Determine if a type implements a logical trait expression^?.

This library defines [impls!], a macro^? that returns a [bool] indicating whether a type implements a boolean-like expression over a set of traits^?.

rust assert!(impls!(String: Clone & !Copy & Send & Sync));

See examples for detailed use cases.

Usage

This crate is available on crates.io and can be used by adding the following to your project's [Cargo.toml]:

toml [dependencies] impls = "1"

and this to your crate root (main.rs or lib.rs):

```rust

[macro_use]

extern crate impls; ```

When using Rust 2018 edition, the following import can help if having #[macro_use] is undesirable.

rust use impls::impls;

Vocabulary

This documentation uses jargon that may be new to inexperienced Rust users. This section exists to make these terms easier to understand. Feel free to skip this section if these are already familiar to you.

Macro

In Rust, macros are functions over the abstract syntax tree (AST). They map input tokens to output tokens by performing some operation over them through a set of rules. Because of this, only their outputs are ever type-checked.

If you wish to learn about implementing macros, I recommend: - The Little Book of Rust Macros - "Macros" - The Rust Programming Language - "Macros" - The Rust Reference - "Macros By Example" - The Rust Reference

To use this crate, you do not need to know how macros are defined.

Trait

In Rust, traits are a way of defining a generalized property. They should be thought of expressing what a type is capable of doing. For example: if a type implements [Into] for some type T, then we know it can be converted into T by just calling the .into() method on it.

If you wish to learn about traits in detail, I recommend: - "Traits: Defining Shared Behavior" - The Rust Programming Language - "Traits" - The Rust Reference

Logical Trait Expression

In this crate, traits should be thought of as [bool]s where the condition is whether the given type implements the trait or not.

An expression can be formed from these trait operations:

And (&): also known as [logical disjunction], this returns true if both operands are true. This is usually defined in Rust via the [BitAnd] trait.
Or (|): also known as [logical conjunction], this returns true if either of two operands is true. This is usually defined in Rust via the [BitOr] trait.
Exclusive-or (^): also known as [exclusive disjunction], this returns true if only one of two operands is true. This is usually defined in Rust via the [BitXor] trait.
Not (!): a negation that returns false if the operand is true, or true if the operand is false. This is usually defined in Rust via the [Not] trait.

See "Precedence and Nesting" for information about the order in which these operations are performed.

Examples

This macro works in every type context. See below for use cases.

Constant Evaluation

Because types are [compile-time] constructs, the result of this macro can be used as a const value:

rust const IMPLS: bool = impls!(u8: From<u32>);

Using [static_assertions], we can fail to compile if the trait expression evaluates to false:

rust const_assert!(impls!(*const u8: Send | Sync));

Precedence and Nesting

Trait operations abide by Rust's expression precedence. To define a custom order of operations (e.g. left-to-right), simply nest the expressions with parentheses.

```rust let pre = impls!(u64: From | From ^ From & From); let ltr = impls!(u64: ((From | From) ^ From) & From);

asserteq!(pre, true | true ^ true & true); assertne!(pre, ltr); ```

Mutual Exclusion

Because exclusive-or (^) is a trait operation, we can check that a type implements one of two traits, but not both:

```rust struct T;

trait Foo {} trait Bar {}

impl Foo for T {}

assert!(impls!(T: Foo ^ Bar)); ```

Reference Types

Something that's surprising to many Rust users is that [&mut T] does not implement [Copy] nor [Clone]:

rust assert!(impls!(&mut u32: !Copy & !Clone));

Surely you're thinking now that this macro must be broken, because you've been able to reuse &mut T throughout your lifetime with Rust. This works because, in certain contexts, the compiler silently adds "re-borrows" (&mut *ref) with a shorter lifetime and shadows the original. In reality, &mut T is a move-only type.

Unsized Types

There's a variety of types in Rust that don't implement [Sized]:

```rust // Slices store their size with their pointer. assert!(impls!(str: !Sized)); assert!(impls!([u8]: !Sized));

// Trait objects store their size in a vtable. trait Foo {} assert!(impls!(dyn Foo: !Sized));

// Wrappers around unsized types are also unsized themselves. struct Bar([u8]); assert!(impls!(Bar: !Sized)); ```

Generic Types

When called from a generic function, the returned value is based on the constraints of the generic type:

```rust use std::cell::Cell;

struct Value { // ... }

impl Value { fn do_stuff() { assert!(impls!(Cell: Send)); // ... } } ```

Keep in mind that this can result in false negatives:

```rust const fn is_copy() -> bool { impls!(T: Copy) }

assertne!(iscopy::(), impls!(u32: Copy)); ```

Lifetimes

Traits with lifetimes are also supported:

```rust trait Ref<'a> {} impl<'a, T: ?Sized> Ref<'a> for &'a T {} impl<'a, T: ?Sized> Ref<'a> for &'a mut T {}

assert!(impls!(&'static str: Ref<'static>)); assert!(impls!(&'static mut [u8]: Ref<'static>)); assert!(impls!(String: !Ref<'static>)); ```

Authors

Nikolai Vazquez (GitHub: @nvzqz, Twitter: @NikolaiVazquez)

Implemented the impls! macro with support for all logical operators and without the limitations of the initial does_impl! macro by Nadrieril.
Nadrieril Feneanar (GitHub: @Nadrieril)

Implemented the initial does_impl! macro in nvzqz/static-assertions-rs#28 upon which this crate was originally based.

License

This project is released under either:

at your choosing.