckError::Unsafe { span } => {
                 Diagnostic::simple_warning(error.to_string(), String::new(), *span)
             }
+            TypeCheckError::UnsafeFn { span } => {
+                Diagnostic::simple_warning(error.to_string(), String::new(), *span)
+            }
         }
     }
 }

compiler/noirc_frontend/src/hir_def/types.rs

@@ -205,6 +205,12 @@ impl ResolvedGeneric {
     }
 }
 
+enum FunctionCoercionResult {
+    NoCoercion,
+    Coerced(Type),
+    UnconstrainedMismatch(Type),
+}
+
 impl std::hash::Hash for StructType {
     fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
         self.id.hash(state);
@@ -1776,6 +1782,7 @@ impl Type {
         &self,
         expected: &Type,
         expression: ExprId,
+        span: Span,
         interner: &mut NodeInterner,
         errors: &mut Vec<TypeCheckError>,
         make_error: impl FnOnce() -> TypeCheckError,
@@ -1792,28 +1799,40 @@ impl Type {
         }
 
         // Try to coerce `fn (..) -> T` to `unconstrained fn (..) -> T`
-        if let Some(coerced_self) = self.try_fn_to_unconstrained_fn_coercion(expected) {
-            coerced_self.unify_with_coercions(expected, expression, interner, errors, make_error);
-            return;
-        }
+        match self.try_fn_to_unconstrained_fn_coercion(expected) {
+            FunctionCoercionResult::NoCoercion => errors.push(make_error()),
+            FunctionCoercionResult::Coerced(coerced_self) => {
+                coerced_self
+                    .unify_with_coercions(expected, expression, span, interner, errors, make_error);
+            }
+            FunctionCoercionResult::UnconstrainedMismatch(coerced_self) => {
+                errors.push(TypeCheckError::UnsafeFn { span });
 
-        errors.push(make_error());
+                coerced_self
+                    .unify_with_coercions(expected, expression, span, interner, errors, make_error);
+            }
+        }
     }
 
     // If `self` and `expected` are function types, tries to coerce `self` to `expected`.
     // Returns None if no coercion can be applied, otherwise returns `self` coerced to `expected`.
-    fn try_fn_to_unconstrained_fn_coercion(&self, expected: &Type) -> Option<Type> {
+    fn try_fn_to_unconstrained_fn_coercion(&self, expected: &Type) -> FunctionCoercionResult {
         // If `self` and `expected` are function types, `self` can be coerced to `expected`
         // if `self` is unconstrained and `expected` is not. The other way around is an error, though.
         if let (
             Type::Function(params, ret, env, unconstrained_self),
             Type::Function(_, _, _, unconstrained_expected),
         ) = (self.follow_bindings(), expected.follow_bindings())
         {
-            (!unconstrained_self && unconstrained_expected)
-                .then(|| Type::Function(params, ret, env, unconstrained_expected))
+            let coerced_type = Type::Function(params, ret, env, unconstrained_expected);
+
+            match (unconstrained_self, unconstrained_expected) {
+                (true, true) | (false, false) => FunctionCoercionResult::NoCoercion,
+                (false, true) => FunctionCoercionResult::Coerced(coerced_type),
+                (true, false) => FunctionCoercionResult::UnconstrainedMismatch(coerced_type),
+            }
         } else {
-            None
+            FunctionCoercionResult::NoCoercion
         }
     }
 

compiler/noirc_frontend/src/tests.rs

@@ -2562,16 +2562,8 @@ fn cannot_pass_unconstrained_function_to_regular_function() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    if let CompilationError::TypeError(TypeCheckError::TypeMismatch {
-        expected_typ,
-        expr_typ,
-        ..
-    }) = &errors[0].0
-    {
-        assert_eq!(expected_typ, "fn() -> ()");
-        assert_eq!(expr_typ, "unconstrained fn() -> ()");
-    } else {
-        panic!("Expected a type mismatch error, got {:?}", errors[0].0);
+    let CompilationError::TypeError(TypeCheckError::UnsafeFn { .. }) = &errors[0].0 else {
+        panic!("Expected an UnsafeFn error, got {:?}", errors[0].0);
     };
 }
 
@@ -2630,8 +2622,8 @@ fn cannot_pass_unconstrained_function_to_constrained_function() {
     let errors = get_program_errors(src);
     assert_eq!(errors.len(), 1);
 
-    let CompilationError::TypeError(TypeCheckError::TypeMismatch { .. }) = &errors[0].0 else {
-        panic!("Expected a type mismatch error, got {:?}", errors[0].0);
+    let CompilationError::TypeError(TypeCheckError::UnsafeFn { .. }) = &errors[0].0 else {
+        panic!("Expected an UnsafeFn error, got {:?}", errors[0].0);
     };
 }