13. Macros — Code That Writes Code 🟡

// A simple macro that creates a HashMap
macro_rules! hashmap {
    // Match: key => value pairs separated by commas
    ( $( $key:expr => $value:expr ),* $(,)? ) => {
        {
            let mut map = std::collections::HashMap::new();
            $( map.insert($key, $value); )*
            map
        }
    };
}

let scores = hashmap! {
    "Alice" => 95,
    "Bob" => 87,
    "Carol" => 92,
};
// Expands to:
// let mut map = HashMap::new();
// map.insert("Alice", 95);
// map.insert("Bob", 87);
// map.insert("Carol", 92);
// map

// Generate test functions automatically
macro_rules! test_cases {
    ( $( $name:ident: $input:expr => $expected:expr ),* $(,)? ) => {
        $(
            #[test]
            fn $name() {
                assert_eq!(process($input), $expected);
            }
        )*
    };
}

test_cases! {
    test_empty: "" => "",
    test_hello: "hello" => "HELLO",
    test_trim: "  spaces  " => "SPACES",
}
// Generates three separate #[test] functions

// ❌ Unnecessary macro — a function works fine:
macro_rules! double {
    ($x:expr) => { $x * 2 };
}

// ✅ Just use a function:
fn double(x: i32) -> i32 { x * 2 }

// ✅ Good macro use — variadic, can't be a function:
macro_rules! println {
    ($($arg:tt)*) => { /* format string + args */ };
}

// Three types of proc macros:

// 1. Derive macros — #[derive(MyTrait)]
// Generate trait implementations from struct definitions
#[derive(Debug, Clone, Serialize, Deserialize)]
struct Config {
    name: String,
    port: u16,
}

// 2. Attribute macros — #[my_attribute]
// Transform the annotated item
#[route(GET, "/api/users")]
async fn list_users() -> Json<Vec<User>> { /* ... */ }

// 3. Function-like macros — my_macro!(...)
// Custom syntax
let query = sql!(SELECT * FROM users WHERE id = ?);

// Input (your struct):
#[derive(Debug)]
struct Point {
    x: f64,
    y: f64,
}

// The derive macro generates:
impl std::fmt::Debug for Point {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Point")
            .field("x", &self.x)
            .field("y", &self.y)
            .finish()
    }
}

macro_rules! make_var {
    () => {
        let x = 42; // This 'x' is in the MACRO's scope
    };
}

fn main() {
    let x = 10;
    make_var!();   // Creates a different 'x' (hygienic)
    println!("{x}"); // Prints 10, not 42 — macro's x doesn't leak
}

// In my_diagnostics crate:

pub fn log_result(msg: &str) {
    println!("[diag] {msg}");
}

#[macro_export]
macro_rules! diag_log {
    ($($arg:tt)*) => {
        // ✅ $crate always resolves to my_diagnostics, even if the user
        // renamed the crate in their Cargo.toml
        $crate::log_result(&format!($($arg)*))
    };
}

// ❌ Without $crate:
// my_diagnostics::log_result(...)  ← breaks if user writes:
//   [dependencies]
//   diag = { package = "my_diagnostics", version = "1" }

// Count the number of expressions passed to the macro
macro_rules! count {
    // Base case: no tokens left
    () => { 0usize };
    // Recursive case: consume one expression, count the rest
    ($head:expr $(, $tail:expr)* $(,)?) => {
        1usize + count!($($tail),*)
    };
}

fn main() {
    let n = count!("a", "b", "c", "d");
    assert_eq!(n, 4);

    // Works at compile time too:
    const N: usize = count!(1, 2, 3);
    assert_eq!(N, 3);
}

// Build a heterogeneous tuple from a list of expressions:
macro_rules! tuple_from {
    // Base: single element
    ($single:expr $(,)?) => { ($single,) };
    // Recursive: first element + rest
    ($head:expr, $($tail:expr),+ $(,)?) => {
        ($head, tuple_from!($($tail),+))
    };
}

let t = tuple_from!(1, "hello", 3.14, true);
// Expands to: (1, ("hello", (3.14, (true,))))

# my_derive/Cargo.toml
[lib]
proc-macro = true

[dependencies]
syn = { version = "2", features = ["full"] }
quote = "1"
proc-macro2 = "1"

// my_derive/src/lib.rs
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};

/// Derive macro that generates a `describe()` method
/// returning the struct name and field names.
#[proc_macro_derive(Describe)]
pub fn derive_describe(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let name = &input.ident;
    let name_str = name.to_string();

    // Extract field names (only for structs with named fields)
    let fields = match &input.data {
        syn::Data::Struct(data) => {
            data.fields.iter()
                .filter_map(|f| f.ident.as_ref())
                .map(|id| id.to_string())
                .collect::<Vec<_>>()
        }
        _ => vec![],
    };

    let field_list = fields.join(", ");

    let expanded = quote! {
        impl #name {
            pub fn describe() -> String {
                format!("{} {{ {} }}", #name_str, #field_list)
            }
        }
    };

    TokenStream::from(expanded)
}

// In the application crate:
use my_derive::Describe;

#[derive(Describe)]
struct SensorReading {
    sensor_id: u16,
    value: f64,
    timestamp: u64,
}

fn main() {
    println!("{}", SensorReading::describe());
    // "SensorReading { sensor_id, value, timestamp }"
}

let m = map! {
    "host" => "localhost",
    "port" => "8080",
};
assert_eq!(m.get("host"), Some(&"localhost"));