### Install constime Rust Crate

Source: https://github.com/codebycruz/constime/blob/master/README.md

Add constime to a Rust project using cargo. The crate has zero dependencies and works with dependencies defined in either [dependencies] or [build-dependencies] sections of Cargo.toml.

```bash
cargo add constime
```

--------------------------------

### External Dependencies and Network Requests with comptime!

Source: https://context7.com/codebycruz/constime/llms.txt

Execute code from external crates at compile time by declaring them with extern crate within the comptime! macro. Dependencies can be listed in [dependencies] or [build-dependencies] in Cargo.toml. This enables making HTTP requests, fetching remote data, and using third-party libraries during compilation without runtime overhead.

```rust
use constime::comptime;

fn main() {
    // Make HTTP requests at compile time
    let fact = comptime! {
        extern crate ureq;

        ureq::get("http://numbersapi.com/5/math")
            .call()
            .unwrap()
            .into_string()
            .unwrap()
    };
    println!("Fact: {}", fact);

    // Fetch data from remote sources during compilation
    let content = comptime! {
        extern crate ureq;

        ureq::get("https://example.com/config.txt")
            .call()
            .unwrap()
            .into_string()
            .unwrap()
    };
    println!("Config: {}", content);
}
```

--------------------------------

### Compile-Time Code Execution with comptime! Macro

Source: https://github.com/codebycruz/constime/blob/master/README.md

Use the comptime! macro to execute Rust code at compile-time, including external crate dependencies and standard library calls. The macro evaluates expressions during compilation and embeds the results in the final binary. Temporary binaries are created in the system temp directory for each invocation.

```rust
fn main() {
	use constime::comptime;

	// Let's use a pure-build time dependency
	println!("Here's a fact about the number 5: {}", comptime! {
		extern crate ureq;

		ureq::get("http://numbersapi.com/5/math")
			.call()
			.unwrap()
			.into_string()
			.unwrap()
	});

	// Standard library works fine too.
	println!(
		"Compiled {} seconds after unix epoch.",
		comptime! {
			std::time::SystemTime::now()
				.duration_since(std::time::UNIX_EPOCH)
				.expect("Time went backwards")
				.as_secs()
		}
	);
}
```

--------------------------------

### Basic Arithmetic and String Operations with comptime! Macro

Source: https://context7.com/codebycruz/constime/llms.txt

The comptime! macro evaluates Rust expressions at compile time and replaces them with their computed values. It can perform mathematical calculations, format strings, access environment variables, and use standard library functions during compilation. The macro spawns a rustc subprocess to compile and execute the provided code, capturing output as tokens.

```rust
use constime::comptime;

fn main() {
    // Compute mathematical expressions at compile time
    let result = comptime! { 239 + 259 * 23 };
    println!("Result: {}", result); // Result: 6196

    // Use standard library functions
    let timestamp = comptime! {
        std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .expect("Time went backwards")
            .as_secs()
    };
    println!("Compiled at: {} seconds after unix epoch", timestamp);

    // Format strings at compile time
    let formatted = comptime! { format!("abc{}", 218) };
    println!("{}", formatted); // abc218

    // Access environment variables during compilation
    let pkg_name: Result<String, std::env::VarError> = comptime! {
        std::env::var("CARGO_PKG_NAME")
    };
    println!("Package: {:?}", pkg_name);
}
```

--------------------------------

### Constant Declarations with comptime! Macro

Source: https://context7.com/codebycruz/constime/llms.txt

Use comptime! to initialize constants with complex expressions that aren't supported by regular const fn. The macro evaluates expressions at compile time and injects results as literal values into the binary. This enables creating compile-time constants from String types, formatted output, and complex computations without runtime evaluation.

```rust
use constime::comptime;

// Initialize constants with compile-time evaluated expressions
const FORMATTED: &str = comptime! { format!("abc{}", 218) };
const CALCULATION: i32 = comptime! { 2 + 259 * 7 };
const STRING_VALUE: String = comptime! { String::from("test") };

fn main() {
    println!("Constant string: {}", FORMATTED); // abc218
    println!("Constant calculation: {}", CALCULATION); // 1815
    println!("Constant string value: {}", STRING_VALUE); // test

    // These are true compile-time constants, not runtime computations
    assert_eq!(FORMATTED, "abc218");
    assert_eq!(CALCULATION, 1815);
    assert_eq!(STRING_VALUE, "test");
}
```

--------------------------------

### Error Handling and Result Types in comptime!

Source: https://context7.com/codebycruz/constime/llms.txt

The comptime! macro provides compile-time error feedback when code fails to compile or execute. Errors are reported as compile errors with full stack traces, and rust-analyzer provides autocomplete and type checking within the macro. Handle Results and panics gracefully within compile-time code blocks.

```rust
use constime::comptime;

fn main() {
    // Successful execution with Result handling
    let result: Result<&str, std::env::VarError> = comptime! {
        std::env::var("CARGO_PKG_NAME")
    };

    match result {
        Ok(name) => println!("Package name: {}", name),
        Err(_) => println!("Variable not found"),
    }

    // Handle panics and errors gracefully
    let safe_operation = comptime! {
        let value = 42;
        if value > 0 {
            format!("Positive: {}", value)
        } else {
            panic!("Negative value not allowed")
        }
    };
    println!("{}", safe_operation); // Positive: 42
}
```

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