feat: add Quoted::as_expr and Expr::as_function_call (#5708)

# Description

## Problem

Part of #5668

## Summary

What the title says.

## Additional Context

I didn't know how to test the exprs. I tried implementing `impl Eq for
Expr` but even though two exprs looked the same (same tokens) they
weren't equal... but I didn't dig deeper into why that is (I didn't know
if `Eq` was desired in this case)

## 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

@@ -6,7 +6,7 @@ use std::{
 use acvm::{AcirField, FieldElement};
 use builtin_helpers::{
     check_argument_count, check_function_not_yet_resolved, check_one_argument,
-    check_three_arguments, check_two_arguments, get_function_def, get_module, get_quoted,
+    check_three_arguments, check_two_arguments, get_expr, get_function_def, get_module, get_quoted,
     get_slice, get_struct, get_trait_constraint, get_trait_def, get_tuple, get_type, get_u32,
     hir_pattern_to_tokens, mutate_func_meta_type, parse, parse_tokens,
     replace_func_meta_parameters, replace_func_meta_return_type,
@@ -17,8 +17,8 @@ use rustc_hash::FxHashMap as HashMap;
 
 use crate::{
     ast::{
-        FunctionKind, FunctionReturnType, IntegerBitSize, UnresolvedType, UnresolvedTypeData,
-        Visibility,
+        ExpressionKind, FunctionKind, FunctionReturnType, IntegerBitSize, UnresolvedType,
+        UnresolvedTypeData, Visibility,
     },
     hir::comptime::{errors::IResult, value::add_token_spans, InterpreterError, Value},
     hir_def::function::FunctionBody,
@@ -47,6 +47,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "array_as_str_unchecked" => array_as_str_unchecked(interner, arguments, location),
             "array_len" => array_len(interner, arguments, location),
             "as_slice" => as_slice(interner, arguments, location),
+            "expr_as_function_call" => expr_as_function_call(arguments, return_type, location),
             "is_unconstrained" => Ok(Value::Bool(true)),
             "function_def_name" => function_def_name(interner, arguments, location),
             "function_def_parameters" => function_def_parameters(interner, arguments, location),
@@ -64,6 +65,7 @@ impl<'local, 'context> Interpreter<'local, 'context> {
             "modulus_le_bits" => modulus_le_bits(interner, arguments, location),
             "modulus_le_bytes" => modulus_le_bytes(interner, arguments, location),
             "modulus_num_bits" => modulus_num_bits(interner, arguments, location),
+            "quoted_as_expr" => quoted_as_expr(arguments, return_type, location),
             "quoted_as_module" => quoted_as_module(self, arguments, return_type, location),
             "quoted_as_trait_constraint" => quoted_as_trait_constraint(self, arguments, location),
             "quoted_as_type" => quoted_as_type(self, arguments, location),
@@ -312,6 +314,20 @@ fn slice_insert(
     Ok(Value::Slice(values, typ))
 }
 
+// fn as_expr(quoted: Quoted) -> Option<Expr>
+fn quoted_as_expr(
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+) -> IResult<Value> {
+    let argument = check_one_argument(arguments, location)?;
+
+    let expr = parse(argument, parser::expression(), "an expression").ok();
+    let value = expr.map(|expr| Value::Expr(expr.kind));
+
+    option(return_type, value)
+}
+
 // fn as_module(quoted: Quoted) -> Option<Module>
 fn quoted_as_module(
     interpreter: &mut Interpreter,
@@ -672,6 +688,42 @@ fn zeroed(return_type: Type) -> IResult<Value> {
     }
 }
 
+// fn as_function_call(self) -> Option<(Expr, [Expr])>
+fn expr_as_function_call(
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+) -> IResult<Value> {
+    expr_as(arguments, return_type, location, |expr| {
+        if let ExpressionKind::Call(call_expression) = expr {
+            let function = Value::Expr(call_expression.func.kind);
+            let arguments = call_expression.arguments.into_iter();
+            let arguments = arguments.map(|argument| Value::Expr(argument.kind)).collect();
+            let arguments =
+                Value::Slice(arguments, Type::Slice(Box::new(Type::Quoted(QuotedType::Expr))));
+            Some(Value::Tuple(vec![function, arguments]))
+        } else {
+            None
+        }
+    })
+}
+
+// Helper function for implementing the `expr_as_...` functions.
+fn expr_as<F>(
+    arguments: Vec<(Value, Location)>,
+    return_type: Type,
+    location: Location,
+    f: F,
+) -> IResult<Value>
+where
+    F: FnOnce(ExpressionKind) -> Option<Value>,
+{
+    let self_argument = check_one_argument(arguments, location)?;
+    let expr = get_expr(self_argument)?;
+    let option_value = f(expr);
+    option(return_type, option_value)
+}
+
 // fn name(self) -> Quoted
 fn function_def_name(
     interner: &NodeInterner,

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

@@ -4,7 +4,7 @@ use acvm::FieldElement;
 use noirc_errors::Location;
 
 use crate::{
-    ast::{IntegerBitSize, Signedness},
+    ast::{ExpressionKind, IntegerBitSize, Signedness},
     hir::{
         comptime::{errors::IResult, value::add_token_spans, Interpreter, InterpreterError, Value},
         def_map::ModuleId,
@@ -145,6 +145,17 @@ pub(crate) fn get_u32((value, location): (Value, Location)) -> IResult<u32> {
     }
 }
 
+pub(crate) fn get_expr((value, location): (Value, Location)) -> IResult<ExpressionKind> {
+    match value {
+        Value::Expr(expr) => Ok(expr),
+        value => {
+            let expected = Type::Quoted(QuotedType::Expr);
+            let actual = value.get_type().into_owned();
+            Err(InterpreterError::TypeMismatch { expected, actual, location })
+        }
+    }
+}
+
 pub(crate) fn get_function_def((value, location): (Value, Location)) -> IResult<FuncId> {
     match value {
         Value::FunctionDefinition(id) => Ok(id),

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

@@ -57,6 +57,7 @@ pub enum Value {
     ModuleDefinition(ModuleId),
     Type(Type),
     Zeroed(Type),
+    Expr(ExpressionKind),
 }
 
 impl Value {
@@ -103,6 +104,7 @@ impl Value {
             Value::ModuleDefinition(_) => Type::Quoted(QuotedType::Module),
             Value::Type(_) => Type::Quoted(QuotedType::Type),
             Value::Zeroed(typ) => return Cow::Borrowed(typ),
+            Value::Expr(_) => Type::Quoted(QuotedType::Expr),
         })
     }
 
@@ -223,6 +225,7 @@ impl Value {
                     }
                 };
             }
+            Value::Expr(expr) => expr,
             Value::Pointer(..)
             | Value::StructDefinition(_)
             | Value::TraitConstraint(..)
@@ -345,7 +348,8 @@ impl Value {
                 HirExpression::Literal(HirLiteral::Slice(HirArrayLiteral::Standard(elements)))
             }
             Value::Quoted(tokens) => HirExpression::Unquote(add_token_spans(tokens, location.span)),
-            Value::Pointer(..)
+            Value::Expr(..)
+            | Value::Pointer(..)
             | Value::StructDefinition(_)
             | Value::TraitConstraint(..)
             | Value::TraitDefinition(_)
@@ -530,6 +534,7 @@ impl<'value, 'interner> Display for ValuePrinter<'value, 'interner> {
             Value::ModuleDefinition(_) => write!(f, "(module)"),
             Value::Zeroed(typ) => write!(f, "(zeroed {typ})"),
             Value::Type(typ) => write!(f, "{}", typ),
+            Value::Expr(expr) => write!(f, "{}", expr),
         }
     }
 }

noir_stdlib/src/meta/expr.nr

@@ -0,0 +1,6 @@
+use crate::option::Option;
+
+impl Expr {
+    #[builtin(expr_as_function_call)]
+    fn as_function_call(self) -> Option<(Expr, [Expr])> {}
+}

noir_stdlib/src/meta/mod.nr

@@ -2,6 +2,7 @@ use crate::collections::umap::UHashMap;
 use crate::hash::BuildHasherDefault;
 use crate::hash::poseidon2::Poseidon2Hasher;
 
+mod expr;
 mod function_def;
 mod module;
 mod struct_def;

noir_stdlib/src/meta/quoted.nr

@@ -2,6 +2,9 @@ use crate::cmp::Eq;
 use crate::option::Option;
 
 impl Quoted {
+    #[builtin(quoted_as_expr)]
+    fn as_expr(self) -> Option<Expr> {}
+
     #[builtin(quoted_as_module)]
     fn as_module(self) -> Option<Module> {}
 

test_programs/compile_success_empty/comptime_exp/Nargo.toml

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

test_programs/compile_success_empty/comptime_exp/src/main.nr

@@ -0,0 +1,8 @@
+fn main() {
+    comptime
+    {
+        let expr = quote { foo(bar) }.as_expr().unwrap();
+        let (_function, args) = expr.as_function_call().unwrap();
+        assert_eq(args.len(), 1);
+    }
+}