feat: Add some metaprogramming methods on TypeDefinition (#5310)

# Description

## Problem\*

Resolves #5285 

## Summary\*

Implements `as_type`, `generics`, and `fields` on the `TypeDefinition`
type.

The `Type` type isn't really supported yet so `as_type` and `fields`
return `Quoted` token streams instead of `Type`s for now.

## Additional Context

A few bugs still need to be fixed after this PR for the vertical slice
to work.

## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [x] **[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.

---------

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

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

@@ -98,14 +98,7 @@ impl<'a> Interpreter<'a> {
             .expect("all builtin functions must contain a function  attribute which contains the opcode which it links to");
 
         if let Some(builtin) = func_attrs.builtin() {
-            match builtin.as_str() {
-                "array_len" => builtin::array_len(&arguments),
-                "as_slice" => builtin::as_slice(arguments),
-                _ => {
-                    let item = format!("Comptime evaluation for builtin function {builtin}");
-                    Err(InterpreterError::Unimplemented { item, location })
-                }
-            }
+            builtin::call_builtin(self.interner, builtin, arguments, location)
         } else if let Some(foreign) = func_attrs.foreign() {
             let item = format!("Comptime evaluation for foreign functions like {foreign}");
             Err(InterpreterError::Unimplemented { item, location })
@@ -934,6 +927,18 @@ impl<'a> Interpreter<'a> {
     fn evaluate_access(&mut self, access: HirMemberAccess, id: ExprId) -> IResult<Value> {
         let (fields, struct_type) = match self.evaluate(access.lhs)? {
             Value::Struct(fields, typ) => (fields, typ),
+            Value::Tuple(fields) => {
+                let (fields, field_types): (HashMap<Rc<String>, Value>, Vec<Type>) = fields
+                    .into_iter()
+                    .enumerate()
+                    .map(|(i, field)| {
+                        let field_type = field.get_type().into_owned();
+                        let key_val_pair = (Rc::new(i.to_string()), field);
+                        (key_val_pair, field_type)
+                    })
+                    .unzip();
+                (fields, Type::Tuple(field_types))
+            }
             value => {
                 let location = self.interner.expr_location(&id);
                 return Err(InterpreterError::NonTupleOrStructInMemberAccess { value, location });

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

@@ -1,11 +1,36 @@
+use std::rc::Rc;
+
 use noirc_errors::Location;
 
 use crate::{
-    hir::comptime::{errors::IResult, Value},
-    Type,
+    hir::comptime::{errors::IResult, InterpreterError, Value},
+    lexer::Lexer,
+    macros_api::NodeInterner,
+    token::{SpannedToken, Token, Tokens},
+    QuotedType, Type,
 };
 
-pub(super) fn array_len(arguments: &[(Value, Location)]) -> IResult<Value> {
+pub(super) fn call_builtin(
+    interner: &NodeInterner,
+    name: &str,
+    arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> IResult<Value> {
+    match name {
+        "array_len" => array_len(&arguments),
+        "as_slice" => as_slice(arguments),
+        "slice_push_back" => slice_push_back(arguments),
+        "type_def_as_type" => type_def_as_type(interner, arguments),
+        "type_def_generics" => type_def_generics(interner, arguments),
+        "type_def_fields" => type_def_fields(interner, arguments),
+        _ => {
+            let item = format!("Comptime evaluation for builtin function {name}");
+            Err(InterpreterError::Unimplemented { item, location })
+        }
+    }
+}
+
+fn array_len(arguments: &[(Value, Location)]) -> IResult<Value> {
     assert_eq!(arguments.len(), 1, "ICE: `array_len` should only receive a single argument");
     match &arguments[0].0 {
         Value::Array(values, _) | Value::Slice(values, _) => Ok(Value::U32(values.len() as u32)),
@@ -14,7 +39,7 @@ pub(super) fn array_len(arguments: &[(Value, Location)]) -> IResult<Value> {
     }
 }
 
-pub(super) fn as_slice(mut arguments: Vec<(Value, Location)>) -> IResult<Value> {
+fn as_slice(mut arguments: Vec<(Value, Location)>) -> IResult<Value> {
     assert_eq!(arguments.len(), 1, "ICE: `as_slice` should only receive a single argument");
     let (array, _) = arguments.pop().unwrap();
     match array {
@@ -23,3 +48,129 @@ pub(super) fn as_slice(mut arguments: Vec<(Value, Location)>) -> IResult<Value>
         _ => unreachable!("ICE: Cannot convert types other than arrays into slices"),
     }
 }
+
+fn slice_push_back(mut arguments: Vec<(Value, Location)>) -> IResult<Value> {
+    assert_eq!(arguments.len(), 2, "ICE: `slice_push_back` should only receive two arguments");
+    let (element, _) = arguments.pop().unwrap();
+    let (slice, _) = arguments.pop().unwrap();
+    match slice {
+        Value::Slice(mut values, typ) => {
+            values.push_back(element);
+            Ok(Value::Slice(values, typ))
+        }
+        // Type checking should prevent this branch being taken.
+        _ => unreachable!("ICE: `slice_push_back` expects a slice as its first argument"),
+    }
+}
+
+/// fn as_type(self) -> Quoted
+fn type_def_as_type(
+    interner: &NodeInterner,
+    mut arguments: Vec<(Value, Location)>,
+) -> IResult<Value> {
+    assert_eq!(arguments.len(), 1, "ICE: `generics` should only receive a single argument");
+    let (type_def, span) = match arguments.pop() {
+        Some((Value::TypeDefinition(id), location)) => (id, location.span),
+        other => {
+            unreachable!("ICE: `as_type` expected a `TypeDefinition` argument, found {other:?}")
+        }
+    };
+
+    let struct_def = interner.get_struct(type_def);
+    let struct_def = struct_def.borrow();
+    let make_token = |name| SpannedToken::new(Token::Str(name), span);
+
+    let mut tokens = vec![make_token(struct_def.name.to_string())];
+
+    for (i, generic) in struct_def.generics.iter().enumerate() {
+        if i != 0 {
+            tokens.push(SpannedToken::new(Token::Comma, span));
+        }
+        tokens.push(make_token(generic.borrow().to_string()));
+    }
+
+    Ok(Value::Code(Rc::new(Tokens(tokens))))
+}
+
+/// fn generics(self) -> [Quoted]
+fn type_def_generics(
+    interner: &NodeInterner,
+    mut arguments: Vec<(Value, Location)>,
+) -> IResult<Value> {
+    assert_eq!(arguments.len(), 1, "ICE: `generics` should only receive a single argument");
+    let (type_def, span) = match arguments.pop() {
+        Some((Value::TypeDefinition(id), location)) => (id, location.span),
+        other => {
+            unreachable!("ICE: `as_type` expected a `TypeDefinition` argument, found {other:?}")
+        }
+    };
+
+    let struct_def = interner.get_struct(type_def);
+
+    let generics = struct_def
+        .borrow()
+        .generics
+        .iter()
+        .map(|generic| {
+            let name = SpannedToken::new(Token::Str(generic.borrow().to_string()), span);
+            Value::Code(Rc::new(Tokens(vec![name])))
+        })
+        .collect();
+
+    let typ = Type::Slice(Box::new(Type::Quoted(QuotedType::Quoted)));
+    Ok(Value::Slice(generics, typ))
+}
+
+/// fn fields(self) -> [(Quoted, Quoted)]
+/// Returns (name, type) pairs of each field of this TypeDefinition
+fn type_def_fields(
+    interner: &NodeInterner,
+    mut arguments: Vec<(Value, Location)>,
+) -> IResult<Value> {
+    assert_eq!(arguments.len(), 1, "ICE: `generics` should only receive a single argument");
+    let (type_def, span) = match arguments.pop() {
+        Some((Value::TypeDefinition(id), location)) => (id, location.span),
+        other => {
+            unreachable!("ICE: `as_type` expected a `TypeDefinition` argument, found {other:?}")
+        }
+    };
+
+    let struct_def = interner.get_struct(type_def);
+    let struct_def = struct_def.borrow();
+
+    let make_token = |name| SpannedToken::new(Token::Str(name), span);
+    let make_quoted = |tokens| Value::Code(Rc::new(Tokens(tokens)));
+
+    let mut fields = im::Vector::new();
+
+    for (name, typ) in struct_def.get_fields_as_written() {
+        let name = make_quoted(vec![make_token(name)]);
+        let typ = Value::Code(Rc::new(type_to_tokens(&typ)?));
+        fields.push_back(Value::Tuple(vec![name, typ]));
+    }
+
+    let typ = Type::Slice(Box::new(Type::Tuple(vec![
+        Type::Quoted(QuotedType::Quoted),
+        Type::Quoted(QuotedType::Quoted),
+    ])));
+    Ok(Value::Slice(fields, typ))
+}
+
+/// FIXME(https://github.com/noir-lang/noir/issues/5309): This code is temporary.
+/// It will produce poor results for type variables and will result in incorrect
+/// spans on the returned tokens.
+fn type_to_tokens(typ: &Type) -> IResult<Tokens> {
+    let (mut tokens, mut errors) = Lexer::lex(&typ.to_string());
+
+    if let Some(last) = tokens.0.last() {
+        if matches!(last.token(), Token::EOF) {
+            tokens.0.pop();
+        }
+    }
+
+    if !errors.is_empty() {
+        let error = errors.swap_remove(0);
+        todo!("Got lexer error: {error}")
+    }
+    Ok(tokens)
+}

compiler/noirc_frontend/src/hir_def/types.rs

@@ -296,6 +296,16 @@ impl StructType {
         })
     }
 
+    /// Returns the name and raw types of each field of this type.
+    /// This will not substitute any generic arguments so a generic field like `x`
+    /// in `struct Foo<T> { x: T }` will return a `("x", T)` pair.
+    ///
+    /// This method is almost never what is wanted for type checking or monomorphization,
+    /// prefer to use `get_fields` whenever possible.
+    pub fn get_fields_as_written(&self) -> Vec<(String, Type)> {
+        vecmap(&self.fields, |(name, typ)| (name.0.contents.clone(), typ.clone()))
+    }
+
     pub fn field_names(&self) -> BTreeSet<Ident> {
         self.fields.iter().map(|(name, _)| name.clone()).collect()
     }
@@ -805,10 +815,11 @@ impl Type {
             | Type::FmtString(_, _)
             | Type::Error => true,
 
-            Type::MutableReference(_)
-            | Type::Forall(_, _)
-            | Type::Quoted(_)
-            | Type::TraitAsType(..) => false,
+            // Quoted objects only exist at compile-time where the only execution
+            // environment is the interpreter. In this environment, they are valid.
+            Type::Quoted(_) => true,
+
+            Type::MutableReference(_) | Type::Forall(_, _) | Type::TraitAsType(..) => false,
 
             Type::Alias(alias, generics) => {
                 let alias = alias.borrow();

noir_stdlib/src/lib.nr

@@ -26,6 +26,7 @@ mod prelude;
 mod uint128;
 mod bigint;
 mod runtime;
+mod meta;
 
 // Oracle calls are required to be wrapped in an unconstrained function
 // Thus, the only argument to the `println` oracle is expected to always be an ident

noir_stdlib/src/meta.nr

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

noir_stdlib/src/meta/type_def.nr

@@ -0,0 +1,16 @@
+impl TypeDefinition {
+    /// Return a syntactic version of this type definition as a type.
+    /// For example, `as_type(quote { type Foo<A, B> { ... } })` would return `Foo<A, B>`
+    #[builtin(type_def_as_type)]
+    fn as_type(self) -> Quoted {}
+
+    /// Return each generic on this type. The names of these generics are unchanged
+    /// so users may need to keep name collisions in mind if this is used directly in a macro.
+    #[builtin(type_def_generics)]
+    fn generics(self) -> [Quoted] {}
+
+    /// Returns (name, type) pairs of each field in this type. Each type is as-is
+    /// with any generic arguments unchanged.
+    #[builtin(type_def_fields)]
+    fn fields(self) -> [(Quoted, Quoted)] {}
+}

test_programs/compile_success_empty/comptime_type_definition/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "comptime_type_definition"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.31.0"
+
+[dependencies]

test_programs/compile_success_empty/comptime_type_definition/src/main.nr

@@ -0,0 +1,13 @@
+fn main() {}
+
+#[my_comptime_fn]
+struct MyType<A, B, C> {
+    field1: [A; 10],
+    field2: (B, C),
+}
+
+comptime fn my_comptime_fn(typ: TypeDefinition) {
+    let _ = typ.as_type();
+    assert_eq(typ.generics().len(), 3);
+    assert_eq(typ.fields().len(), 2);
+}

test_programs/compile_success_empty/derive_impl/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "derive_impl"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.30.0"
+
+[dependencies]

test_programs/compile_success_empty/derive_impl/src/main.nr

@@ -0,0 +1,44 @@
+comptime fn derive_default(typ: TypeDefinition) -> Quoted {
+    let generics: [Quoted] = typ.generics();
+    assert_eq(
+        generics.len(), 0, "derive_default: Deriving Default on generic types is currently unimplemented"
+    );
+
+    let type_name = typ.as_type();
+    let fields = typ.fields();
+
+    let fields = join(make_field_exprs(fields));
+
+    quote {
+        impl Default for $type_name {
+            fn default() -> Self {
+                Self { $fields }
+            }
+        }
+    }
+}
+
+#[derive_default]
+struct Foo {
+    x: Field,
+    y: u32,
+}
+
+comptime fn make_field_exprs(fields: [(Quoted, Quoted)]) -> [Quoted] {
+    let mut result = &[];
+    for my_field in fields {
+        let name = my_field.0;
+        result = result.push_back(quote { $name: Default::default(), });
+    }
+    result
+}
+
+comptime fn join(slice: [Quoted]) -> Quoted {
+    let mut result = quote {};
+    for elem in slice {
+        result = quote { $result $elem };
+    }
+    result
+}
+
+fn main() {}