feat: trait aliases (#6431)

Co-authored-by: Tom French <[email protected]>

compiler/noirc_frontend/src/ast/traits.rs

@@ -24,6 +24,7 @@ pub struct NoirTrait {
     pub items: Vec<Documented<TraitItem>>,
     pub attributes: Vec<SecondaryAttribute>,
     pub visibility: ItemVisibility,
+    pub is_alias: bool,
 }
 
 /// Any declaration inside the body of a trait that a user is required to
@@ -77,6 +78,9 @@ pub struct NoirTraitImpl {
     pub where_clause: Vec<UnresolvedTraitConstraint>,
 
     pub items: Vec<Documented<TraitImplItem>>,
+
+    /// true if generated at compile-time, e.g. from a trait alias
+    pub is_synthetic: bool,
 }
 
 /// Represents a simple trait constraint such as `where Foo: TraitY<U, V>`
@@ -130,12 +134,19 @@ impl Display for TypeImpl {
     }
 }
 
+// TODO: display where clauses (follow-up issue)
 impl Display for NoirTrait {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let generics = vecmap(&self.generics, |generic| generic.to_string());
         let generics = if generics.is_empty() { "".into() } else { generics.join(", ") };
 
         write!(f, "trait {}{}", self.name, generics)?;
+
+        if self.is_alias {
+            let bounds = vecmap(&self.bounds, |bound| bound.to_string()).join(" + ");
+            return write!(f, " = {};", bounds);
+        }
+
         if !self.bounds.is_empty() {
             let bounds = vecmap(&self.bounds, |bound| bound.to_string()).join(" + ");
             write!(f, ": {}", bounds)?;
@@ -222,6 +233,11 @@ impl Display for TraitBound {
 
 impl Display for NoirTraitImpl {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // Synthetic NoirTraitImpl's don't get printed
+        if self.is_synthetic {
+            return Ok(());
+        }
+
         write!(f, "impl")?;
         if !self.impl_generics.is_empty() {
             write!(

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

@@ -208,7 +208,7 @@ pub enum TypeCheckError {
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct NoMatchingImplFoundError {
-    constraints: Vec<(Type, String)>,
+    pub(crate) constraints: Vec<(Type, String)>,
     pub span: Span,
 }
 

compiler/noirc_frontend/src/parser/errors.rs

@@ -95,6 +95,8 @@ pub enum ParserErrorReason {
     AssociatedTypesNotAllowedInPaths,
     #[error("Associated types are not allowed on a method call")]
     AssociatedTypesNotAllowedInMethodCalls,
+    #[error("Empty trait alias")]
+    EmptyTraitAlias,
     #[error(
         "Wrong number of arguments for attribute `{}`. Expected {}, found {}",
         name,

compiler/noirc_frontend/src/parser/parser/impls.rs

@@ -113,6 +113,7 @@ impl<'a> Parser<'a> {
         let object_type = self.parse_type_or_error();
         let where_clause = self.parse_where_clause();
         let items = self.parse_trait_impl_body();
+        let is_synthetic = false;
 
         NoirTraitImpl {
             impl_generics,
@@ -121,6 +122,7 @@ impl<'a> Parser<'a> {
             object_type,
             where_clause,
             items,
+            is_synthetic,
         }
     }
 

compiler/noirc_frontend/src/parser/parser/item.rs

@@ -1,3 +1,5 @@
+use iter_extended::vecmap;
+
 use crate::{
     parser::{labels::ParsingRuleLabel, Item, ItemKind},
     token::{Keyword, Token},
@@ -13,31 +15,32 @@ impl<'a> Parser<'a> {
     }
 
     pub(crate) fn parse_module_items(&mut self, nested: bool) -> Vec<Item> {
-        self.parse_many("items", without_separator(), |parser| {
+        self.parse_many_to_many("items", without_separator(), |parser| {
             parser.parse_module_item_in_list(nested)
         })
     }
 
-    fn parse_module_item_in_list(&mut self, nested: bool) -> Option<Item> {
+    fn parse_module_item_in_list(&mut self, nested: bool) -> Vec<Item> {
         loop {
             // We only break out of the loop on `}` if we are inside a `mod { ..`
             if nested && self.at(Token::RightBrace) {
-                return None;
+                return vec![];
             }
 
             // We always break on EOF (we don't error because if we are inside `mod { ..`
             // the outer parsing logic will error instead)
             if self.at_eof() {
-                return None;
+                return vec![];
             }
 
-            let Some(item) = self.parse_item() else {
+            let parsed_items = self.parse_item();
+            if parsed_items.is_empty() {
                 // If we couldn't parse an item we check which token we got
                 match self.token.token() {
                     Token::RightBrace if nested => {
-                        return None;
+                        return vec![];
                     }
-                    Token::EOF => return None,
+                    Token::EOF => return vec![],
                     _ => (),
                 }
 
@@ -47,7 +50,7 @@ impl<'a> Parser<'a> {
                 continue;
             };
 
-            return Some(item);
+            return parsed_items;
         }
     }
 
@@ -85,15 +88,19 @@ impl<'a> Parser<'a> {
     }
 
     /// Item = OuterDocComments ItemKind
-    fn parse_item(&mut self) -> Option<Item> {
+    fn parse_item(&mut self) -> Vec<Item> {
         let start_span = self.current_token_span;
         let doc_comments = self.parse_outer_doc_comments();
-        let kind = self.parse_item_kind()?;
+        let kinds = self.parse_item_kind();
         let span = self.span_since(start_span);
 
-        Some(Item { kind, span, doc_comments })
+        vecmap(kinds, |kind| Item { kind, span, doc_comments: doc_comments.clone() })
     }
 
+    /// This method returns one 'ItemKind' in the majority of cases.
+    /// The current exception is when parsing a trait alias,
+    /// which returns both the trait and the impl.
+    ///
     /// ItemKind
     ///     = InnerAttribute
     ///     | Attributes Modifiers
@@ -106,9 +113,9 @@ impl<'a> Parser<'a> {
     ///         | TypeAlias
     ///         | Function
     ///         )
-    fn parse_item_kind(&mut self) -> Option<ItemKind> {
+    fn parse_item_kind(&mut self) -> Vec<ItemKind> {
         if let Some(kind) = self.parse_inner_attribute() {
-            return Some(ItemKind::InnerAttribute(kind));
+            return vec![ItemKind::InnerAttribute(kind)];
         }
 
         let start_span = self.current_token_span;
@@ -122,78 +129,81 @@ impl<'a> Parser<'a> {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
             let use_tree = self.parse_use_tree();
-            return Some(ItemKind::Import(use_tree, modifiers.visibility));
+            return vec![ItemKind::Import(use_tree, modifiers.visibility)];
         }
 
         if let Some(is_contract) = self.eat_mod_or_contract() {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
-            return Some(self.parse_mod_or_contract(attributes, is_contract, modifiers.visibility));
+            return vec![self.parse_mod_or_contract(attributes, is_contract, modifiers.visibility)];
         }
 
         if self.eat_keyword(Keyword::Struct) {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
-            return Some(ItemKind::Struct(self.parse_struct(
+            return vec![ItemKind::Struct(self.parse_struct(
                 attributes,
                 modifiers.visibility,
                 start_span,
-            )));
+            ))];
         }
 
         if self.eat_keyword(Keyword::Impl) {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
-            return Some(match self.parse_impl() {
+            return vec![match self.parse_impl() {
                 Impl::Impl(type_impl) => ItemKind::Impl(type_impl),
                 Impl::TraitImpl(noir_trait_impl) => ItemKind::TraitImpl(noir_trait_impl),
-            });
+            }];
         }
 
         if self.eat_keyword(Keyword::Trait) {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
-            return Some(ItemKind::Trait(self.parse_trait(
-                attributes,
-                modifiers.visibility,
-                start_span,
-            )));
+            let (noir_trait, noir_impl) =
+                self.parse_trait(attributes, modifiers.visibility, start_span);
+            let mut output = vec![ItemKind::Trait(noir_trait)];
+            if let Some(noir_impl) = noir_impl {
+                output.push(ItemKind::TraitImpl(noir_impl));
+            }
+
+            return output;
         }
 
         if self.eat_keyword(Keyword::Global) {
             self.unconstrained_not_applicable(modifiers);
 
-            return Some(ItemKind::Global(
+            return vec![ItemKind::Global(
                 self.parse_global(
                     attributes,
                     modifiers.comptime.is_some(),
                     modifiers.mutable.is_some(),
                 ),
                 modifiers.visibility,
-            ));
+            )];
         }
 
         if self.eat_keyword(Keyword::Type) {
             self.comptime_mutable_and_unconstrained_not_applicable(modifiers);
 
-            return Some(ItemKind::TypeAlias(
+            return vec![ItemKind::TypeAlias(
                 self.parse_type_alias(modifiers.visibility, start_span),
-            ));
+            )];
         }
 
         if self.eat_keyword(Keyword::Fn) {
             self.mutable_not_applicable(modifiers);
 
-            return Some(ItemKind::Function(self.parse_function(
+            return vec![ItemKind::Function(self.parse_function(
                 attributes,
                 modifiers.visibility,
                 modifiers.comptime.is_some(),
                 modifiers.unconstrained.is_some(),
                 false, // allow_self
-            )));
+            ))];
         }
 
-        None
+        vec![]
     }
 
     fn eat_mod_or_contract(&mut self) -> Option<bool> {

compiler/noirc_frontend/src/parser/parser/parse_many.rs

@@ -18,6 +18,19 @@ impl<'a> Parser<'a> {
         self.parse_many_return_trailing_separator_if_any(items, separated_by, f).0
     }
 
+    /// parse_many, where the given function `f` may return multiple results
+    pub(super) fn parse_many_to_many<T, F>(
+        &mut self,
+        items: &'static str,
+        separated_by: SeparatedBy,
+        f: F,
+    ) -> Vec<T>
+    where
+        F: FnMut(&mut Parser<'a>) -> Vec<T>,
+    {
+        self.parse_many_to_many_return_trailing_separator_if_any(items, separated_by, f).0
+    }
+
     /// Same as parse_many, but returns a bool indicating whether a trailing separator was found.
     pub(super) fn parse_many_return_trailing_separator_if_any<T, F>(
         &mut self,
@@ -27,6 +40,26 @@ impl<'a> Parser<'a> {
     ) -> (Vec<T>, bool)
     where
         F: FnMut(&mut Parser<'a>) -> Option<T>,
+    {
+        let f = |x: &mut Parser<'a>| {
+            if let Some(result) = f(x) {
+                vec![result]
+            } else {
+                vec![]
+            }
+        };
+        self.parse_many_to_many_return_trailing_separator_if_any(items, separated_by, f)
+    }
+
+    /// Same as parse_many, but returns a bool indicating whether a trailing separator was found.
+    fn parse_many_to_many_return_trailing_separator_if_any<T, F>(
+        &mut self,
+        items: &'static str,
+        separated_by: SeparatedBy,
+        mut f: F,
+    ) -> (Vec<T>, bool)
+    where
+        F: FnMut(&mut Parser<'a>) -> Vec<T>,
     {
         let mut elements: Vec<T> = Vec::new();
         let mut trailing_separator = false;
@@ -38,20 +71,21 @@ impl<'a> Parser<'a> {
             }
 
             let start_span = self.current_token_span;
-            let Some(element) = f(self) else {
+            let mut new_elements = f(self);
+            if new_elements.is_empty() {
                 if let Some(end) = &separated_by.until {
                     self.eat(end.clone());
                 }
                 break;
-            };
+            }
 
             if let Some(separator) = &separated_by.token {
                 if !trailing_separator && !elements.is_empty() {
                     self.expected_token_separating_items(separator.clone(), items, start_span);
                 }
             }
 
-            elements.push(element);
+            elements.append(&mut new_elements);
 
             trailing_separator = if let Some(separator) = &separated_by.token {
                 self.eat(separator.clone())