impl_trait_for_tuples/

lib.rs

/*!
[![](https://docs.rs/impl-trait-for-tuples/badge.svg)](https://docs.rs/impl-trait-for-tuples/) [![](https://img.shields.io/crates/v/impl-trait-for-tuples.svg)](https://crates.io/crates/impl-trait-for-tuples) [![](https://img.shields.io/crates/d/impl-trait-for-tuples.png)](https://crates.io/crates/impl-trait-for-tuples)

Attribute macro to implement a trait for tuples

* [Introduction](#introduction)
* [Syntax](#syntax)
* [Limitations](#limitations)
* [Example](#example)
* [License](#license)

## Introduction

When wanting to implement a trait for combinations of tuples, Rust requires the trait to be implemented
for each combination manually. With this crate you just need to place `#[impl_for_tuples(5)]` above
your trait declaration (in full-automatic mode) to implement the trait for the tuple combinations
`(), (T0), (T0, T1), (T0, T1, T2), (T0, T1, T2, T3), (T0, T1, T2, T3, T4, T5)`. The number of tuples is the
parameter given to the attribute and can be chosen freely.

This crate provides two modes full-automatic and semi-automatic. The full-automatic mode just requires
the trait definition to implement the trait for the tuple combinations. While being much easier to
use, it also comes with some restrictions like no associated types, no return values or no associated
consts. To support these, the semi-automatic mode is provided. This mode requires a dummy implementation
block of the trait that is expanded to all the tuple combinations implementations. To express the
tuple access in this dummy implementation a special syntax is required `for_tuples!( #( Tuple::function(); )* )`.
This would expand to `Tuple::function();` for each tuple while `Tuple` is chosen by the user and will be
replaced by the corresponding tuple identifier per iteration.

## Syntax

The attribute macro can be called with one `#[impl_for_tuples(5)]` or with two `#[impl_for_tuples(2, 5)]`
parameters. The former instructs the macro to generate up to a tuple of five elements and the later instructs it
to generate from a tuple with two element up to five elements.

### Semi-automatic syntax

```
# use impl_trait_for_tuples::impl_for_tuples;
trait Trait {
    type Ret;
    type Arg;
    type FixedType;
    const VALUE: u32;

    fn test(arg: Self::Arg) -> Self::Ret;

    fn test_with_self(&self) -> Result<(), ()>;
}

#[impl_for_tuples(1, 5)]
impl Trait for Tuple {
    // Here we expand the `Ret` and `Arg` associated types.
    for_tuples!( type Ret = ( #( Tuple::Ret ),* ); );
    for_tuples!( type Arg = ( #( Tuple::Arg ),* ); );
    for_tuples!( const VALUE: u32 = #( Tuple::VALUE )+*; );

    // Here we set the `FixedType` to `u32` and add a custom where bound that forces the same
    // `FixedType` for all tuple types.
    type FixedType = u32;
    for_tuples!( where #( Tuple: Trait<FixedType=u32> )* );

    fn test(arg: Self::Arg) -> Self::Ret {
        for_tuples!( ( #( Tuple::test(arg.Tuple) ),* ) )
    }

    fn test_with_self(&self) -> Result<(), ()> {
        for_tuples!( #( Tuple.test_with_self()?; )* );
        Ok(())
    }
}

# fn main() {}
```

The given example shows all supported combinations of `for_tuples!`. When accessing a method from the
`self` tuple instance, `self.Tuple` is the required syntax and is replaced by `self.0`, `self.1`, etc.
The placeholder tuple identifer is taken from the self type given to the implementation block. So, it
is up to the user to chose any valid identifier.

The separator given to `#( Tuple::something() )SEPARATOR*` can be chosen from `,`, `+`, `-`,
`*`, `/`, `|`, `&` or nothing for no separator.

By adding the `#[tuple_types_no_default_trait_bound]` above the impl block, the macro will not add the
automatic bound to the implemented trait for each tuple type.

The trait bound can be customized using `#[tuple_types_custom_trait_bound(NewBound)]`.
The new bound will be used instead of the impleted trait for each tuple type.

## Limitations

The macro does not supports `for_tuples!` calls in a different macro, so stuff like
`vec![ for_tuples!( bla ) ]` will generate invalid code.

## Example

### Full-automatic

```
# use impl_trait_for_tuples::impl_for_tuples;
#[impl_for_tuples(5)]
trait Notify {
    fn notify(&self);
}

# fn main() {}
```

### Semi-automatic

```
# use impl_trait_for_tuples::impl_for_tuples;
trait Notify {
    fn notify(&self) -> Result<(), ()>;
}

#[impl_for_tuples(5)]
impl Notify for TupleIdentifier {
    fn notify(&self) -> Result<(), ()> {
        for_tuples!( #( TupleIdentifier.notify()?; )* );
        Ok(())
    }
}

# fn main() {}
```

## License
Licensed under either of
 * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
 * [MIT license](http://opensource.org/licenses/MIT)
at your option.
*/

extern crate proc_macro;

use proc_macro2::{Span, TokenStream};

use syn::{
    parse::{Parse, ParseStream},
    parse_macro_input,
    punctuated::Punctuated,
    token, Attribute, Error, Ident, ItemImpl, ItemTrait, LitInt, Result,
};

mod full_automatic;
mod semi_automatic;
mod utils;

/// Enum to parse the input and to distinguish between full/semi-automatic mode.
enum FullOrSemiAutomatic {
    /// Full-automatic trait implementation for tuples uses the trait definition.
    Full(ItemTrait),
    /// Sem-automatic trait implementation for tuples uses a trait implementation.
    Semi(ItemImpl),
}

impl Parse for FullOrSemiAutomatic {
    fn parse(input: ParseStream) -> Result<Self> {
        // We need to parse any attributes first, before we know what we actually can parse.
        let fork = input.fork();
        fork.call(Attribute::parse_outer)?;

        // If there is a `unsafe` in front of, just skip it.
        if fork.peek(token::Unsafe) {
            fork.parse::<token::Unsafe>()?;
        }

        let lookahead1 = fork.lookahead1();

        if lookahead1.peek(token::Impl) {
            Ok(Self::Semi(input.parse()?))
        } else if lookahead1.peek(token::Trait) || lookahead1.peek(token::Pub) {
            Ok(Self::Full(input.parse()?))
        } else {
            Err(lookahead1.error())
        }
    }
}

/// The minimum and maximum given as two `LitInt`'s to the macro as arguments.
struct MinMax {
    min: Option<usize>,
    max: usize,
}

impl Parse for MinMax {
    fn parse(input: ParseStream) -> Result<Self> {
        let args = Punctuated::<LitInt, token::Comma>::parse_terminated(input)?;

        if args.is_empty() {
            Err(Error::new(
                Span::call_site(),
                "Expected at least one argument to the macro!",
            ))
        } else if args.len() == 1 {
            Ok(Self {
                max: args[0].base10_parse()?,
                min: None,
            })
        } else if args.len() == 2 {
            let min = args[0].base10_parse()?;
            let max = args[1].base10_parse()?;

            if min >= max {
                Err(Error::new(
                    Span::call_site(),
                    "It is expected that `min` comes before `max` and that `max > min` is true!",
                ))
            } else {
                Ok(Self {
                    min: Some(min),
                    max,
                })
            }
        } else {
            Err(Error::new(
                Span::call_site(),
                "Too many arguments given to the macro!",
            ))
        }
    }
}

/// See [crate](index.html) documentation.
#[proc_macro_attribute]
pub fn impl_for_tuples(
    args: proc_macro::TokenStream,
    input: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let input = parse_macro_input!(input as FullOrSemiAutomatic);
    let min_max = parse_macro_input!(args as MinMax);

    impl_for_tuples_impl(input, min_max)
        .unwrap_or_else(|e| e.to_compile_error())
        .into()
}

fn impl_for_tuples_impl(input: FullOrSemiAutomatic, min_max: MinMax) -> Result<TokenStream> {
    let tuple_elements = (0usize..min_max.max)
        .map(|i| generate_tuple_element_ident(i))
        .collect::<Vec<_>>();

    match input {
        FullOrSemiAutomatic::Full(definition) => {
            full_automatic::full_automatic_impl(definition, tuple_elements, min_max.min)
        }
        FullOrSemiAutomatic::Semi(trait_impl) => {
            semi_automatic::semi_automatic_impl(trait_impl, tuple_elements, min_max.min)
        }
    }
}

fn generate_tuple_element_ident(num: usize) -> Ident {
    Ident::new(&format!("TupleElement{}", num), Span::call_site())
}