comptime interpreter handles scopes incorrectly when calling builtin/foreign/oracle functions

[TomAFrench]

The below program compiles just fine

fn main() {
    comptime {
        let ws: [Field; 1] = [1];
        let ws_as_slice: [Field] = ws.as_slice();

        assert_eq(ws_as_slice[0], 1);
    }
}

however if I change the assertion to act on ws rather than ws_as_slice then the program fails to compile with the error message below.

fn main() {
    comptime {
        let ws: [Field; 1] = [1];
        let ws_as_slice: [Field] = ws.as_slice();

        assert_eq(ws[0], 1);
    }
}

$ nargo compile
warning: unused variable ws_as_slice
  ┌─ /home/tom/Programming/aztec/noir/test_programs/compile_success_empty/comptime_as_slice/src/main.nr:4:13
  │
4 │         let ws_as_slice: [Field] = ws.as_slice();
  │             ----------- unused variable 
  │

error: Non-comptime variable `ws` referenced in comptime code
  ┌─ /home/tom/Programming/aztec/noir/test_programs/compile_success_empty/comptime_as_slice/src/main.nr:6:19
  │
6 │         assert_eq(ws[0], 1);
  │                   -- Non-comptime variables can't be used in comptime code
  │

[TomAFrench]

Actually, this seems to be specific to as_slice.

[jfecher]

@TomAFrench It is also because of the fact that any time a variable is not in scope the interpreter assumes the variable is not comptime. I'm not sure why it'd be happening here but we should probably make that error more general.

[TomAFrench]

I've fixed this issue in https://github.com/noir-lang/noir/pull/5276. We were performing an early return in the situation where we hit a builtin/foreign/oracle function so we missed the call to exit_function

[jfecher]

@TomAFrench Thanks, I was about to say I had this fixed as well yesterday but it looks like I never made the PR. We found the same issue though.