feat: Add TraitDefinition::as_trait_constraint() (#5541)

# Description

## Problem\*

Part of work towards
#4594 (comment),
forgot an issue for this one.

## Summary\*



## 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.

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

@@ -5,12 +5,14 @@ use std::{
 
 use acvm::{AcirField, FieldElement};
 use chumsky::Parser;
+use iter_extended::vecmap;
 use noirc_errors::Location;
 
 use crate::{
     ast::{IntegerBitSize, TraitBound},
     hir::comptime::{errors::IResult, InterpreterError, Value},
-    macros_api::{NodeInterner, Signedness},
+    macros_api::{NodeInterner, Path, Signedness, UnresolvedTypeData},
+    node_interner::TraitId,
     parser,
     token::{SpannedToken, Token, Tokens},
     QuotedType, Type,
@@ -42,6 +44,9 @@ pub(super) fn call_builtin(
         "struct_def_generics" => struct_def_generics(interner, arguments, location),
         "trait_constraint_eq" => trait_constraint_eq(interner, arguments, location),
         "trait_constraint_hash" => trait_constraint_hash(interner, arguments, location),
+        "trait_def_as_trait_constraint" => {
+            trait_def_as_trait_constraint(interner, arguments, location)
+        }
         "quoted_as_trait_constraint" => quoted_as_trait_constraint(interner, arguments, location),
         _ => {
             let item = format!("Comptime evaluation for builtin function {name}");
@@ -103,6 +108,16 @@ fn get_trait_constraint(value: Value, location: Location) -> IResult<TraitBound>
     }
 }
 
+fn get_trait_def(value: Value, location: Location) -> IResult<TraitId> {
+    match value {
+        Value::TraitDefinition(id) => Ok(id),
+        value => {
+            let expected = Type::Quoted(QuotedType::TraitDefinition);
+            Err(InterpreterError::TypeMismatch { expected, value, location })
+        }
+    }
+}
+
 fn get_quoted(value: Value, location: Location) -> IResult<Rc<Tokens>> {
     match value {
         Value::Code(tokens) => Ok(tokens),
@@ -456,3 +471,24 @@ fn modulus_num_bits(
     let bits = FieldElement::max_num_bits().into();
     Ok(Value::U64(bits))
 }
+
+fn trait_def_as_trait_constraint(
+    interner: &mut NodeInterner,
+    mut arguments: Vec<(Value, Location)>,
+    location: Location,
+) -> Result<Value, InterpreterError> {
+    check_argument_count(1, &arguments, location)?;
+
+    let trait_id = get_trait_def(arguments.pop().unwrap().0, location)?;
+    let the_trait = interner.get_trait(trait_id);
+
+    let trait_path = Path::from_ident(the_trait.name.clone());
+
+    let trait_generics = vecmap(&the_trait.generics, |generic| {
+        let name = Path::from_single(generic.name.as_ref().clone(), generic.span);
+        UnresolvedTypeData::Named(name, Vec::new(), false).with_span(generic.span)
+    });
+
+    let trait_id = Some(trait_id);
+    Ok(Value::TraitConstraint(TraitBound { trait_path, trait_id, trait_generics }))
+}

noir_stdlib/src/meta/mod.nr

@@ -1,5 +1,6 @@
-mod type_def;
 mod trait_constraint;
+mod trait_def;
+mod type_def;
 mod quoted;
 
 /// Calling unquote as a macro (via `unquote!(arg)`) will unquote

noir_stdlib/src/meta/trait_def.nr

@@ -0,0 +1,4 @@
+impl TraitDefinition {
+    #[builtin(trait_def_as_trait_constraint)]
+    fn as_trait_constraint(_self: Self) -> TraitConstraint {}
+}

test_programs/compile_success_empty/trait_as_constraint/Nargo.toml

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

test_programs/compile_success_empty/trait_as_constraint/src/main.nr

@@ -0,0 +1,9 @@
+#[test_as_constraint]
+trait Foo<T> {}
+
+comptime fn test_as_constraint(t: TraitDefinition) {
+    let constraint = t.as_trait_constraint();
+    assert(constraint == constraint);
+}
+
+fn main() {}