feat: implement comptime support for as_slice builtin (#5276)

# Description

## Problem\*

Resolves #5274

## Summary\*

This PR implements the `as_slice` builtin function for the comptime
interpreter. I've also fixed an issue where due to
builtin/foreign/oracle functions having an early return, we don't call
`exit_function` which results in the scopes being left in an
inconsistent state.

## 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.

Author: TomAFrench

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

@@ -62,8 +62,6 @@ impl<'a> Interpreter<'a> {
         arguments: Vec<(Value, Location)>,
         location: Location,
     ) -> IResult<Value> {
-        let previous_state = self.enter_function();
-
         let meta = self.interner.function_meta(&function);
         if meta.parameters.len() != arguments.len() {
             return Err(InterpreterError::ArgumentCountMismatch {
@@ -78,6 +76,8 @@ impl<'a> Interpreter<'a> {
         }
 
         let parameters = meta.parameters.0.clone();
+        let previous_state = self.enter_function();
+
         for ((parameter, typ, _), (argument, arg_location)) in parameters.iter().zip(arguments) {
             self.define_pattern(parameter, typ, argument, arg_location)?;
         }
@@ -102,6 +102,7 @@ impl<'a> Interpreter<'a> {
         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!("Evaluation for builtin function {builtin}");
                     Err(InterpreterError::Unimplemented { item, location })

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

@@ -1,6 +1,9 @@
 use noirc_errors::Location;
 
-use crate::hir::comptime::{errors::IResult, Value};
+use crate::{
+    hir::comptime::{errors::IResult, Value},
+    Type,
+};
 
 pub(super) fn array_len(arguments: &[(Value, Location)]) -> IResult<Value> {
     assert_eq!(arguments.len(), 1, "ICE: `array_len` should only receive a single argument");
@@ -10,3 +13,13 @@ pub(super) fn array_len(arguments: &[(Value, Location)]) -> IResult<Value> {
         _ => unreachable!("ICE: Cannot query length of types other than arrays or slices"),
     }
 }
+
+pub(super) 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 {
+        Value::Array(values, Type::Array(_, typ)) => Ok(Value::Slice(values, Type::Slice(typ))),
+        // Type checking should prevent this branch being taken.
+        _ => unreachable!("ICE: Cannot convert types other than arrays into slices"),
+    }
+}

test_programs/compile_success_empty/comptime_as_slice/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "array_to_slice"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.24.0"
+
+[dependencies]

test_programs/compile_success_empty/comptime_as_slice/src/main.nr

@@ -0,0 +1,9 @@
+fn main() {
+    comptime
+    {
+        let ws: [Field; 3] = [1; 3];
+        let ws_as_slice: [Field] = ws.as_slice();
+
+        assert_eq(ws[0], ws_as_slice[0]);
+    }
+}