feat: Derive Eq & Default (#5667)

# Description

## Problem\*

Resolves #4594 

First traits to derive in the stdlib!

## Summary\*

Lets us derive the `Eq` and `Default` traits for almost any type.

I anticipate there will be issues with deriving generic types. I'll fix
this in another PR.

## Additional Context

Since derive is in the stdlib and we can derive traits now, I'm
arbitrarily marking the epic for metaprogramming as completed. There are
still bugs to fix and many helper functions we can add but the bulk of
the work and infrastructure is all in.

## 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\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

noir_stdlib/src/cmp.nr

@@ -1,9 +1,37 @@
+use crate::meta::derive_via;
+
+#[derive_via(derive_eq)]
 // docs:start:eq-trait
 trait Eq {
     fn eq(self, other: Self) -> bool;
 }
 // docs:end:eq-trait
 
+comptime fn derive_eq(s: StructDefinition) -> Quoted {
+    let typ = s.as_type();
+
+    let impl_generics = s.generics().join(quote {,});
+
+    let where_clause = s.generics().map(|name| quote { $name: Default }).join(quote {,});
+
+    // `(self.a == other.a) & (self.b == other.b) & ...`
+    let equalities = s.fields().map(
+        |f: (Quoted, Type)| {
+        let name = f.0;
+        quote { (self.$name == other.$name) }
+    }
+    );
+    let body = equalities.join(quote { & });
+
+    quote {
+        impl<$impl_generics> Eq for $typ where $where_clause {
+            fn eq(self, other: Self) -> bool {
+                $body
+            }
+        }
+    }
+}
+
 impl Eq for Field { fn eq(self, other: Field) -> bool { self == other } }
 
 impl Eq for u64 { fn eq(self, other: u64) -> bool { self == other } }

noir_stdlib/src/default.nr

@@ -1,9 +1,37 @@
+use crate::meta::derive_via;
+
+#[derive_via(derive_default)]
 // docs:start:default-trait
 trait Default {
     fn default() -> Self;
 }
 // docs:end:default-trait
 
+comptime fn derive_default(s: StructDefinition) -> Quoted {
+    let typ = s.as_type();
+
+    let impl_generics = s.generics().join(quote {,});
+
+    let where_clause = s.generics().map(|name| quote { $name: Default }).join(quote {,});
+
+    // `foo: Default::default(), bar: Default::default(), ...`
+    let fields = s.fields().map(
+        |f: (Quoted, Type)| {
+        let name = f.0;
+        quote { $name: Default::default() }
+    }
+    );
+    let fields = fields.join(quote {,});
+
+    quote {
+        impl<$impl_generics> Default for $typ where $where_clause {
+            fn default() -> Self {
+                Self { $fields }
+            }
+        }
+    }
+}
+
 impl Default for Field { fn default() -> Field { 0 } }
 
 impl Default for u8 { fn default() -> u8 { 0 } }

noir_stdlib/src/hash/poseidon2.nr

@@ -1,7 +1,7 @@
 use crate::hash::Hasher;
 use crate::default::Default;
 
-global RATE: u32 = 3;
+comptime global RATE: u32 = 3;
 
 struct Poseidon2 {
     cache: [Field;3],

noir_stdlib/src/meta/mod.nr

@@ -2,9 +2,9 @@ use crate::collections::umap::UHashMap;
 use crate::hash::BuildHasherDefault;
 use crate::hash::poseidon2::Poseidon2Hasher;
 
+mod struct_def;
 mod trait_constraint;
 mod trait_def;
-mod type_def;
 mod quoted;
 
 /// Calling unquote as a macro (via `unquote!(arg)`) will unquote

test_programs/execution_success/derive/src/main.nr

@@ -19,7 +19,17 @@ comptime fn derive_do_nothing(s: StructDefinition) -> Quoted {
     }
 }
 
+// Test stdlib derive fns & multiple traits
+#[derive(Eq, Default)]
+struct MyOtherStruct {
+    field1: u32,
+    field2: u64,
+}
+
 fn main() {
     let s = MyStruct { my_field: 1 };
     s.do_nothing();
+
+    let o = MyOtherStruct::default();
+    assert_eq(o, o);
 }