chore: Add Hir -> Ast conversion (#4788)

# Description

## Problem\*

Resolves #4796

## Summary\*

 Adds a way to go from the Hir back to the Ast.
This conversion isn't exact and information is lost currently. Notably
many spans are lost and some constructs like `Type::TypeVariable` have
no equivalent in the Ast.

This also isn't expected to be idempotent. Different nodes may be
desugared, `let` statements may have explicit types added, etc.

## Additional Context

This is unused currently and can't be triggered from the current
compiler at all. I'm adding this PR now to split off required changes
for metaprogramming into smaller chunks.

## 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.

compiler/noirc_frontend/src/ast/mod.rs

@@ -112,6 +112,9 @@ pub enum UnresolvedTypeData {
         /*env:*/ Box<UnresolvedType>,
     ),
 
+    // The type of quoted code for metaprogramming
+    Code,
+
     Unspecified, // This is for when the user declares a variable without specifying it's type
     Error,
 }
@@ -200,6 +203,7 @@ impl std::fmt::Display for UnresolvedTypeData {
                 }
             }
             MutableReference(element) => write!(f, "&mut {element}"),
+            Code => write!(f, "Code"),
             Unit => write!(f, "()"),
             Error => write!(f, "error"),
             Unspecified => write!(f, "unspecified"),

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

@@ -0,0 +1,350 @@
+use iter_extended::vecmap;
+use noirc_errors::{Span, Spanned};
+
+use crate::ast::{ConstrainStatement, Expression, Statement, StatementKind};
+use crate::hir_def::expr::{HirArrayLiteral, HirExpression, HirIdent};
+use crate::hir_def::stmt::{HirLValue, HirPattern, HirStatement};
+use crate::macros_api::HirLiteral;
+use crate::node_interner::{ExprId, NodeInterner, StmtId};
+use crate::{
+    ArrayLiteral, AssignStatement, BlockExpression, CallExpression, CastExpression, ConstrainKind,
+    ConstructorExpression, ExpressionKind, ForLoopStatement, ForRange, Ident, IfExpression,
+    IndexExpression, InfixExpression, LValue, Lambda, LetStatement, Literal,
+    MemberAccessExpression, MethodCallExpression, Path, Pattern, PrefixExpression, Type,
+    UnresolvedType, UnresolvedTypeData, UnresolvedTypeExpression,
+};
+
+// TODO:
+// - Full path for idents & types
+// - Assert/AssertEq information lost
+// - The type name span is lost in constructor patterns & expressions
+// - All type spans are lost
+// - Type::TypeVariable has no equivalent in the Ast
+
+impl StmtId {
+    #[allow(unused)]
+    fn to_ast(self, interner: &NodeInterner) -> Statement {
+        let statement = interner.statement(&self);
+        let span = interner.statement_span(&self);
+
+        let kind = match statement {
+            HirStatement::Let(let_stmt) => {
+                let pattern = let_stmt.pattern.into_ast(interner);
+                let r#type = interner.id_type(let_stmt.expression).to_ast();
+                let expression = let_stmt.expression.to_ast(interner);
+                StatementKind::Let(LetStatement {
+                    pattern,
+                    r#type,
+                    expression,
+                    attributes: Vec::new(),
+                })
+            }
+            HirStatement::Constrain(constrain) => {
+                let expr = constrain.0.to_ast(interner);
+                let message = constrain.2.map(|message| message.to_ast(interner));
+
+                // TODO: Find difference in usage between Assert & AssertEq
+                StatementKind::Constrain(ConstrainStatement(expr, message, ConstrainKind::Assert))
+            }
+            HirStatement::Assign(assign) => StatementKind::Assign(AssignStatement {
+                lvalue: assign.lvalue.into_ast(interner),
+                expression: assign.expression.to_ast(interner),
+            }),
+            HirStatement::For(for_stmt) => StatementKind::For(ForLoopStatement {
+                identifier: for_stmt.identifier.to_ast(interner),
+                range: ForRange::Range(
+                    for_stmt.start_range.to_ast(interner),
+                    for_stmt.end_range.to_ast(interner),
+                ),
+                block: for_stmt.block.to_ast(interner),
+                span,
+            }),
+            HirStatement::Break => StatementKind::Break,
+            HirStatement::Continue => StatementKind::Continue,
+            HirStatement::Expression(expr) => StatementKind::Expression(expr.to_ast(interner)),
+            HirStatement::Semi(expr) => StatementKind::Semi(expr.to_ast(interner)),
+            HirStatement::Error => StatementKind::Error,
+        };
+
+        Statement { kind, span }
+    }
+}
+
+impl ExprId {
+    #[allow(unused)]
+    fn to_ast(self, interner: &NodeInterner) -> Expression {
+        let expression = interner.expression(&self);
+        let span = interner.expr_span(&self);
+
+        let kind = match expression {
+            HirExpression::Ident(ident) => {
+                let path = Path::from_ident(ident.to_ast(interner));
+                ExpressionKind::Variable(path)
+            }
+            HirExpression::Literal(HirLiteral::Array(array)) => {
+                let array = array.into_ast(interner, span);
+                ExpressionKind::Literal(Literal::Array(array))
+            }
+            HirExpression::Literal(HirLiteral::Slice(array)) => {
+                let array = array.into_ast(interner, span);
+                ExpressionKind::Literal(Literal::Slice(array))
+            }
+            HirExpression::Literal(HirLiteral::Bool(value)) => {
+                ExpressionKind::Literal(Literal::Bool(value))
+            }
+            HirExpression::Literal(HirLiteral::Integer(value, sign)) => {
+                ExpressionKind::Literal(Literal::Integer(value, sign))
+            }
+            HirExpression::Literal(HirLiteral::Str(string)) => {
+                ExpressionKind::Literal(Literal::Str(string))
+            }
+            HirExpression::Literal(HirLiteral::FmtStr(string, _exprs)) => {
+                // TODO: Is throwing away the exprs here valid?
+                ExpressionKind::Literal(Literal::FmtStr(string))
+            }
+            HirExpression::Literal(HirLiteral::Unit) => ExpressionKind::Literal(Literal::Unit),
+            HirExpression::Block(expr) => {
+                let statements = vecmap(expr.statements, |statement| statement.to_ast(interner));
+                ExpressionKind::Block(BlockExpression { statements })
+            }
+            HirExpression::Prefix(prefix) => ExpressionKind::Prefix(Box::new(PrefixExpression {
+                operator: prefix.operator,
+                rhs: prefix.rhs.to_ast(interner),
+            })),
+            HirExpression::Infix(infix) => ExpressionKind::Infix(Box::new(InfixExpression {
+                lhs: infix.lhs.to_ast(interner),
+                operator: Spanned::from(infix.operator.location.span, infix.operator.kind),
+                rhs: infix.rhs.to_ast(interner),
+            })),
+            HirExpression::Index(index) => ExpressionKind::Index(Box::new(IndexExpression {
+                collection: index.collection.to_ast(interner),
+                index: index.index.to_ast(interner),
+            })),
+            HirExpression::Constructor(constructor) => {
+                let type_name = constructor.r#type.borrow().name.to_string();
+                let type_name = Path::from_single(type_name, span);
+                let fields =
+                    vecmap(constructor.fields, |(name, expr)| (name, expr.to_ast(interner)));
+
+                ExpressionKind::Constructor(Box::new(ConstructorExpression { type_name, fields }))
+            }
+            HirExpression::MemberAccess(access) => {
+                ExpressionKind::MemberAccess(Box::new(MemberAccessExpression {
+                    lhs: access.lhs.to_ast(interner),
+                    rhs: access.rhs,
+                }))
+            }
+            HirExpression::Call(call) => {
+                let func = Box::new(call.func.to_ast(interner));
+                let arguments = vecmap(call.arguments, |arg| arg.to_ast(interner));
+                ExpressionKind::Call(Box::new(CallExpression { func, arguments }))
+            }
+            HirExpression::MethodCall(method_call) => {
+                ExpressionKind::MethodCall(Box::new(MethodCallExpression {
+                    object: method_call.object.to_ast(interner),
+                    method_name: method_call.method,
+                    arguments: vecmap(method_call.arguments, |arg| arg.to_ast(interner)),
+                }))
+            }
+            HirExpression::Cast(cast) => {
+                let lhs = cast.lhs.to_ast(interner);
+                let r#type = cast.r#type.to_ast();
+                ExpressionKind::Cast(Box::new(CastExpression { lhs, r#type }))
+            }
+            HirExpression::If(if_expr) => ExpressionKind::If(Box::new(IfExpression {
+                condition: if_expr.condition.to_ast(interner),
+                consequence: if_expr.consequence.to_ast(interner),
+                alternative: if_expr.alternative.map(|expr| expr.to_ast(interner)),
+            })),
+            HirExpression::Tuple(fields) => {
+                ExpressionKind::Tuple(vecmap(fields, |field| field.to_ast(interner)))
+            }
+            HirExpression::Lambda(lambda) => {
+                let parameters = vecmap(lambda.parameters, |(pattern, typ)| {
+                    (pattern.into_ast(interner), typ.to_ast())
+                });
+                let return_type = lambda.return_type.to_ast();
+                let body = lambda.body.to_ast(interner);
+                ExpressionKind::Lambda(Box::new(Lambda { parameters, return_type, body }))
+            }
+            HirExpression::Quote(block) => ExpressionKind::Quote(block),
+            HirExpression::Error => ExpressionKind::Error,
+        };
+
+        Expression::new(kind, span)
+    }
+}
+
+impl HirPattern {
+    fn into_ast(self, interner: &NodeInterner) -> Pattern {
+        match self {
+            HirPattern::Identifier(ident) => Pattern::Identifier(ident.to_ast(interner)),
+            HirPattern::Mutable(pattern, location) => {
+                let pattern = Box::new(pattern.into_ast(interner));
+                Pattern::Mutable(pattern, location.span, false)
+            }
+            HirPattern::Tuple(patterns, location) => {
+                let patterns = vecmap(patterns, |pattern| pattern.into_ast(interner));
+                Pattern::Tuple(patterns, location.span)
+            }
+            HirPattern::Struct(typ, patterns, location) => {
+                let patterns =
+                    vecmap(patterns, |(name, pattern)| (name, pattern.into_ast(interner)));
+                let name = match typ.follow_bindings() {
+                    Type::Struct(struct_def, _) => {
+                        let struct_def = struct_def.borrow();
+                        struct_def.name.0.contents.clone()
+                    }
+                    // This pass shouldn't error so if the type isn't a struct we just get a string
+                    // representation of any other type and use that. We're relying on name
+                    // resolution to fail later when this Ast is re-converted to Hir.
+                    other => other.to_string(),
+                };
+                // The name span is lost here
+                let path = Path::from_single(name, location.span);
+                Pattern::Struct(path, patterns, location.span)
+            }
+        }
+    }
+}
+
+impl HirIdent {
+    fn to_ast(&self, interner: &NodeInterner) -> Ident {
+        let name = interner.definition_name(self.id).to_owned();
+        Ident(Spanned::from(self.location.span, name))
+    }
+}
+
+impl Type {
+    fn to_ast(&self) -> UnresolvedType {
+        let typ = match self {
+            Type::FieldElement => UnresolvedTypeData::FieldElement,
+            Type::Array(length, element) => {
+                let length = length.to_type_expression();
+                let element = Box::new(element.to_ast());
+                UnresolvedTypeData::Array(length, element)
+            }
+            Type::Slice(element) => {
+                let element = Box::new(element.to_ast());
+                UnresolvedTypeData::Slice(element)
+            }
+            Type::Integer(sign, bit_size) => UnresolvedTypeData::Integer(*sign, *bit_size),
+            Type::Bool => UnresolvedTypeData::Bool,
+            Type::String(length) => {
+                let length = length.to_type_expression();
+                UnresolvedTypeData::String(Some(length))
+            }
+            Type::FmtString(length, element) => {
+                let length = length.to_type_expression();
+                let element = Box::new(element.to_ast());
+                UnresolvedTypeData::FormatString(length, element)
+            }
+            Type::Unit => UnresolvedTypeData::Unit,
+            Type::Tuple(fields) => {
+                let fields = vecmap(fields, |field| field.to_ast());
+                UnresolvedTypeData::Tuple(fields)
+            }
+            Type::Struct(def, generics) => {
+                let struct_def = def.borrow();
+                let generics = vecmap(generics, |generic| generic.to_ast());
+                let name = Path::from_ident(struct_def.name.clone());
+                UnresolvedTypeData::Named(name, generics, false)
+            }
+            Type::Alias(type_def, generics) => {
+                // Keep the alias name instead of expanding this in case the
+                // alias' definition was changed
+                let type_def = type_def.borrow();
+                let generics = vecmap(generics, |generic| generic.to_ast());
+                let name = Path::from_ident(type_def.name.clone());
+                UnresolvedTypeData::Named(name, generics, false)
+            }
+            Type::TypeVariable(_, _) => todo!("Convert Type::TypeVariable Hir -> Ast"),
+            Type::TraitAsType(_, name, generics) => {
+                let generics = vecmap(generics, |generic| generic.to_ast());
+                let name = Path::from_single(name.as_ref().clone(), Span::default());
+                UnresolvedTypeData::TraitAsType(name, generics)
+            }
+            Type::NamedGeneric(_, name) => {
+                let name = Path::from_single(name.as_ref().clone(), Span::default());
+                UnresolvedTypeData::TraitAsType(name, Vec::new())
+            }
+            Type::Function(args, ret, env) => {
+                let args = vecmap(args, |arg| arg.to_ast());
+                let ret = Box::new(ret.to_ast());
+                let env = Box::new(env.to_ast());
+                UnresolvedTypeData::Function(args, ret, env)
+            }
+            Type::MutableReference(element) => {
+                let element = Box::new(element.to_ast());
+                UnresolvedTypeData::MutableReference(element)
+            }
+            // Type::Forall is only for generic functions which don't store a type
+            // in their Ast so they don't need to call to_ast for their Forall type.
+            // Since there is no UnresolvedTypeData equivalent for Type::Forall, we use
+            // this to ignore this case since it shouldn't be needed anyway.
+            Type::Forall(_, typ) => return typ.to_ast(),
+            Type::Constant(_) => panic!("Type::Constant where a type was expected: {self:?}"),
+            Type::Code => UnresolvedTypeData::Code,
+            Type::Error => UnresolvedTypeData::Error,
+        };
+
+        UnresolvedType { typ, span: None }
+    }
+
+    fn to_type_expression(&self) -> UnresolvedTypeExpression {
+        let span = Span::default();
+
+        match self.follow_bindings() {
+            Type::Constant(length) => UnresolvedTypeExpression::Constant(length, span),
+            Type::NamedGeneric(_, name) => {
+                let path = Path::from_single(name.as_ref().clone(), span);
+                UnresolvedTypeExpression::Variable(path)
+            }
+            // TODO: This should be turned into a proper error.
+            other => panic!("Cannot represent {other:?} as type expression"),
+        }
+    }
+}
+
+impl HirLValue {
+    fn into_ast(self, interner: &NodeInterner) -> LValue {
+        match self {
+            HirLValue::Ident(ident, _) => LValue::Ident(ident.to_ast(interner)),
+            HirLValue::MemberAccess { object, field_name, field_index: _, typ: _, location } => {
+                let object = Box::new(object.into_ast(interner));
+                LValue::MemberAccess { object, field_name, span: location.span }
+            }
+            HirLValue::Index { array, index, typ: _, location } => {
+                let array = Box::new(array.into_ast(interner));
+                let index = index.to_ast(interner);
+                LValue::Index { array, index, span: location.span }
+            }
+            HirLValue::Dereference { lvalue, element_type: _, location } => {
+                let lvalue = Box::new(lvalue.into_ast(interner));
+                LValue::Dereference(lvalue, location.span)
+            }
+        }
+    }
+}
+
+impl HirArrayLiteral {
+    fn into_ast(self, interner: &NodeInterner, span: Span) -> ArrayLiteral {
+        match self {
+            HirArrayLiteral::Standard(elements) => {
+                ArrayLiteral::Standard(vecmap(elements, |element| element.to_ast(interner)))
+            }
+            HirArrayLiteral::Repeated { repeated_element, length } => {
+                let repeated_element = Box::new(repeated_element.to_ast(interner));
+                let length = match length {
+                    Type::Constant(length) => {
+                        let literal = Literal::Integer((length as u128).into(), false);
+                        let kind = ExpressionKind::Literal(literal);
+                        Box::new(Expression::new(kind, span))
+                    }
+                    other => panic!("Cannot convert non-constant type for repeated array literal from Hir -> Ast: {other:?}"),
+                };
+                ArrayLiteral::Repeated { repeated_element, length }
+            }
+        }
+    }
+}

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

@@ -0,0 +1 @@
+mod hir_to_ast;

compiler/noirc_frontend/src/hir/mod.rs

@@ -1,3 +1,4 @@
+pub mod comptime;
 pub mod def_collector;
 pub mod def_map;
 pub mod resolution;

compiler/noirc_frontend/src/hir/resolution/resolver.rs

@@ -502,6 +502,7 @@ impl<'a> Resolver<'a> {
                 let fields = self.resolve_type_inner(*fields, new_variables);
                 Type::FmtString(Box::new(resolved_size), Box::new(fields))
             }
+            Code => Type::Code,
             Unit => Type::Unit,
             Unspecified => Type::Error,
             Error => Type::Error,

compiler/noirc_frontend/src/node_interner.rs

@@ -917,6 +917,10 @@ impl NodeInterner {
         self.id_location(expr_id)
     }
 
+    pub fn statement_span(&self, stmt_id: &StmtId) -> Span {
+        self.id_location(stmt_id).span
+    }
+
     pub fn get_struct(&self, id: StructId) -> Shared<StructType> {
         self.structs[&id].clone()
     }