feat: Add a comptime string type for string handling at compile-time (

#6026)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

Adds CtString, a dynamically sized string type available at compile-time
only.

## Additional Context

- Named `CtString` because I didn't want to use up the more general
`String` in case we wanted one later. `CtString` also avoids users
accidentally using this in runtime code before they know what it is.
- Had to make a small change to allow `comptime` on trait functions so
that I could have one return `CtString` which is only allowed in
comptime code.

## 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\*

- [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

@@ -108,7 +108,17 @@ fn empty_noir_function(noir_function: &mut NoirFunction) {
 
 fn empty_trait_item(trait_item: &mut TraitItem) {
     match trait_item {
-        TraitItem::Function { name, generics, parameters, return_type, where_clause, body } => {
+        TraitItem::Function {
+            name,
+            generics,
+            parameters,
+            return_type,
+            where_clause,
+            body,
+            is_unconstrained: _,
+            visibility: _,
+            is_comptime: _,
+        } => {
             empty_ident(name);
             empty_unresolved_generics(generics);
             for (name, typ) in parameters.iter_mut() {

compiler/noirc_frontend/src/ast/traits.rs

@@ -10,7 +10,7 @@ use crate::ast::{
 use crate::macros_api::SecondaryAttribute;
 use crate::node_interner::TraitId;
 
-use super::{Documented, GenericTypeArgs};
+use super::{Documented, GenericTypeArgs, ItemVisibility};
 
 /// AST node for trait definitions:
 /// `trait name<generics> { ... items ... }`
@@ -29,6 +29,9 @@ pub struct NoirTrait {
 #[derive(Clone, Debug)]
 pub enum TraitItem {
     Function {
+        is_unconstrained: bool,
+        visibility: ItemVisibility,
+        is_comptime: bool,
         name: Ident,
         generics: UnresolvedGenerics,
         parameters: Vec<(Ident, UnresolvedType)>,
@@ -146,7 +149,17 @@ impl Display for NoirTrait {
 impl Display for TraitItem {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            TraitItem::Function { name, generics, parameters, return_type, where_clause, body } => {
+            TraitItem::Function {
+                name,
+                generics,
+                parameters,
+                return_type,
+                where_clause,
+                body,
+                is_unconstrained,
+                visibility,
+                is_comptime,
+            } => {
                 let generics = vecmap(generics, |generic| generic.to_string());
                 let parameters = vecmap(parameters, |(name, typ)| format!("{name}: {typ}"));
                 let where_clause = vecmap(where_clause, ToString::to_string);
@@ -155,9 +168,17 @@ impl Display for TraitItem {
                 let parameters = parameters.join(", ");
                 let where_clause = where_clause.join(", ");
 
+                let unconstrained = if *is_unconstrained { "unconstrained " } else { "" };
+                let visibility = if *visibility == ItemVisibility::Private {
+                    "".to_string()
+                } else {
+                    visibility.to_string()
+                };
+                let is_comptime = if *is_comptime { "comptime " } else { "" };
+
                 write!(
                     f,
-                    "fn {name}<{generics}>({parameters}) -> {return_type} where {where_clause}"
+                    "{unconstrained}{visibility}{is_comptime}fn {name}<{generics}>({parameters}) -> {return_type} where {where_clause}"
                 )?;
 
                 if let Some(body) = body {

compiler/noirc_frontend/src/ast/visitor.rs

@@ -655,7 +655,17 @@ impl TraitItem {
 
     pub fn accept_children(&self, visitor: &mut impl Visitor) {
         match self {
-            TraitItem::Function { name, generics, parameters, return_type, where_clause, body } => {
+            TraitItem::Function {
+                name,
+                generics,
+                parameters,
+                return_type,
+                where_clause,
+                body,
+                is_unconstrained: _,
+                visibility: _,
+                is_comptime: _,
+            } => {
                 if visitor.visit_trait_item_function(
                     name,
                     generics,

compiler/noirc_frontend/src/elaborator/traits.rs

@@ -74,6 +74,9 @@ impl<'context> Elaborator<'context> {
                 return_type,
                 where_clause,
                 body: _,
+                is_unconstrained,
+                visibility: _,
+                is_comptime: _,
             } = &item.item
             {
                 self.recover_generics(|this| {
@@ -134,9 +137,12 @@ impl<'context> Elaborator<'context> {
                     };
 
                     let no_environment = Box::new(Type::Unit);
-                    // TODO: unconstrained
-                    let function_type =
-                        Type::Function(arguments, Box::new(return_type), no_environment, false);
+                    let function_type = Type::Function(
+                        arguments,
+                        Box::new(return_type),
+                        no_environment,
+                        *is_unconstrained,
+                    );
 
                     functions.push(TraitFunction {
                         name: name.clone(),

compiler/noirc_frontend/src/hir/comptime/interpreter/builtin.rs

@@ -40,7 +40,7 @@ use crate::{
     Kind, QuotedType, ResolvedGeneric, Shared, Type, TypeVariable,
 };
 
-use self::builtin_helpers::{eq_item, get_array, get_str, get_u8, hash_item};
+use self::builtin_helpers::{eq_item, get_array, get_ctstring, get_str, get_u8, hash_item};
 use super::Interpreter;
 
 pub(crate) mod builtin_helpers;
@@ -60,6 +60,8 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "array_len" => array_len(interner, arguments, location),
             "assert_constant" => Ok(Value::Bool(true)),
             "as_slice" => as_slice(interner, arguments, location),
+            "ctstring_eq" => ctstring_eq(arguments, location),
+            "ctstring_hash" => ctstring_hash(arguments, location),
             "expr_as_array" => expr_as_array(interner, arguments, return_type, location),
             "expr_as_assert" => expr_as_assert(interner, arguments, return_type, location),
             "expr_as_assert_eq" => expr_as_assert_eq(interner, arguments, return_type, location),
@@ -97,6 +99,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "expr_is_continue" => expr_is_continue(interner, arguments, location),
             "expr_resolve" => expr_resolve(self, arguments, location),
             "is_unconstrained" => Ok(Value::Bool(true)),
+            "fmtstr_as_ctstring" => fmtstr_as_ctstring(interner, arguments, location),
             "fmtstr_quoted_contents" => fmtstr_quoted_contents(interner, arguments, location),
             "fresh_type_variable" => fresh_type_variable(interner),
             "function_def_add_attribute" => function_def_add_attribute(self, arguments, location),
@@ -151,6 +154,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "slice_push_front" => slice_push_front(interner, arguments, location),
             "slice_remove" => slice_remove(interner, arguments, location, call_stack),
             "str_as_bytes" => str_as_bytes(interner, arguments, location),
+            "str_as_ctstring" => str_as_ctstring(interner, arguments, location),
             "struct_def_add_attribute" => struct_def_add_attribute(interner, arguments, location),
             "struct_def_add_generic" => struct_def_add_generic(interner, arguments, location),
             "struct_def_as_type" => struct_def_as_type(interner, arguments, location),
@@ -297,6 +301,17 @@ fn str_as_bytes(
     Ok(Value::Array(bytes, byte_array_type))
 }
 
+// fn str_as_ctstring(self) -> CtString
+fn str_as_ctstring(
+    interner: &NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> IResult<Value> {
+    let self_argument = check_one_argument(arguments, location)?;
+    let string = get_str(interner, self_argument)?;
+    Ok(Value::CtString(string))
+}
+
 // fn add_attribute<let N: u32>(self, attribute: str<N>)
 fn struct_def_add_attribute(
     interner: &mut NodeInterner,
@@ -1863,6 +1878,17 @@ fn unwrap_expr_value(interner: &NodeInterner, mut expr_value: ExprValue) -> Expr
     expr_value
 }
 
+// fn fmtstr_as_ctstring(self) -> CtString
+fn fmtstr_as_ctstring(
+    interner: &NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> IResult<Value> {
+    let self_argument = check_one_argument(arguments, location)?;
+    let (string, _) = get_format_string(interner, self_argument)?;
+    Ok(Value::CtString(string))
+}
+
 // fn quoted_contents(self) -> Quoted
 fn fmtstr_quoted_contents(
     interner: &NodeInterner,
@@ -2435,3 +2461,11 @@ pub(crate) fn extract_option_generic_type(typ: Type) -> Type {
 
     generics.pop().expect("Expected Option to have a T generic type")
 }
+
+fn ctstring_eq(arguments: Vec<(Value, Location)>, location: Location) -> IResult<Value> {
+    eq_item(arguments, location, get_ctstring)
+}
+
+fn ctstring_hash(arguments: Vec<(Value, Location)>, location: Location) -> IResult<Value> {
+    hash_item(arguments, location, get_ctstring)
+}

compiler/noirc_frontend/src/hir/comptime/interpreter/builtin/builtin_helpers.rs

@@ -125,6 +125,13 @@ pub(crate) fn get_str(
     }
 }
 
+pub(crate) fn get_ctstring((value, location): (Value, Location)) -> IResult<Rc<String>> {
+    match value {
+        Value::CtString(string) => Ok(string),
+        value => type_mismatch(value, Type::Quoted(QuotedType::CtString), location),
+    }
+}
+
 pub(crate) fn get_tuple(
     interner: &NodeInterner,
     (value, location): (Value, Location),

compiler/noirc_frontend/src/hir/comptime/value.rs

@@ -49,6 +49,7 @@ pub enum Value {
     U64(u64),
     String(Rc<String>),
     FormatString(Rc<String>, Type),
+    CtString(Rc<String>),
     Function(FuncId, Type, Rc<TypeBindings>),
     Closure(HirLambda, Vec<Value>, Type),
     Tuple(Vec<Value>),
@@ -151,6 +152,7 @@ impl Value {
             Value::Expr(_) => Type::Quoted(QuotedType::Expr),
             Value::TypedExpr(_) => Type::Quoted(QuotedType::TypedExpr),
             Value::UnresolvedType(_) => Type::Quoted(QuotedType::UnresolvedType),
+            Value::CtString(_) => Type::Quoted(QuotedType::CtString),
         })
     }
 
@@ -202,7 +204,9 @@ impl Value {
             Value::U64(value) => {
                 ExpressionKind::Literal(Literal::Integer((value as u128).into(), false))
             }
-            Value::String(value) => ExpressionKind::Literal(Literal::Str(unwrap_rc(value))),
+            Value::String(value) | Value::CtString(value) => {
+                ExpressionKind::Literal(Literal::Str(unwrap_rc(value)))
+            }
             // Format strings are lowered as normal strings since they are already interpolated.
             Value::FormatString(value, _) => {
                 ExpressionKind::Literal(Literal::Str(unwrap_rc(value)))
@@ -349,7 +353,9 @@ impl Value {
             Value::U64(value) => {
                 HirExpression::Literal(HirLiteral::Integer((value as u128).into(), false))
             }
-            Value::String(value) => HirExpression::Literal(HirLiteral::Str(unwrap_rc(value))),
+            Value::String(value) | Value::CtString(value) => {
+                HirExpression::Literal(HirLiteral::Str(unwrap_rc(value)))
+            }
             // Format strings are lowered as normal strings since they are already interpolated.
             Value::FormatString(value, _) => {
                 HirExpression::Literal(HirLiteral::Str(unwrap_rc(value)))
@@ -589,6 +595,7 @@ impl<'value, 'interner> Display for ValuePrinter<'value, 'interner> {
             Value::U32(value) => write!(f, "{value}"),
             Value::U64(value) => write!(f, "{value}"),
             Value::String(value) => write!(f, "{value}"),
+            Value::CtString(value) => write!(f, "{value}"),
             Value::FormatString(value, _) => write!(f, "{value}"),
             Value::Function(..) => write!(f, "(function)"),
             Value::Closure(_, _, _) => write!(f, "(closure)"),

compiler/noirc_frontend/src/hir/def_collector/dc_mod.rs

@@ -424,6 +424,9 @@ impl<'a> ModCollector<'a> {
                         return_type,
                         where_clause,
                         body,
+                        is_unconstrained,
+                        visibility: _,
+                        is_comptime,
                     } => {
                         let func_id = context.def_interner.push_empty_fn();
                         method_ids.insert(name.to_string(), func_id);
@@ -434,9 +437,9 @@ impl<'a> ModCollector<'a> {
                             visibility: ItemVisibility::Public,
                             // TODO(Maddiaa): Investigate trait implementations with attributes see: https://github.com/noir-lang/noir/issues/2629
                             attributes: crate::token::Attributes::empty(),
-                            is_unconstrained: false,
+                            is_unconstrained: *is_unconstrained,
                             generic_count: generics.len(),
-                            is_comptime: false,
+                            is_comptime: *is_comptime,
                             name_location: location,
                         };
 

compiler/noirc_frontend/src/hir_def/types.rs

@@ -161,6 +161,7 @@ pub enum QuotedType {
     UnresolvedType,
     FunctionDefinition,
     Module,
+    CtString,
 }
 
 /// A list of TypeVariableIds to bind to a type. Storing the
@@ -759,6 +760,7 @@ impl std::fmt::Display for QuotedType {
             QuotedType::UnresolvedType => write!(f, "UnresolvedType"),
             QuotedType::FunctionDefinition => write!(f, "FunctionDefinition"),
             QuotedType::Module => write!(f, "Module"),
+            QuotedType::CtString => write!(f, "CtString"),
         }
     }
 }

compiler/noirc_frontend/src/lexer/token.rs

@@ -1023,6 +1023,7 @@ pub enum Keyword {
     Continue,
     Contract,
     Crate,
+    CtString,
     Dep,
     Else,
     Expr,
@@ -1079,6 +1080,7 @@ impl fmt::Display for Keyword {
             Keyword::Continue => write!(f, "continue"),
             Keyword::Contract => write!(f, "contract"),
             Keyword::Crate => write!(f, "crate"),
+            Keyword::CtString => write!(f, "CtString"),
             Keyword::Dep => write!(f, "dep"),
             Keyword::Else => write!(f, "else"),
             Keyword::Expr => write!(f, "Expr"),
@@ -1138,6 +1140,7 @@ impl Keyword {
             "continue" => Keyword::Continue,
             "contract" => Keyword::Contract,
             "crate" => Keyword::Crate,
+            "CtString" => Keyword::CtString,
             "dep" => Keyword::Dep,
             "else" => Keyword::Else,
             "Expr" => Keyword::Expr,

compiler/noirc_frontend/src/parser/parser/function.rs

@@ -114,7 +114,7 @@ pub(super) fn function_definition(allow_self: bool) -> impl NoirParser<NoirFunct
 /// function_modifiers: 'unconstrained'? (visibility)?
 ///
 /// returns (is_unconstrained, visibility) for whether each keyword was present
-fn function_modifiers() -> impl NoirParser<(bool, ItemVisibility, bool)> {
+pub(super) fn function_modifiers() -> impl NoirParser<(bool, ItemVisibility, bool)> {
     keyword(Keyword::Unconstrained).or_not().then(item_visibility()).then(maybe_comp_time()).map(
         |((unconstrained, visibility), comptime)| (unconstrained.is_some(), visibility, comptime),
     )

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

@@ -2,7 +2,7 @@ use chumsky::prelude::*;
 
 use super::attributes::{attributes, validate_secondary_attributes};
 use super::doc_comments::outer_doc_comments;
-use super::function::function_return_type;
+use super::function::{function_modifiers, function_return_type};
 use super::path::path_no_turbofish;
 use super::{
     block, expression, fresh_statement, function, function_declaration_parameters, let_statement,
@@ -101,16 +101,29 @@ fn trait_function_declaration() -> impl NoirParser<TraitItem> {
             }
         });
 
-    keyword(Keyword::Fn)
-        .ignore_then(ident())
+    function_modifiers()
+        .then_ignore(keyword(Keyword::Fn))
+        .then(ident())
         .then(function::generics())
         .then(parenthesized(function_declaration_parameters()))
         .then(function_return_type().map(|(_, typ)| typ))
         .then(where_clause())
         .then(trait_function_body_or_semicolon_or_error)
-        .map(|(((((name, generics), parameters), return_type), where_clause), body)| {
-            TraitItem::Function { name, generics, parameters, return_type, where_clause, body }
-        })
+        .map(
+            |((((((modifiers, name), generics), parameters), return_type), where_clause), body)| {
+                TraitItem::Function {
+                    name,
+                    generics,
+                    parameters,
+                    return_type,
+                    where_clause,
+                    body,
+                    is_unconstrained: modifiers.0,
+                    visibility: modifiers.1,
+                    is_comptime: modifiers.2,
+                }
+            },
+        )
 }
 
 /// trait_type_declaration: 'type' ident generics

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

@@ -89,6 +89,7 @@ pub(super) fn comptime_type() -> impl NoirParser<UnresolvedType> {
         top_level_item_type(),
         quoted_type(),
         typed_expr_type(),
+        comptime_string_type(),
     ))
 }
 
@@ -166,6 +167,12 @@ fn typed_expr_type() -> impl NoirParser<UnresolvedType> {
         .map_with_span(|_, span| UnresolvedTypeData::Quoted(QuotedType::TypedExpr).with_span(span))
 }
 
+/// This is the `CtString` type for dynamically-sized compile-time strings
+fn comptime_string_type() -> impl NoirParser<UnresolvedType> {
+    keyword(Keyword::CtString)
+        .map_with_span(|_, span| UnresolvedTypeData::Quoted(QuotedType::CtString).with_span(span))
+}
+
 /// This is the type of an already resolved type.
 /// The only way this can appear in the token input is if an already resolved `Type` object
 /// was spliced into a macro's token stream via the `$` operator.