feat: add Expr::as_assert_eq (#5880)

# Description

## Problem

Part of #5668

## 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.

---------

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

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

@@ -61,6 +61,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "as_slice" => as_slice(interner, 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),
             "expr_as_assign" => expr_as_assign(interner, arguments, return_type, location),
             "expr_as_binary_op" => expr_as_binary_op(interner, arguments, return_type, location),
             "expr_as_block" => expr_as_block(interner, arguments, return_type, location),
@@ -953,6 +954,43 @@ fn expr_as_assert(
     })
 }
 
+// fn as_assert_eq(self) -> Option<(Expr, Expr, Option<Expr>)>
+fn expr_as_assert_eq(
+    interner: &NodeInterner,
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+) -> IResult<Value> {
+    expr_as(interner, arguments, return_type.clone(), location, |expr| {
+        if let ExprValue::Statement(StatementKind::Constrain(constrain)) = expr {
+            if constrain.2 == ConstrainKind::AssertEq {
+                let ExpressionKind::Infix(infix) = constrain.0.kind else {
+                    panic!("Expected AssertEq constrain statement to have an infix expression");
+                };
+
+                let lhs = Value::expression(infix.lhs.kind);
+                let rhs = Value::expression(infix.rhs.kind);
+
+                let option_type = extract_option_generic_type(return_type);
+                let Type::Tuple(mut tuple_types) = option_type else {
+                    panic!("Expected the return type option generic arg to be a tuple");
+                };
+                assert_eq!(tuple_types.len(), 3);
+
+                let option_type = tuple_types.pop().unwrap();
+                let message = constrain.1.map(|message| Value::expression(message.kind));
+                let message = option(option_type, message).ok()?;
+
+                Some(Value::Tuple(vec![lhs, rhs, message]))
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    })
+}
+
 // fn as_assign(self) -> Option<(Expr, Expr)>
 fn expr_as_assign(
     interner: &NodeInterner,

docs/docs/noir/standard_library/meta/expr.md

@@ -18,6 +18,13 @@ If this expression is an array, this returns a slice of each element in the arra
 
 If this expression is an assert, this returns the assert expression and the optional message.
 
+### as_assert_eq
+
+#include_code as_assert_eq noir_stdlib/src/meta/expr.nr rust
+
+If this expression is an assert_eq, this returns the left-hand-side and right-hand-side
+expressions, together with the optional message.
+
 ### as_assign
 
 #include_code as_assign noir_stdlib/src/meta/expr.nr rust

noir_stdlib/src/meta/expr.nr

@@ -13,6 +13,11 @@ impl Expr {
     fn as_assert(self) -> Option<(Expr, Option<Expr>)> {}
     // docs:end:as_assert
 
+    #[builtin(expr_as_assert_eq)]
+    // docs:start:as_assert_eq
+    fn as_assert_eq(self) -> Option<(Expr, Expr, Option<Expr>)> {}
+    // docs:end:as_assert_eq
+
     #[builtin(expr_as_assign)]
     // docs:start:as_assign
     fn as_assign(self) -> Option<(Expr, Expr)> {}
@@ -121,6 +126,7 @@ impl Expr {
         // docs:end:modify
         let result = modify_array(self, f);
         let result = result.or_else(|| modify_assert(self, f));
+        let result = result.or_else(|| modify_assert_eq(self, f));
         let result = result.or_else(|| modify_assign(self, f));
         let result = result.or_else(|| modify_binary_op(self, f));
         let result = result.or_else(|| modify_block(self, f));
@@ -178,6 +184,18 @@ fn modify_assert<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>) -> Option<Ex
     )
 }
 
+fn modify_assert_eq<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>) -> Option<Expr> {
+    expr.as_assert_eq().map(
+        |expr: (Expr, Expr, Option<Expr>)| {
+        let (lhs, rhs, msg) = expr;
+        let lhs = lhs.modify(f);
+        let rhs = rhs.modify(f);
+        let msg = msg.map(|msg: Expr| msg.modify(f));
+        new_assert_eq(lhs, rhs, msg)
+    }
+    )
+}
+
 fn modify_assign<Env>(expr: Expr, f: fn[Env](Expr) -> Option<Expr>) -> Option<Expr> {
     expr.as_assign().map(
         |expr: (Expr, Expr)| {
@@ -360,6 +378,15 @@ fn new_assert(predicate: Expr, msg: Option<Expr>) -> Expr {
     }
 }
 
+fn new_assert_eq(lhs: Expr, rhs: Expr, msg: Option<Expr>) -> Expr {
+    if msg.is_some() {
+        let msg = msg.unwrap();
+        quote { assert_eq($lhs, $rhs, $msg) }.as_expr().unwrap()
+    } else {
+        quote { assert_eq($lhs, $rhs) }.as_expr().unwrap()
+    }
+}
+
 fn new_assign(lhs: Expr, rhs: Expr) -> Expr {
     quote { $lhs = $rhs }.as_expr().unwrap()
 }

test_programs/noir_test_success/comptime_expr/src/main.nr

@@ -63,6 +63,44 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_expr_as_assert_eq() {
+        comptime
+        {
+            let expr = quote { assert_eq(true, false) }.as_expr().unwrap();
+            let (lhs, rhs, msg) = expr.as_assert_eq().unwrap();
+            assert_eq(lhs.as_bool().unwrap(), true);
+            assert_eq(rhs.as_bool().unwrap(), false);
+            assert(msg.is_none());
+
+            let expr = quote { assert_eq(false, true, "oops") }.as_expr().unwrap();
+            let (lhs, rhs, msg) = expr.as_assert_eq().unwrap();
+            assert_eq(lhs.as_bool().unwrap(), false);
+            assert_eq(rhs.as_bool().unwrap(), true);
+            assert(msg.is_some());
+        }
+    }
+
+    #[test]
+    fn test_expr_mutate_for_assert_eq() {
+        comptime
+        {
+            let expr = quote { assert_eq(1, 2) }.as_expr().unwrap();
+            let expr = expr.modify(times_two);
+            let (lhs, rhs, msg) = expr.as_assert_eq().unwrap();
+            assert_eq(lhs.as_integer().unwrap(), (2, false));
+            assert_eq(rhs.as_integer().unwrap(), (4, false));
+            assert(msg.is_none());
+
+            let expr = quote { assert_eq(1, 2, 3) }.as_expr().unwrap();
+            let expr = expr.modify(times_two);
+            let (lhs, rhs, msg) = expr.as_assert_eq().unwrap();
+            assert_eq(lhs.as_integer().unwrap(), (2, false));
+            assert_eq(rhs.as_integer().unwrap(), (4, false));
+            assert_eq(msg.unwrap().as_integer().unwrap(), (6, false));
+        }
+    }
+
     #[test]
     fn test_expr_as_assign() {
         comptime