feat: trait inheritance (#6252)

# Description

## Problem

Resolves #3949

## Summary

Still pending documentation.

## Additional Context

I copied part of the documentation from
[here](https://practice.course.rs/generics-traits/advanced-traits.html#supertraits),
I hope it's fine.

## Documentation\*

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

# PR Checklist\*

- [ ] 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 <[email protected]>

compiler/noirc_frontend/src/ast/traits.rs

@@ -18,6 +18,7 @@ use super::{Documented, GenericTypeArgs, ItemVisibility};
 pub struct NoirTrait {
     pub name: Ident,
     pub generics: UnresolvedGenerics,
+    pub bounds: Vec<TraitBound>,
     pub where_clause: Vec<UnresolvedTraitConstraint>,
     pub span: Span,
     pub items: Vec<Documented<TraitItem>>,
@@ -134,7 +135,12 @@ impl Display for NoirTrait {
         let generics = vecmap(&self.generics, |generic| generic.to_string());
         let generics = if generics.is_empty() { "".into() } else { generics.join(", ") };
 
-        writeln!(f, "trait {}{} {{", self.name, generics)?;
+        write!(f, "trait {}{}", self.name, generics)?;
+        if !self.bounds.is_empty() {
+            let bounds = vecmap(&self.bounds, |bound| bound.to_string()).join(" + ");
+            write!(f, ": {}", bounds)?;
+        }
+        writeln!(f, " {{")?;
 
         for item in self.items.iter() {
             let item = item.to_string();

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -26,7 +26,7 @@ use crate::{
             HirPrefixExpression,
         },
         stmt::HirStatement,
-        traits::TraitConstraint,
+        traits::{ResolvedTraitBound, TraitConstraint},
     },
     node_interner::{DefinitionKind, ExprId, FuncId, InternedStatementKind, TraitMethodId},
     token::Tokens,
@@ -743,9 +743,11 @@ impl<'context> Elaborator<'context> {
                     // that implements the trait.
                     let constraint = TraitConstraint {
                         typ: operand_type.clone(),
-                        trait_id: trait_id.trait_id,
-                        trait_generics: TraitGenerics::default(),
-                        span,
+                        trait_bound: ResolvedTraitBound {
+                            trait_id: trait_id.trait_id,
+                            trait_generics: TraitGenerics::default(),
+                            span,
+                        },
                     };
                     self.push_trait_constraint(constraint, expr_id);
                     self.type_check_operator_method(expr_id, trait_id, operand_type, span);

compiler/noirc_frontend/src/elaborator/mod.rs

@@ -3,7 +3,7 @@ use std::{
     rc::Rc,
 };
 
-use crate::{ast::ItemVisibility, StructField};
+use crate::{ast::ItemVisibility, hir_def::traits::ResolvedTraitBound, StructField, TypeBindings};
 use crate::{
     ast::{
         BlockExpression, FunctionKind, GenericTypeArgs, Ident, NoirFunction, NoirStruct, Param,
@@ -54,6 +54,7 @@ mod unquote;
 use fm::FileId;
 use iter_extended::vecmap;
 use noirc_errors::{Location, Span};
+use types::bind_ordered_generics;
 
 use self::traits::check_trait_impl_method_matches_declaration;
 
@@ -433,7 +434,8 @@ impl<'context> Elaborator<'context> {
 
         // Now remove all the `where` clause constraints we added
         for constraint in &func_meta.trait_constraints {
-            self.interner.remove_assumed_trait_implementations_for_trait(constraint.trait_id);
+            self.interner
+                .remove_assumed_trait_implementations_for_trait(constraint.trait_bound.trait_id);
         }
 
         let func_scope_tree = self.scopes.end_function();
@@ -479,9 +481,9 @@ impl<'context> Elaborator<'context> {
 
             self.verify_trait_constraint(
                 &constraint.typ,
-                constraint.trait_id,
-                &constraint.trait_generics.ordered,
-                &constraint.trait_generics.named,
+                constraint.trait_bound.trait_id,
+                &constraint.trait_bound.trait_generics.ordered,
+                &constraint.trait_bound.trait_generics.named,
                 expr_id,
                 span,
             );
@@ -510,7 +512,8 @@ impl<'context> Elaborator<'context> {
         let generic_type = Type::NamedGeneric(new_generic, Rc::new(name));
         let trait_bound = TraitBound { trait_path, trait_id: None, trait_generics };
 
-        if let Some(new_constraint) = self.resolve_trait_bound(&trait_bound, generic_type.clone()) {
+        if let Some(trait_bound) = self.resolve_trait_bound(&trait_bound) {
+            let new_constraint = TraitConstraint { typ: generic_type.clone(), trait_bound };
             trait_constraints.push(new_constraint);
         }
 
@@ -668,22 +671,19 @@ impl<'context> Elaborator<'context> {
         constraint: &UnresolvedTraitConstraint,
     ) -> Option<TraitConstraint> {
         let typ = self.resolve_type(constraint.typ.clone());
-        self.resolve_trait_bound(&constraint.trait_bound, typ)
+        let trait_bound = self.resolve_trait_bound(&constraint.trait_bound)?;
+        Some(TraitConstraint { typ, trait_bound })
     }
 
-    pub fn resolve_trait_bound(
-        &mut self,
-        bound: &TraitBound,
-        typ: Type,
-    ) -> Option<TraitConstraint> {
+    pub fn resolve_trait_bound(&mut self, bound: &TraitBound) -> Option<ResolvedTraitBound> {
         let the_trait = self.lookup_trait_or_error(bound.trait_path.clone())?;
         let trait_id = the_trait.id;
         let span = bound.trait_path.span;
 
         let (ordered, named) = self.resolve_type_args(bound.trait_generics.clone(), trait_id, span);
 
         let trait_generics = TraitGenerics { ordered, named };
-        Some(TraitConstraint { typ, trait_id, trait_generics, span })
+        Some(ResolvedTraitBound { trait_id, trait_generics, span })
     }
 
     /// Extract metadata from a NoirFunction
@@ -942,21 +942,52 @@ impl<'context> Elaborator<'context> {
 
     fn add_trait_constraints_to_scope(&mut self, func_meta: &FuncMeta) {
         for constraint in &func_meta.trait_constraints {
-            let object = constraint.typ.clone();
-            let trait_id = constraint.trait_id;
-            let generics = constraint.trait_generics.clone();
-
-            if !self.interner.add_assumed_trait_implementation(object, trait_id, generics) {
-                if let Some(the_trait) = self.interner.try_get_trait(trait_id) {
-                    let trait_name = the_trait.name.to_string();
-                    let typ = constraint.typ.clone();
-                    let span = func_meta.location.span;
-                    self.push_err(TypeCheckError::UnneededTraitConstraint {
-                        trait_name,
-                        typ,
-                        span,
-                    });
+            self.add_trait_bound_to_scope(
+                func_meta,
+                &constraint.typ,
+                &constraint.trait_bound,
+                constraint.trait_bound.trait_id,
+            );
+        }
+    }
+
+    fn add_trait_bound_to_scope(
+        &mut self,
+        func_meta: &FuncMeta,
+        object: &Type,
+        trait_bound: &ResolvedTraitBound,
+        starting_trait_id: TraitId,
+    ) {
+        let trait_id = trait_bound.trait_id;
+        let generics = trait_bound.trait_generics.clone();
+
+        if !self.interner.add_assumed_trait_implementation(object.clone(), trait_id, generics) {
+            if let Some(the_trait) = self.interner.try_get_trait(trait_id) {
+                let trait_name = the_trait.name.to_string();
+                let typ = object.clone();
+                let span = func_meta.location.span;
+                self.push_err(TypeCheckError::UnneededTraitConstraint { trait_name, typ, span });
+            }
+        }
+
+        // Also add assumed implementations for the parent traits, if any
+        if let Some(trait_bounds) =
+            self.interner.try_get_trait(trait_id).map(|the_trait| the_trait.trait_bounds.clone())
+        {
+            for parent_trait_bound in trait_bounds {
+                // Avoid looping forever in case there are cycles
+                if parent_trait_bound.trait_id == starting_trait_id {
+                    continue;
                 }
+
+                let parent_trait_bound =
+                    self.instantiate_parent_trait_bound(trait_bound, &parent_trait_bound);
+                self.add_trait_bound_to_scope(
+                    func_meta,
+                    object,
+                    &parent_trait_bound,
+                    starting_trait_id,
+                );
             }
         }
     }
@@ -972,6 +1003,8 @@ impl<'context> Elaborator<'context> {
         self.file = trait_impl.file_id;
         self.local_module = trait_impl.module_id;
 
+        self.check_parent_traits_are_implemented(&trait_impl);
+
         self.generics = trait_impl.resolved_generics;
         self.current_trait_impl = trait_impl.impl_id;
 
@@ -988,6 +1021,73 @@ impl<'context> Elaborator<'context> {
         self.generics.clear();
     }
 
+    fn check_parent_traits_are_implemented(&mut self, trait_impl: &UnresolvedTraitImpl) {
+        let Some(trait_id) = trait_impl.trait_id else {
+            return;
+        };
+
+        let Some(object_type) = &trait_impl.resolved_object_type else {
+            return;
+        };
+
+        let Some(the_trait) = self.interner.try_get_trait(trait_id) else {
+            return;
+        };
+
+        if the_trait.trait_bounds.is_empty() {
+            return;
+        }
+
+        let impl_trait = the_trait.name.to_string();
+        let the_trait_file = the_trait.location.file;
+
+        let mut bindings = TypeBindings::new();
+        bind_ordered_generics(
+            &the_trait.generics,
+            &trait_impl.resolved_trait_generics,
+            &mut bindings,
+        );
+
+        // Note: we only check if the immediate parents are implemented, we don't check recursively.
+        // Why? If a parent isn't implemented, we get an error. If a parent is implemented, we'll
+        // do the same check for the parent, so this trait's parents parents will be checked, so the
+        // recursion is guaranteed.
+        for parent_trait_bound in the_trait.trait_bounds.clone() {
+            let Some(parent_trait) = self.interner.try_get_trait(parent_trait_bound.trait_id)
+            else {
+                continue;
+            };
+
+            let parent_trait_bound = ResolvedTraitBound {
+                trait_generics: parent_trait_bound
+                    .trait_generics
+                    .map(|typ| typ.substitute(&bindings)),
+                ..parent_trait_bound
+            };
+
+            if self
+                .interner
+                .try_lookup_trait_implementation(
+                    object_type,
+                    parent_trait_bound.trait_id,
+                    &parent_trait_bound.trait_generics.ordered,
+                    &parent_trait_bound.trait_generics.named,
+                )
+                .is_err()
+            {
+                let missing_trait =
+                    format!("{}{}", parent_trait.name, parent_trait_bound.trait_generics);
+                self.push_err(ResolverError::TraitNotImplemented {
+                    impl_trait: impl_trait.clone(),
+                    missing_trait,
+                    type_missing_trait: trait_impl.object_type.to_string(),
+                    span: trait_impl.object_type.span,
+                    missing_trait_location: Location::new(parent_trait_bound.span, the_trait_file),
+                });
+            }
+        }
+    }
+
     fn collect_impls(
         &mut self,
         module: LocalModuleId,

compiler/noirc_frontend/src/elaborator/patterns.rs

@@ -655,7 +655,7 @@ impl<'context> Elaborator<'context> {
         if let ImplKind::TraitMethod(mut method) = ident.impl_kind {
             method.constraint.apply_bindings(&bindings);
             if method.assumed {
-                let trait_generics = method.constraint.trait_generics.clone();
+                let trait_generics = method.constraint.trait_bound.trait_generics.clone();
                 let object_type = method.constraint.typ;
                 let trait_impl = TraitImplKind::Assumed { object_type, trait_generics };
                 self.interner.select_impl_for_expression(expr_id, trait_impl);
@@ -748,7 +748,7 @@ impl<'context> Elaborator<'context> {
             HirMethodReference::TraitMethodId(method_id, generics) => {
                 let mut constraint =
                     self.interner.get_trait(method_id.trait_id).as_constraint(span);
-                constraint.trait_generics = generics;
+                constraint.trait_bound.trait_generics = generics;
                 ImplKind::TraitMethod(TraitMethod { method_id, constraint, assumed: false })
             }
         };

compiler/noirc_frontend/src/elaborator/trait_impls.rs

@@ -167,36 +167,40 @@ impl<'context> Elaborator<'context> {
         let mut substituted_method_ids = HashSet::default();
         for method_constraint in method.trait_constraints.iter() {
             let substituted_constraint_type = method_constraint.typ.substitute(&bindings);
-            let substituted_trait_generics =
-                method_constraint.trait_generics.map(|generic| generic.substitute(&bindings));
+            let substituted_trait_generics = method_constraint
+                .trait_bound
+                .trait_generics
+                .map(|generic| generic.substitute(&bindings));
 
             substituted_method_ids.insert((
                 substituted_constraint_type,
-                method_constraint.trait_id,
+                method_constraint.trait_bound.trait_id,
                 substituted_trait_generics,
             ));
         }
 
         for override_trait_constraint in override_meta.trait_constraints.clone() {
             let override_constraint_is_from_impl =
                 trait_impl_where_clause.iter().any(|impl_constraint| {
-                    impl_constraint.trait_id == override_trait_constraint.trait_id
+                    impl_constraint.trait_bound.trait_id
+                        == override_trait_constraint.trait_bound.trait_id
                 });
             if override_constraint_is_from_impl {
                 continue;
             }
 
             if !substituted_method_ids.contains(&(
                 override_trait_constraint.typ.clone(),
-                override_trait_constraint.trait_id,
-                override_trait_constraint.trait_generics.clone(),
+                override_trait_constraint.trait_bound.trait_id,
+                override_trait_constraint.trait_bound.trait_generics.clone(),
             )) {
-                let the_trait = self.interner.get_trait(override_trait_constraint.trait_id);
+                let the_trait =
+                    self.interner.get_trait(override_trait_constraint.trait_bound.trait_id);
                 self.push_err(DefCollectorErrorKind::ImplIsStricterThanTrait {
                     constraint_typ: override_trait_constraint.typ,
                     constraint_name: the_trait.name.0.contents.clone(),
-                    constraint_generics: override_trait_constraint.trait_generics,
-                    constraint_span: override_trait_constraint.span,
+                    constraint_generics: override_trait_constraint.trait_bound.trait_generics,
+                    constraint_span: override_trait_constraint.trait_bound.span,
                     trait_method_name: method.name.0.contents.clone(),
                     trait_method_span: method.location.span,
                 });

compiler/noirc_frontend/src/elaborator/traits.rs

@@ -10,8 +10,11 @@ use crate::{
         UnresolvedTraitConstraint, UnresolvedType,
     },
     hir::{def_collector::dc_crate::UnresolvedTrait, type_check::TypeCheckError},
-    hir_def::{function::Parameters, traits::TraitFunction},
-    node_interner::{FuncId, NodeInterner, ReferenceId, TraitId},
+    hir_def::{
+        function::Parameters,
+        traits::{ResolvedTraitBound, TraitFunction},
+    },
+    node_interner::{DependencyId, FuncId, NodeInterner, ReferenceId, TraitId},
     ResolvedGeneric, Type, TypeBindings,
 };
 
@@ -34,10 +37,17 @@ impl<'context> Elaborator<'context> {
                     this.generics.push(associated_type.clone());
                 }
 
+                let resolved_trait_bounds = this.resolve_trait_bounds(unresolved_trait);
+                for bound in &resolved_trait_bounds {
+                    this.interner
+                        .add_trait_dependency(DependencyId::Trait(bound.trait_id), *trait_id);
+                }
+
                 let methods = this.resolve_trait_methods(*trait_id, unresolved_trait);
 
                 this.interner.update_trait(*trait_id, |trait_def| {
                     trait_def.set_methods(methods);
+                    trait_def.set_trait_bounds(resolved_trait_bounds);
                 });
             });
 
@@ -53,6 +63,14 @@ impl<'context> Elaborator<'context> {
         self.current_trait = None;
     }
 
+    fn resolve_trait_bounds(
+        &mut self,
+        unresolved_trait: &UnresolvedTrait,
+    ) -> Vec<ResolvedTraitBound> {
+        let bounds = &unresolved_trait.trait_def.bounds;
+        bounds.iter().filter_map(|bound| self.resolve_trait_bound(bound)).collect()
+    }
+
     fn resolve_trait_methods(
         &mut self,
         trait_id: TraitId,