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#![deny(
    missing_docs,
    missing_debug_implementations,
    missing_copy_implementations,
    trivial_casts,
    trivial_numeric_casts,
    unsafe_code,
    unstable_features,
    unused_import_braces,
    unused_qualifications
)]

//! Tailcall is a library that adds safe, zero-cost [tail recursion] to stable Rust.
//! Eventually, it will be superseded by the [`become` keyword].
//!
//! # Usage
//!
//! To guarantee that recursive calls a function will reuse the same stack frame,
//! annotate it with the [`tailcall`] attribute.
//!
//! ```
//! use tailcall::tailcall;
//!
//! fn factorial(input: u64) -> u64 {
//!     #[tailcall]
//!     fn factorial_inner(accumulator: u64, input: u64) -> u64 {
//!         if input > 0 {
//!             factorial_inner(accumulator * input, input - 1)
//!         } else {
//!             accumulator
//!         }
//!     }
//!
//!     factorial_inner(1, input)
//! }
//! ```
//!
//! Recursive calls which are not in tail form will result in a compile-time error.
//!
//! ```compile_fail
//! use tailcall::tailcall;
//!   
//! #[tailcall]
//! fn factorial(input: u64) -> u64 {
//!     if input > 0 {
//!         input * factorial(input - 1)
//!     } else {
//!         1
//!     }
//! }
//! ```
//!
//! [tail recursion]: https://en.wikipedia.org/wiki/Tail_call
//! [`become` keyword]: https://internals.rust-lang.org/t/pre-rfc-explicit-proper-tail-calls/3797/16
//! [`tailcall`]: attr.tailcall.html

extern crate proc_macro;

mod helpers;
mod transforms;

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, ItemFn};

/// Transforms a [function definition] so that all recursive calls within the body are
/// guaranteed to use a single stack frame (via [tail recursion]).
///
/// [function definition]: https://docs.rs/syn/1.0.9/syn/struct.ItemFn.html
/// [tail recursion]: https://en.wikipedia.org/wiki/Tail_call
///
/// # Example
///
/// ```
/// use tailcall::tailcall;
///
/// fn factorial(input: u64) -> u64 {
///     #[tailcall]
///     fn factorial_inner(accumulator: u64, input: u64) -> u64 {
///         if input > 0 {
///             factorial_inner(accumulator * input, input - 1)
///         } else {
///             accumulator
///         }
///     }
///
///     factorial_inner(1, input)
/// }
/// ```
///
/// # Requirements
///
/// - All recursive calls must be in [tail form]:
///
/// ```compile_fail
/// use tailcall::tailcall;
///   
/// #[tailcall]
/// fn factorial(input: u64) -> u64 {
///     if input > 0 {
///         input * factorial(input - 1)
/// //      ^^^^^^^ This is not allowed.
///     } else {
///         1
///     }
/// }
/// ```
///
/// - Methods (functions which bind `self` in the arguments list) are not supported:
///
/// ```compile_fail
/// trait Factorialable {
///     fn factorial(self) -> Self {
///         self.calc_factorial(1)
///     }
///
///     fn calc_factorial(self, accumulator: u64) -> u64;
/// }
///
/// impl Factorialable for u64 {
///     #[tailcall]
///     fn calc_factorial(self, accumulator: u64) -> u64 {
/// //                    ^^^^ This is not allowed.
///         if self > 0 {
///             (self - 1).calc_factorial(self * accumulator)
///         } else {
///             accumulator
///         }
///     }
/// }
/// ```
///
/// - No identifier with the name `___tailcall___` may appear anywhere within the body:
///
/// ```compile_fail
/// use tailcall::tailcall;
///
/// fn factorial(input: u64) -> u64 {
///     #[tailcall]
///     fn factorial_inner(accumulator: u64, input: u64) -> u64 {
///         mod ___tailcall___ {
/// //          ^^^^^^^^^^^^^^ This is not allowed.
///             pub fn one() -> u64 {
///                 1
///             }
///         }
///
///         if input > 0 {
///             factorial_inner(accumulator * input, input - ___tailcall___::one())
///         } else {
///             accumulator
///         }
///     }
///
///     factorial_inner(1, input)
/// }
/// ```
///
/// [tail form]: https://en.wikipedia.org/wiki/Tail_call
#[proc_macro_attribute]
pub fn tailcall(_attr: TokenStream, tokens: TokenStream) -> TokenStream {
    let input = parse_macro_input!(tokens as ItemFn);
    let output = transforms::apply_fn_tailcall_transform(input);

    TokenStream::from(quote! {
        #output
    })
}