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// Copyright 2020 Louis Garczynski // // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or // http://opensource.org/licenses/MIT>, at your option. This file may not be // copied, modified, or distributed except according to those terms. #![no_std] /*! A macro to generate numerical constants in multiple types at once. You can have the number Pi be available in f64 or f32 It was designed with three goals in mind: * Catch all overflow errors on compile-time * Minimize the code footprint * Be readable and easy to use This is meant as a temporary fix to the year-long debate over [Rust Pre-RFC #1337](https://internals.rust-lang.org/t/pre-rfc-untyped-constants/1337/) # Syntax ```rust use polymorphic_constant::polymorphic_constant; polymorphic_constant! { const PI: f32 | f64 = 3.141592653589793; } // Which can then be used as fn pi_squared () -> f64 { PI.f64 * PI.f64 } ``` A few features are supported: ```rust use polymorphic_constant::polymorphic_constant; polymorphic_constant! { /// Doc comment attributes const PI: f32 | f64 = 3.141592653589793; // Visibility modifiers (for both constant and type) pub (crate) const E: f32 | f64 = 2.7182818284590452; // Nonzero numeric types (NonZeroI32, NonZeroU8, etc) const ASCII_LINE_RETURN: u8 | nz_u8 = 10; } // You can handle constants like any const struct const PI_COPY: PI = PI; const PI_F32: f32 = PI.f32; // Into is implemented for every variant of the constant fn times_pi<T: std::ops::Mul<T>> (value: T) -> <T as std::ops::Mul>::Output where PI: Into<T>, { value * PI.into() } assert_eq!(times_pi(2.0), 6.283185307179586f64); ``` # Safety This system ensures that you keep all the safeties and warnings given by rust, but no more Any incompatible type will prevent compilation: * Float literals cannot be stored if it would convert them to infinity ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAILS: f32 | f64 = 3141592653589793238462643383279502884197.0; # } ``` * Literals cannot be stored in a type too small to hold them ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAILS: u64 | nz_i8 = 128; # } ``` * Negative numbers cannot be stored in unsigned types ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAILS: i64 | u8 = -1; # } ``` * 0 cannot be stored in non-zero types ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAILS: nz_u8 | nz_u16 | nz_u32 = 0; # } ``` * However, floats may lose precision, and a lot of it ```rust # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const SUCCEEDS: f32 = 3.141592653589793238462643383279; # } ``` # Warnings Currently, the same constant cannot hold both int and float variants ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAIL: i32 = 0.1; # } ``` ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAIL: f32 = 0; # } ``` The constant also has to be initialized with an untyped literal ```compile_fail # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const FAIL: i32 = 0u32; # } ``` It is still unclear if accepting the examples above could be dangerous, thus the conservative choice. # Example ``` use polymorphic_constant::polymorphic_constant; polymorphic_constant! { const HEIGHT: i8 | u8 | i16 | u16 | i32 | u32 = 16; const WIDTH: i8 | u8 | i16 | u16 | i32 | u32 = 32; } fn main() { let size = HEIGHT.i16 * WIDTH.i16; assert_eq!(size, 16 * 32); let height_copy:i32 = HEIGHT.into(); assert_eq!(HEIGHT.i32, height_copy); } ``` # Support I would love any feedback on usage, for future ameliorations and features. */ /** Define one or more polymorphic numerical constants. A constant X of value 10, available in i32 and u32 will read: ``` # use polymorphic_constant::polymorphic_constant; polymorphic_constant! { const X: i32 | u32 = 10; } ``` and be used like: ``` # use polymorphic_constant::polymorphic_constant; # polymorphic_constant! { const X: i32 | u32 = 10; } let x_i32 = X.i32; ``` */ #[macro_export(local_inner_macros)] macro_rules! polymorphic_constant { // Handle the const (pub?) CONST format ($(#[$attr:meta])* ($($vis:tt)*) const $name:ident : $( $numeric_type:ident )|* = $lit:literal; $($nextLine:tt)*) => { // Generate the struct to hold the constant // Remove warnings #[allow(non_camel_case_types)] // Derive the common traits, but only if std is available #[cfg_attr(not(no_std), derive(Debug, Clone, Copy))] // Expend the attributes passed by the user $(#[$attr])* // Add the visibility attributes $($vis)* // Create the struct struct $name { // For each type (f32, ...) create a new property $($numeric_type: __nz_impl!(@GET_TYPE $numeric_type),)* } // Implement `into` for every type $(impl ::core::convert::Into<__nz_impl!(@GET_TYPE $numeric_type)> for $name { fn into(self) -> __nz_impl!(@GET_TYPE $numeric_type) { self.$numeric_type } })* // Expand the visibility, this time for the constant $($vis)* // Instantiate the struct and create the constant const $name: $name = $name { $($numeric_type: __nz_impl!(@MAKE_VAL $lit, $numeric_type ),)* }; // Keep munching until the next ; polymorphic_constant!($($nextLine)*); }; // Handle `const CONST` format ($(#[$attr:meta])* const $($t:tt)*) => { // use `()` to explicitly forward the information about private items polymorphic_constant!($(#[$attr])* () const $($t)*); }; // Handle `pub const CONST` format ($(#[$attr:meta])* pub const $($t:tt)*) => { polymorphic_constant!($(#[$attr])* (pub) const $($t)*); }; // Handle `pub (crate) CONST` format and similar ($(#[$attr:meta])* pub ($($vis:tt)+) const $($t:tt)*) => { polymorphic_constant!($(#[$attr])* (pub ($($vis)+)) const $($t)*); }; () => {}; } #[macro_export] #[doc(hidden)] macro_rules! __nz_impl { // constally obtain a nonzero struct // Surprisingly fails to compile if $lit is 0 or not in range (@MAKE_VAL $lit:literal, nz_i8 ) => { unsafe { ::std::num::NonZeroI8::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_i16 ) => { unsafe { ::std::num::NonZeroI16::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_i32 ) => { unsafe { ::std::num::NonZeroI32::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_i64 ) => { unsafe { ::std::num::NonZeroI64::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_i128 ) => { unsafe { ::std::num::NonZeroI128::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_isize) => { unsafe { ::std::num::NonZeroIsize::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_u8 ) => { unsafe { ::std::num::NonZeroU8::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_u16 ) => { unsafe { ::std::num::NonZeroU16::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_u32 ) => { unsafe { ::std::num::NonZeroU32::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_u64 ) => { unsafe { ::std::num::NonZeroU64::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_u128 ) => { unsafe { ::std::num::NonZeroU128::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, nz_usize) => { unsafe { ::std::num::NonZeroUsize::new_unchecked($lit) } }; (@MAKE_VAL $lit:literal, $numeric_type:ident) => { $lit }; // Get the full nonzero type from shorthand // Fails in nonstd (@GET_TYPE nz_i8 ) => { ::std::num::NonZeroI8 }; (@GET_TYPE nz_i16 ) => { ::std::num::NonZeroI16 }; (@GET_TYPE nz_i32 ) => { ::std::num::NonZeroI32 }; (@GET_TYPE nz_i64 ) => { ::std::num::NonZeroI64 }; (@GET_TYPE nz_i128 ) => { ::std::num::NonZeroI128 }; (@GET_TYPE nz_isize) => { ::std::num::NonZeroIsize }; (@GET_TYPE nz_u8 ) => { ::std::num::NonZeroU8 }; (@GET_TYPE nz_u16 ) => { ::std::num::NonZeroU16 }; (@GET_TYPE nz_u32 ) => { ::std::num::NonZeroU32 }; (@GET_TYPE nz_u64 ) => { ::std::num::NonZeroU64 }; (@GET_TYPE nz_u128 ) => { ::std::num::NonZeroU128 }; (@GET_TYPE nz_usize) => { ::std::num::NonZeroUsize }; (@GET_TYPE $numeric_type:ident) => { $numeric_type }; }