chore: Add parser support for <MyType as Trait>::ident (#5688)

# Description

## Problem\*

## Summary\*

In the interest of smaller PRs I'll be breaking up the associated
constants & types work into multiple smaller PRs.
This PR adds parser support for `<Type as Trait>::ident` syntax since we
already parse both associated constants & types in traits & impls.

## Additional Context

## 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.

aztec_macros/src/utils/parse_utils.rs

@@ -268,6 +268,11 @@ fn empty_expression(expression: &mut Expression) {
             empty_block_expression(block_expression);
         }
         ExpressionKind::Quote(..) | ExpressionKind::Resolved(_) | ExpressionKind::Error => (),
+        ExpressionKind::AsTraitPath(path) => {
+            empty_unresolved_type(&mut path.typ);
+            empty_path(&mut path.trait_path);
+            empty_ident(&mut path.impl_item);
+        }
     }
 }
 
@@ -324,6 +329,11 @@ fn empty_unresolved_type(unresolved_type: &mut UnresolvedType) {
             empty_unresolved_types(args);
             empty_unresolved_type(ret);
         }
+        UnresolvedTypeData::AsTraitPath(path) => {
+            empty_unresolved_type(&mut path.typ);
+            empty_path(&mut path.trait_path);
+            empty_ident(&mut path.impl_item);
+        }
         UnresolvedTypeData::FieldElement
         | UnresolvedTypeData::Integer(_, _)
         | UnresolvedTypeData::Bool

compiler/noirc_frontend/src/ast/expression.rs

@@ -14,7 +14,7 @@ use acvm::{acir::AcirField, FieldElement};
 use iter_extended::vecmap;
 use noirc_errors::{Span, Spanned};
 
-use super::UnaryRhsMemberAccess;
+use super::{AsTraitPath, UnaryRhsMemberAccess};
 
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub enum ExpressionKind {
@@ -36,6 +36,7 @@ pub enum ExpressionKind {
     Quote(Tokens),
     Unquote(Box<Expression>),
     Comptime(BlockExpression, Span),
+    AsTraitPath(AsTraitPath),
 
     // This variant is only emitted when inlining the result of comptime
     // code. It is used to translate function values back into the AST while
@@ -593,6 +594,7 @@ impl Display for ExpressionKind {
                 let tokens = vecmap(&tokens.0, ToString::to_string);
                 write!(f, "quote {{ {} }}", tokens.join(" "))
             }
+            AsTraitPath(path) => write!(f, "{path}"),
         }
     }
 }
@@ -752,6 +754,12 @@ impl Display for Lambda {
     }
 }
 
+impl Display for AsTraitPath {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "<{} as {}>::{}", self.typ, self.trait_path, self.impl_item)
+    }
+}
+
 impl FunctionDefinition {
     pub fn normal(
         name: &Ident,

compiler/noirc_frontend/src/ast/mod.rs

@@ -129,9 +129,13 @@ pub enum UnresolvedTypeData {
         /*env:*/ Box<UnresolvedType>,
     ),
 
-    // The type of quoted code for metaprogramming
+    /// The type of quoted code for metaprogramming
     Quoted(crate::QuotedType),
 
+    /// An "as Trait" path leading to an associated type.
+    /// E.g. `<Foo as Trait>::Bar`
+    AsTraitPath(Box<crate::ast::AsTraitPath>),
+
     /// An already resolved type. These can only be parsed if they were present in the token stream
     /// as a result of being spliced into a macro's token stream input.
     Resolved(QuotedTypeId),
@@ -239,6 +243,7 @@ impl std::fmt::Display for UnresolvedTypeData {
             Unspecified => write!(f, "unspecified"),
             Parenthesized(typ) => write!(f, "({typ})"),
             Resolved(_) => write!(f, "(resolved type)"),
+            AsTraitPath(path) => write!(f, "{path}"),
         }
     }
 }

compiler/noirc_frontend/src/ast/statement.rs

@@ -358,6 +358,20 @@ impl UseTree {
     }
 }
 
+/// A special kind of path in the form `<MyType as Trait>::ident`.
+/// Note that this path must consist of exactly two segments.
+///
+/// An AsTraitPath may be used in either a type context where `ident`
+/// refers to an associated type of a particular impl, or in a value
+/// context where `ident` may refer to an associated constant or a
+/// function within the impl.
+#[derive(Debug, PartialEq, Eq, Clone, Hash)]
+pub struct AsTraitPath {
+    pub typ: UnresolvedType,
+    pub trait_path: Path,
+    pub impl_item: Ident,
+}
+
 // Note: Path deliberately doesn't implement Recoverable.
 // No matter which default value we could give in Recoverable::error,
 // it would most likely cause further errors during name resolution

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -64,6 +64,7 @@ impl<'context> Elaborator<'context> {
                 self.push_err(ResolverError::UnquoteUsedOutsideQuote { span: expr.span });
                 (HirExpression::Error, Type::Error)
             }
+            ExpressionKind::AsTraitPath(_) => todo!("Implement AsTraitPath"),
         };
         let id = self.interner.push_expr(hir_expr);
         self.interner.push_expr_location(id, expr.span, self.file);

compiler/noirc_frontend/src/elaborator/types.rs

@@ -155,6 +155,7 @@ impl<'context> Elaborator<'context> {
             }
             Parenthesized(typ) => self.resolve_type_inner(*typ, kind),
             Resolved(id) => self.interner.get_quoted_type(id).clone(),
+            AsTraitPath(_) => todo!("Resolve AsTraitPath"),
         };
 
         if let Some(unresolved_span) = typ.span {

compiler/noirc_frontend/src/parser/errors.rs

@@ -26,6 +26,8 @@ pub enum ParserErrorReason {
     EarlyReturn,
     #[error("Patterns aren't allowed in a trait's function declarations")]
     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("comptime keyword is deprecated")]

compiler/noirc_frontend/src/parser/parser.rs

@@ -23,6 +23,7 @@
 //! prevent other parsers from being tried afterward since there is no longer an error. Thus, they should
 //! be limited to cases like the above `fn` example where it is clear we shouldn't back out of the
 //! current parser to try alternative parsers in a `choice` expression.
+use self::path::as_trait_path;
 use self::primitives::{keyword, macro_quote_marker, mutable_reference, variable};
 use self::types::{generic_type_args, maybe_comp_time};
 pub use types::parse_type;
@@ -518,16 +519,24 @@ where
         .map(|(block, span)| ExpressionKind::Comptime(block, span))
 }
 
-fn declaration<'a, P>(expr_parser: P) -> impl NoirParser<StatementKind> + 'a
+fn let_statement<'a, P>(
+    expr_parser: P,
+) -> impl NoirParser<((Pattern, UnresolvedType), Expression)> + 'a
 where
     P: ExprParser + 'a,
 {
     let p =
         ignore_then_commit(keyword(Keyword::Let).labelled(ParsingRuleLabel::Statement), pattern());
     let p = p.then(optional_type_annotation());
     let p = then_commit_ignore(p, just(Token::Assign));
-    let p = then_commit(p, expr_parser);
-    p.map(StatementKind::new_let)
+    then_commit(p, expr_parser)
+}
+
+fn declaration<'a, P>(expr_parser: P) -> impl NoirParser<StatementKind> + 'a
+where
+    P: ExprParser + 'a,
+{
+    let_statement(expr_parser).map(StatementKind::new_let)
 }
 
 fn pattern() -> impl NoirParser<Pattern> {
@@ -1076,6 +1085,7 @@ where
         unquote(expr_parser.clone()),
         variable(),
         literal(),
+        as_trait_path(parse_type()).map(ExpressionKind::AsTraitPath),
         macro_quote_marker(),
     ))
     .map_with_span(Expression::new)

compiler/noirc_frontend/src/parser/parser/path.rs

@@ -1,12 +1,12 @@
-use crate::ast::{Path, PathKind, PathSegment, UnresolvedType};
-use crate::parser::NoirParser;
+use crate::ast::{AsTraitPath, Path, PathKind, PathSegment, UnresolvedType};
+use crate::parser::{NoirParser, ParserError, ParserErrorReason};
 
 use crate::token::{Keyword, Token};
 
 use chumsky::prelude::*;
 
 use super::keyword;
-use super::primitives::{path_segment, path_segment_no_turbofish};
+use super::primitives::{ident, path_segment, path_segment_no_turbofish};
 
 pub(super) fn path<'a>(
     type_parser: impl NoirParser<UnresolvedType> + 'a,
@@ -34,6 +34,25 @@ fn path_inner<'a>(segment: impl NoirParser<PathSegment> + 'a) -> impl NoirParser
     ))
 }
 
+/// Parses `<MyType as Trait>::path_segment`
+/// These paths only support exactly two segments.
+pub(super) fn as_trait_path<'a>(
+    type_parser: impl NoirParser<UnresolvedType> + 'a,
+) -> impl NoirParser<AsTraitPath> + 'a {
+    just(Token::Less)
+        .ignore_then(type_parser.clone())
+        .then_ignore(keyword(Keyword::As))
+        .then(path(type_parser))
+        .then_ignore(just(Token::Greater))
+        .then_ignore(just(Token::DoubleColon))
+        .then(ident())
+        .validate(|((typ, trait_path), impl_item), span, emit| {
+            let reason = ParserErrorReason::ExperimentalFeature("Fully qualified trait impl paths");
+            emit(ParserError::with_reason(reason, span));
+            AsTraitPath { typ, trait_path, impl_item }
+        })
+}
+
 fn empty_path() -> impl NoirParser<Path> {
     let make_path = |kind| move |_, span| Path { segments: Vec::new(), kind, span };
     let path_kind = |key, kind| keyword(key).map_with_span(make_path(kind));

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

@@ -3,12 +3,15 @@ use chumsky::prelude::*;
 use super::attributes::{attributes, validate_secondary_attributes};
 use super::function::function_return_type;
 use super::path::path_no_turbofish;
-use super::{block, expression, fresh_statement, function, function_declaration_parameters};
+use super::{
+    block, expression, fresh_statement, function, function_declaration_parameters, let_statement,
+};
 
 use crate::ast::{
     Expression, ItemVisibility, NoirTrait, NoirTraitImpl, TraitBound, TraitImplItem, TraitItem,
     UnresolvedTraitConstraint, UnresolvedType,
 };
+use crate::macros_api::Pattern;
 use crate::{
     parser::{
         ignore_then_commit, parenthesized, parser::primitives::keyword, NoirParser, ParserError,
@@ -59,13 +62,7 @@ fn trait_constant_declaration() -> impl NoirParser<TraitItem> {
         .then(parse_type())
         .then(optional_default_value())
         .then_ignore(just(Token::Semicolon))
-        .validate(|((name, typ), default_value), span, emit| {
-            emit(ParserError::with_reason(
-                ParserErrorReason::ExperimentalFeature("Associated constants"),
-                span,
-            ));
-            TraitItem::Constant { name, typ, default_value }
-        })
+        .map(|((name, typ), default_value)| TraitItem::Constant { name, typ, default_value })
 }
 
 /// trait_function_declaration: 'fn' ident generics '(' declaration_parameters ')' function_return_type
@@ -146,7 +143,17 @@ fn trait_implementation_body() -> impl NoirParser<Vec<TraitImplItem>> {
         .then_ignore(just(Token::Semicolon))
         .map(|(name, alias)| TraitImplItem::Type { name, alias });
 
-    function.or(alias).repeated()
+    let let_statement = let_statement(expression()).then_ignore(just(Token::Semicolon)).try_map(
+        |((pattern, typ), expr), span| match pattern {
+            Pattern::Identifier(ident) => Ok(TraitImplItem::Constant(ident, typ, expr)),
+            _ => Err(ParserError::with_reason(
+                ParserErrorReason::PatternInTraitFunctionParameter,
+                span,
+            )),
+        },
+    );
+
+    choice((function, alias, let_statement)).repeated()
 }
 
 pub(super) fn where_clause() -> impl NoirParser<Vec<UnresolvedTraitConstraint>> {

compiler/noirc_frontend/src/parser/parser/types.rs

@@ -1,4 +1,4 @@
-use super::path::path_no_turbofish;
+use super::path::{as_trait_path, path_no_turbofish};
 use super::primitives::token_kind;
 use super::{
     expression_with_precedence, keyword, nothing, parenthesized, NoirParser, ParserError,
@@ -45,10 +45,18 @@ pub(super) fn parse_type_inner<'a>(
         parenthesized_type(recursive_type_parser.clone()),
         tuple_type(recursive_type_parser.clone()),
         function_type(recursive_type_parser.clone()),
-        mutable_reference_type(recursive_type_parser),
+        mutable_reference_type(recursive_type_parser.clone()),
+        as_trait_path_type(recursive_type_parser),
     ))
 }
 
+fn as_trait_path_type<'a>(
+    type_parser: impl NoirParser<UnresolvedType> + 'a,
+) -> impl NoirParser<UnresolvedType> + 'a {
+    as_trait_path(type_parser)
+        .map_with_span(|path, span| UnresolvedTypeData::AsTraitPath(Box::new(path)).with_span(span))
+}
+
 pub(super) fn parenthesized_type(
     recursive_type_parser: impl NoirParser<UnresolvedType>,
 ) -> impl NoirParser<UnresolvedType> {

tooling/lsp/src/requests/inlay_hint.rs

@@ -294,6 +294,9 @@ impl<'a> InlayHintCollector<'a> {
             ExpressionKind::Comptime(block_expression, _span) => {
                 self.collect_in_block_expression(block_expression);
             }
+            ExpressionKind::AsTraitPath(path) => {
+                self.collect_in_ident(&path.impl_item, true);
+            }
             ExpressionKind::Literal(..)
             | ExpressionKind::Variable(..)
             | ExpressionKind::Quote(..)
@@ -671,6 +674,7 @@ fn get_expression_name(expression: &Expression) -> Option<String> {
         ExpressionKind::MethodCall(method_call) => Some(method_call.method_name.to_string()),
         ExpressionKind::Cast(cast) => get_expression_name(&cast.lhs),
         ExpressionKind::Parenthesized(expr) => get_expression_name(expr),
+        ExpressionKind::AsTraitPath(path) => Some(path.impl_item.to_string()),
         ExpressionKind::Constructor(..)
         | ExpressionKind::Infix(..)
         | ExpressionKind::Index(..)

tooling/nargo_fmt/src/rewrite/expr.rs

@@ -179,6 +179,10 @@ pub(crate) fn rewrite(
                 format!("$({})", rewrite_sub_expr(visitor, shape, *expr))
             }
         }
+        ExpressionKind::AsTraitPath(path) => {
+            let trait_path = rewrite_path(visitor, shape, path.trait_path);
+            format!("<{} as {}>::{}", path.typ, trait_path, path.impl_item)
+        }
     }
 }
 

tooling/nargo_fmt/src/rewrite/typ.rs

@@ -58,6 +58,7 @@ pub(crate) fn rewrite(visitor: &FmtVisitor, _shape: Shape, typ: UnresolvedType)
         UnresolvedTypeData::Resolved(_) => {
             unreachable!("Unexpected macro expansion of a type in nargo fmt input")
         }
+        UnresolvedTypeData::AsTraitPath(path) => path.to_string(),
 
         UnresolvedTypeData::Unspecified => todo!(),
         UnresolvedTypeData::FieldElement