Broken dereference after any blocked scope (if, for, {})

[vezenovm]

Aim

I want to be able to run this test:

#[test]
fn test_mutable_ref()
{
    let x = mutable_return_syntax_error();
    assert(x[0] == 1);
}

fn mutable_return_syntax_error() -> [Field] {
    let slice : &mut [Field] = &mut [];

    slice = &mut (*slice).push_back(1);

    for i in 0..2 { }  // or if (...) { } // or  { } -- same idea

    *slice
}

Expected Behavior

I should be able to return the dereferenced slice.

Bug

Instead I get this bug:

If I fix the scope by adding a semicolon to the end of {} block like so:

fn mutable_return_syntax_error() -> [Field] {
    let slice : &mut [Field] = &mut [];

    slice = &mut (*slice).push_back(1);

    for i in 0..2 { };  // or if (...) { } // or  { } -- same idea

    *slice
}

I will instead get this panic in SSA:

Message:  internal error: entered unreachable code: Cannot assign to a variable without a reference
Location: compiler/noirc_evaluator/src/ssa/ssa_gen/context.rs:662

To Reproduce

1. Copy the snippet in the issue over to a Noir program.
2. Run nargo test

Installation Method

Compiled from source

Nargo Version

nargo 0.15.0 (git version hash: 9854416, is dirty: true)

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[jfecher]

I'm able to replicate the panic.

I believe the problem is this line:

    slice = &mut (*slice).push_back(1);

Which is rather odd. Usually it is written:

    *slice = slice.push_back(1);

To avoid creating the new allocation, but it seems something must have went wrong with creating the new allocation here, leading to the panic.

[jfecher]

It turns out the panic was from slice not being declared as mutable. Normally you can mutate variables of type &mut T even if the variable itself is not declared as mutable - but crucially to mutate them you need to dereference them on the lhs of an assignment. Swapping out the reference to another reference as in this example should not have been possible, so the panic was correct and we were missing an extra check for when a variable can be mutated.