Rust Cfg Attribute

// Outer attribute: applies only to the item immediately following it
#[cfg(predicate)]
struct ConditionalStruct;

// Inner attribute: applies to the entire enclosing scope (e.g., top of a file)
#![cfg(predicate)]

2. **Name-Value Pairs:** Evaluates to true if the specific configuration key matches the provided string value. These are typically populated by the compiler based on the target triple.
   ```rust
#[cfg(target_os = "linux")]
#[cfg(target_arch = "x86_64")]
#[cfg(target_env = "msvc")]

## Logical Operators
Predicates can be combined or inverted using built-in logical operators. These operators can be nested arbitrarily deep.
* **`all(...)`**: Evaluates to true if *every* comma-separated predicate within the parentheses evaluates to true.
  ```rust
#[cfg(all(target_os = "windows", target_arch = "x86_64"))]
fn windows_x64_only() {}

* **`not(...)`**: Evaluates to true if the single predicate within the parentheses evaluates to false.
  ```rust
#[cfg(not(test))]
fn exclude_from_tests() {}

// If the "serialization" feature is enabled, this expands to:
// #[derive(Serialize, Deserialize)]
// struct MyData;
#[cfg_attr(feature = "serialization", derive(Serialize, Deserialize))]
struct MyData;

# Passing a custom flag via rustc
rustc --cfg custom_flag main.rs

// Evaluates to true based on the compiler flag above
#[cfg(custom_flag)]
fn custom_implementation() {}

fn check_os() {
    let is_windows = cfg!(target_os = "windows");
    
    if is_windows {
        // This block is still parsed and type-checked on non-Windows platforms,
        // but the compiler will optimize the branch away.
    }
}