Unexpected token: expected `;`, found `#`.

[Jerrody]

#[cfg(feature = "validation")]
[debug_utils_ext, (*surface_extensions).to_owned()].concat()

#[cfg(not(feature = "validation"))]
surface_extensions.to_owned()

Error code:

error: expected `;`, found `#`
  --> src\engine\renderer\context\instance.rs:44:73
   |
44 |             [debug_utils_ext, (*surface_extensions).to_owned()].concat()
   |                                                                         ^ help: add `;` here
45 |
46 |             #[cfg(not(feature = "validation"))]
   |             - unexpected token

Metadata:

rustc 1.67.0-nightly (e1d819583 2022-12-05)
binary: rustc
commit-hash: e1d819583f0bf13b016b119c1c2c43e6d3979450
commit-date: 2022-12-05
host: x86_64-pc-windows-msvc
release: 1.67.0-nightly
LLVM version: 15.0.4

[compiler-errors]

What is the issue you are reporting here @Jerrody ?

[Jerrody]

What is the issue you are reporting here @Jerrody ?

Well, I thought that compiler can understand that #[cfg()] is related to the block of code, but it fails.

[Noratrieb]

Can you post a minimal reproducible example that demonstrates your issue? Or at least mention exactly what you'd expect to happen and why.

Even code that is cfged out is still required to parse successfully.

[Jerrody]

Can you post a minimal reproducible example that demonstrates your issue? Or at least mention exactly what you'd expect to happen and why.

Even code that is cfged out is still required to parse successfully.

Let's show you what exactly problem I have.
Full code:

let instance_extensions = {
    let surface_extensions =
        ash_window::enumerate_required_extensions(window.raw_display_handle()).track()?;

    #[cfg(feature = "validation")]
    let debug_utils_ext = vec![ash::extensions::ext::DebugUtils::name().as_ptr()];

    #[cfg(feature = "validation")]
    [debug_utils_ext, (*surface_extensions).to_owned()].concat()

    #[cfg(not(feature = "validation"))]
    surface_extensions.to_owned()
};

And I get this error:

error: expected `;`, found `#`
  --> src\engine\renderer\context\instance.rs:44:73
   |
44 |             [debug_utils_ext, (*surface_extensions).to_owned()].concat()
   |                                                                         ^ help: add `;` here
45 |
46 |             #[cfg(not(feature = "validation"))]
   |             - unexpected token

[SkiFire13]

You seem to already have posted this in this discussion in the internals forum a month ago. What you're asking seems a rather big change to me, I'm not that familiar with how the compiler parser works but I believe it currently requires your program syntax to be correct before #[cfg(...)] macros are expanded, which is not true for your example, so some fundamental changes would be needed.

[Jerrody]

You seem to already have posted this in this discussion in the internals forum a month ago. What you're asking seems a rather big change to me, I'm not that familiar with how the compiler parser works but I believe it currently requires your program syntax to be correct before #[cfg(...)] macros are expanded, which is not true for your example, so some fundamental changes would be needed.

Oh, damn, sorry. I totally forgot that I made a post.
Well, if it's not a bug and parser works like this it is bad, anyway, exists alternative way to write like this.
But for me my issue is a candidate to feature request or a bug. Anyway, I'm not a lang dev, so I don't know what scales of this defect (?).

[lukas-code]

The workaround for this is to wrap the expressions in a block:

let instance_extensions = {
    let surface_extensions =
        ash_window::enumerate_required_extensions(window.raw_display_handle()).track()?;

    #[cfg(feature = "validation")]
    let debug_utils_ext = vec![ash::extensions::ext::DebugUtils::name().as_ptr()];

    #[cfg(feature = "validation")]
    { [debug_utils_ext, (*surface_extensions).to_owned()].concat() }

    #[cfg(not(feature = "validation"))]
    { surface_extensions.to_owned() }
};

[Jerrody]

The workaround for this is to wrap the expressions in a block:

let instance_extensions = {
    let surface_extensions =
        ash_window::enumerate_required_extensions(window.raw_display_handle()).track()?;

    #[cfg(feature = "validation")]
    let debug_utils_ext = vec![ash::extensions::ext::DebugUtils::name().as_ptr()];

    #[cfg(feature = "validation")]
    { [debug_utils_ext, (*surface_extensions).to_owned()].concat() }

    #[cfg(not(feature = "validation"))]
    { surface_extensions.to_owned() }
};

I solved this issue exactly as you did.

[albertlarsan68]

Another way to do this is using the cfg! macro and if ... else.
I personally thinks it is more clear, and should optimize the same.

let instance_extensions = {
    let surface_extensions =
        ash_window::enumerate_required_extensions(window.raw_display_handle()).track()?;

    #[cfg(feature = "validation")]
    let debug_utils_ext = vec![ash::extensions::ext::DebugUtils::name().as_ptr()];

    if cfg!(feature = "validation") {
        [debug_utils_ext, (*surface_extensions).to_owned()].concat()
    } else {
        surface_extensions.to_owned()
    }
};

[Jerrody]

Another way to do this is using the cfg! macro and if ... else. I personally thinks it is more clear, and should optimize the same.

let instance_extensions = {
    let surface_extensions =
        ash_window::enumerate_required_extensions(window.raw_display_handle()).track()?;

    #[cfg(feature = "validation")]
    let debug_utils_ext = vec![ash::extensions::ext::DebugUtils::name().as_ptr()];

    if cfg!(feature = "validation") {
        [debug_utils_ext, (*surface_extensions).to_owned()].concat()
    } else {
        surface_extensions.to_owned()
    }
};

Work is not the same, due to cfg! compile-time evaluating an expression that we pass in it and evaluates only it, but at the same time everything compiles what is inside of if else.