# Make possible to use procedural macros with scopes **URL:** \ **Category:** language design\ **Created:** [November 20, 2022, 6:41am UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778 "2022-11-20T06:41:33Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Jerrody](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/jerrody/32/9983_2.png) [@Jerrody](https://internals.rust-lang.org/u/Jerrody)\ **Post date:** [November 20, 2022, 6:41am UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/1 "2022-11-20T06:41:33Z")
Example usage: ```rust fn main() { let x = { #[cfg(debug_assertions)] 5 #[cfg(not(debug_asserttions))] 3 }; println!("{x}"); } ``` or ```rust let debug_utils = { #[cfg(validation)] Some({ let debug_utils_info = vk::DebugUtilsMessengerCreateInfoEXT::default() .message_severity( vk::DebugUtilsMessageSeverityFlagsEXT::ERROR | vk::DebugUtilsMessageSeverityFlagsEXT::WARNING | vk::DebugUtilsMessageSeverityFlagsEXT::INFO, ) .message_type( vk::DebugUtilsMessageTypeFlagsEXT::GENERAL | vk::DebugUtilsMessageTypeFlagsEXT::PERFORMANCE | vk::DebugUtilsMessageTypeFlagsEXT::VALIDATION, ); let debug_loader = ash::extensions::ext::DebugUtils::new(&entry, &instance); unsafe { debug_loader .create_debug_utils_messenger(&debug_utils_info, None) .unwrap() } }) #[cfg(not(validation))] None }; ``` Currently it gives an error: 1. ```rust error: expected `;`, found `#` --> src/main.rs:4:10 | 4 | 5 | ^ help: add `;` here 5 | 6 | #[cfg(not(debug_asserttions))] | - unexpected token error[E0308]: mismatched types --> src/main.rs:4:9 | 4 | 5 | ^ expected `()`, found integer For more information about this error, try `rustc --explain E0308`. error: could not compile `playground` due to 2 previous errors ``` 1. ```rust error: expected `;`, found `#` --> src\engine\backend\mod.rs:77:15 | 77 | }) | ^ help: add `;` here 78 | 79 | #[cfg(not(validation))] | - unexpected token error: could not compile `vulkan_learning` due to previous error ``` ---
**Author:** ![2e71828](https://avatars.discourse-cdn.com/v4/letter/2/3e96dc/32.png) [@2e71828](https://internals.rust-lang.org/u/2e71828)\ **Post date:** [November 20, 2022, 7:21am UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/2 "2022-11-20T07:21:55Z")
Setting aside the potential issues that arise if the various attributes aren’t exhaustive, the problem with annotating expressions instead of statements is that they’re recursive: Many different expressions of different nesting levels start at the same point. For example, what does this evaluate to when `p` is true, 3 or 8? ```rust #[cfg(p)] 3 #[cfg(not(p))] 4 + 5 ``` Your examples can be made to compile today by lifting the annotation to cover the containing `let` statement. The resulting duplication seems like a small price to pay to avoid the ambiguity above; do you have any examples where lifting the annotation to an entire statement is a significant burden? ---
**Author:** ![CAD97](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cad97/32/3460_2.png) [@CAD97](https://internals.rust-lang.org/u/CAD97)\ **Post date:** [November 20, 2022, 10:01am UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/3 "2022-11-20T10:01:37Z")
You don't even need to lift the cfg to the `let` — you just need to cfg blocks instead of expressions, e.g. ```rust fn main() { let x = { #[cfg(debug_assertions)] { 5 } #[cfg(not(debug_asserttions))] { 3 } }; println!("{x}"); } ``` ---
**Author:** ![kornel](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/kornel/32/2711_2.png) [@kornel](https://internals.rust-lang.org/u/kornel)\ **Post date:** [November 20, 2022, 5:18pm UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/4 "2022-11-20T17:18:17Z")
```rust let x = if cfg!(debug_assertions) { 5 } else { 3 }; ``` ---
**Author:** ![Jerrody](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/jerrody/32/9983_2.png) [@Jerrody](https://internals.rust-lang.org/u/Jerrody)\ **Post date:** [November 23, 2022, 6:34pm UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/5 "2022-11-23T18:34:56Z")
This is a runtime check. But I speak about compile-time. ---
**Author:** ![sahnehaeubchen](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/sahnehaeubchen/32/9709_2.png) [@sahnehaeubchen](https://internals.rust-lang.org/u/sahnehaeubchen)\ **Post date:** [November 23, 2022, 10:10pm UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/6 "2022-11-23T22:10:35Z")
There is no runtime check. The other branch is completly removed even without optimizations: [Compiler Explorer](https://rust.godbolt.org/z/6G397zcfK) ---
**Author:** ![CAD97](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cad97/32/3460_2.png) [@CAD97](https://internals.rust-lang.org/u/CAD97)\ **Post date:** [November 23, 2022, 11:01pm UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/7 "2022-11-23T23:01:53Z")
It's trivially removed, yes, but it still needs to typecheck, and thus can't use `#[cfg]`d functionality. `if cfg!` is typically better if both arms _do_ always compile (not the least because you always check both arms compile), but `#[cfg]` blocks are still sometimes required. ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [February 21, 2023, 11:02pm UTC](https://internals.rust-lang.org/t/make-possible-to-use-procedural-macros-with-scopes/17778/8 "2023-02-21T23:02:04Z")
Closed on February 21, 2023, 11:02pm UTC This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.