feat: check unconstrained trait impl method matches (#6057)

# Description

## Problem

Fixes #5717

## Summary

In a previous PR we started allowing `unconstrained` and `comptime` in
trait impls. But two things remained:
1. Defining an `unconstrained` trait impl method would produce a
warning.
2. We should error if a trait method is unconstrained but the trait impl
method is not, or the other way around.

This PR will also include "unconstrained" in trait imp method stubs in
LSP.

## Additional Context

I initially also errored if there's a mismatch in `comptime`, but that's
probably not good as we have some implementation of `Append` be comptime
and some not. This made me wonder what's the purpose of `comptime` in
functions, if non-comptime functions can also be interpreted.

## 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.

---------

Co-authored-by: jfecher <jake@aztecprotocol.com>

compiler/noirc_frontend/src/elaborator/traits.rs

@@ -229,6 +229,7 @@ pub(crate) fn check_trait_impl_method_matches_declaration(
     function: FuncId,
 ) -> Vec<TypeCheckError> {
     let meta = interner.function_meta(&function);
+    let modifiers = interner.function_modifiers(&function);
     let method_name = interner.function_name(&function);
     let mut errors = Vec::new();
 
@@ -267,6 +268,14 @@ pub(crate) fn check_trait_impl_method_matches_declaration(
     // issue an error for it here.
     if let Some(trait_fn_id) = trait_info.method_ids.get(method_name) {
         let trait_fn_meta = interner.function_meta(trait_fn_id);
+        let trait_fn_modifiers = interner.function_modifiers(trait_fn_id);
+
+        if modifiers.is_unconstrained != trait_fn_modifiers.is_unconstrained {
+            let expected = trait_fn_modifiers.is_unconstrained;
+            let span = meta.name.location.span;
+            let item = method_name.to_string();
+            errors.push(TypeCheckError::UnconstrainedMismatch { item, expected, span });
+        }
 
         if trait_fn_meta.direct_generics.len() != meta.direct_generics.len() {
             let expected = trait_fn_meta.direct_generics.len();

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

@@ -61,6 +61,8 @@ pub enum TypeCheckError {
     ParameterCountMismatch { expected: usize, found: usize, span: Span },
     #[error("{item} expects {expected} generics but {found} were given")]
     GenericCountMismatch { item: String, expected: usize, found: usize, span: Span },
+    #[error("{item} has incompatible `unconstrained`")]
+    UnconstrainedMismatch { item: String, expected: bool, span: Span },
     #[error("Only integer and Field types may be casted to")]
     UnsupportedCast { span: Span },
     #[error("Index {index} is out of bounds for this tuple {lhs_type} of length {length}")]
@@ -264,6 +266,14 @@ impl<'a> From<&'a TypeCheckError> for Diagnostic {
                 let msg = format!("{item} expects {expected} generic{empty_or_s} but {found} {was_or_were} given");
                 Diagnostic::simple_error(msg, String::new(), *span)
             }
+            TypeCheckError::UnconstrainedMismatch { item, expected, span } => {
+                let msg = if *expected {
+                    format!("{item} is expected to be unconstrained")
+                } else {
+                    format!("{item} is not expected to be unconstrained")
+                };
+                Diagnostic::simple_error(msg, String::new(), *span)
+            }
             TypeCheckError::InvalidCast { span, .. }
             | TypeCheckError::ExpectedFunction { span, .. }
             | TypeCheckError::AccessUnknownMember { span, .. }

compiler/noirc_frontend/src/parser/errors.rs

@@ -45,8 +45,8 @@ pub enum ParserErrorReason {
     PatternInTraitFunctionParameter,
     #[error("Patterns aren't allowed in a trait impl's associated constants")]
     PatternInAssociatedConstant,
-    #[error("Modifiers are ignored on a trait impl method")]
-    TraitImplFunctionModifiers,
+    #[error("Visibility is ignored on a trait impl method")]
+    TraitImplVisibilityIgnored,
     #[error("comptime keyword is deprecated")]
     ComptimeDeprecated,
     #[error("{0} are experimental and aren't fully supported yet")]
@@ -202,7 +202,7 @@ impl<'a> From<&'a ParserError> for Diagnostic {
                 ParserErrorReason::ExperimentalFeature(_) => {
                     Diagnostic::simple_warning(reason.to_string(), "".into(), error.span)
                 }
-                ParserErrorReason::TraitImplFunctionModifiers => {
+                ParserErrorReason::TraitImplVisibilityIgnored => {
                     Diagnostic::simple_warning(reason.to_string(), "".into(), error.span)
                 }
                 ParserErrorReason::ExpectedPatternButFoundType(ty) => Diagnostic::simple_error(

compiler/noirc_frontend/src/parser/parser/traits.rs

@@ -187,8 +187,8 @@ pub(super) fn trait_implementation() -> impl NoirParser<TopLevelStatementKind> {
 
 fn trait_implementation_body() -> impl NoirParser<Vec<Documented<TraitImplItem>>> {
     let function = function::function_definition(true).validate(|mut f, span, emit| {
-        if f.def().is_unconstrained || f.def().visibility != ItemVisibility::Private {
-            emit(ParserError::with_reason(ParserErrorReason::TraitImplFunctionModifiers, span));
+        if f.def().visibility != ItemVisibility::Private {
+            emit(ParserError::with_reason(ParserErrorReason::TraitImplVisibilityIgnored, span));
         }
         // Trait impl functions are always public
         f.def_mut().visibility = ItemVisibility::Public;

tooling/lsp/src/requests/code_action/implement_missing_members.rs

@@ -101,10 +101,12 @@ impl<'a> CodeActionFinder<'a> {
 
         for (name, func_id) in method_ids {
             let func_meta = self.interner.function_meta(func_id);
+            let modifiers = self.interner.function_modifiers(func_id);
 
             let mut generator = TraitImplMethodStubGenerator::new(
                 name,
                 func_meta,
+                modifiers,
                 trait_,
                 noir_trait_impl,
                 self.interner,

tooling/lsp/src/requests/completion.rs

@@ -899,10 +899,12 @@ impl<'a> NodeFinder<'a> {
             }
 
             let func_meta = self.interner.function_meta(&func_id);
+            let modifiers = self.interner.function_modifiers(&func_id);
 
             let mut generator = TraitImplMethodStubGenerator::new(
                 &name,
                 func_meta,
+                modifiers,
                 trait_,
                 noir_trait_impl,
                 self.interner,

tooling/lsp/src/trait_impl_method_stub_generator.rs

@@ -9,7 +9,7 @@ use noirc_frontend::{
     },
     hir_def::{function::FuncMeta, stmt::HirPattern, traits::Trait},
     macros_api::{ModuleDefId, NodeInterner},
-    node_interner::ReferenceId,
+    node_interner::{FunctionModifiers, ReferenceId},
     Kind, ResolvedGeneric, Type, TypeVariableKind,
 };
 
@@ -18,6 +18,7 @@ use crate::modules::relative_module_id_path;
 pub(crate) struct TraitImplMethodStubGenerator<'a> {
     name: &'a str,
     func_meta: &'a FuncMeta,
+    modifiers: &'a FunctionModifiers,
     trait_: &'a Trait,
     noir_trait_impl: &'a NoirTraitImpl,
     interner: &'a NodeInterner,
@@ -33,6 +34,7 @@ impl<'a> TraitImplMethodStubGenerator<'a> {
     pub(crate) fn new(
         name: &'a str,
         func_meta: &'a FuncMeta,
+        modifiers: &'a FunctionModifiers,
         trait_: &'a Trait,
         noir_trait_impl: &'a NoirTraitImpl,
         interner: &'a NodeInterner,
@@ -43,6 +45,7 @@ impl<'a> TraitImplMethodStubGenerator<'a> {
         Self {
             name,
             func_meta,
+            modifiers,
             trait_,
             noir_trait_impl,
             interner,
@@ -63,6 +66,9 @@ impl<'a> TraitImplMethodStubGenerator<'a> {
         let indent_string = " ".repeat(self.indent);
 
         self.string.push_str(&indent_string);
+        if self.modifiers.is_unconstrained {
+            self.string.push_str("unconstrained ");
+        }
         self.string.push_str("fn ");
         self.string.push_str(self.name);
         self.append_resolved_generics(&self.func_meta.direct_generics);