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/*! Directed Type Conversion This module provides sibling traits to the `std::convert` module. The standard library puts the type parameter in the trait declaration, which makes those traits generic and suitable for constraint clauses and function calls, but not usable in indeterminate method-call positions. These traits put the type parameter in the function declaration, making the trait non-generic and allowing the function to be called in method-call position without ambiguity. !*/ use core::convert::TryInto; /** Directed Type Conversion This trait is an accessory to [`From`] and [`Into`]. It works by moving the destination type from the trait name (`Into<Target>::into`) into the method name (`Conv::conv::<Target>`). This change makes `Into<_>` the correct trait to use in trait bounds and `.conv::<_>` the correct method to use in expressions. A `conv::<T>` method is automatically available whenever an `Into<T>` implementation exists for a type. `Into<T>` is most commonly implemented by taking advantage of the reflexive blanket implentation using `From`, but can also be manually implemented as desired. `.into()` cannot be used in intermediate expressions, because it is impossible for the compiler’s type engine to select a unique `Into<T>` implementation. This means that expressions like `v.into().use()` will never compile. Users can replace `.into()` with `.conv::<Dest>()` in order to inform the compiler of the type of the expression after the conversion, and make compilation succeed. `Conv` cannot be used in trait bounds, because the trait itself is not generic. All `Sized` types implement `Conv` by default, so specifying that a type must be `Conv` adds no information to the solver. # Examples ## Conversion as methods Conversion with `.into()` will fail to compile, even with the type annotation: ```rust,compile_fail let s: String = "static".into().clone(); // ^^^^^^^^^^^^^^^ cannot infer type for `T` // note: type must be known at this point ``` while the equivalent code with `.conv::<_>` does compile: ```rust # use wyz::conv::Conv; let s = "static".conv::<String>().clone(); ``` ## Conversion as traits Bounding a type with `Conv` will not compile, because the trait itself gives no information: ```rust,compile_fail # use wyz::conv::Conv; fn lift<T: Conv>(src: T) -> String { src.conv::<String>().clone() // ^^^^ the trait `From<T>` is not implemented for `String` // help: consider adding a `where String: From<T>` bound // note: required because of the requirements on the impl of `Into<String>` for `T` } ``` This can be fixed by adding the clause `where String: From<T>`, or by using the bound `Into`: ```rust # use wyz::conv::Conv; fn lift<T: Into<String>>(src: T) -> String { src.conv::<String>().clone() } ``` The `Into<T>` trait bound makes available both the `Into::<T>::into` method and the `Conv::conv::<T>` method. [`From`]: https://doc.rust-lang.org/std/convert/trait.From.html [`Into`]: https://doc.rust-lang.org/std/convert/trait.Into.html **/ pub trait Conv: Sized { /// Converts `self` into a target type. /// /// This method runs `<Self as Into<T>>::into` on `self` to produce the /// desired output. The only difference between using `Conv::conv` and /// `Into::into` is where the target type is placed in the name; `.conv()` /// can be used in intermediate positions of an expression, while `.into()` /// cannot. /// /// # Examples /// /// ```rust /// use wyz::conv::Conv; /// /// let t = "hello".conv::<String>(); /// ``` fn conv<T: Sized>(self) -> T where Self: Into<T> { <Self as Into<T>>::into(self) } } impl<T: Sized> Conv for T { } /** Directed Fallible Type Conversion This trait is an accessory to [`TryFrom`] and [`TryInto`]. It works by moving the destination type from the trait name (`TryInto<Target>::try_into`) into the method name (`TryConv::try_conv::<Target>`). This change makes `TryInto<_>` the correct trait to use in trait bounds and `.try_conv::<_>` the correct method to use in expressions. A `try_conv::<T>` method is automatically available whenever a `TryInto<T>` implementation exists for a type. `TryInto<T>` is most commonly implemented by taking advantage of the reflexive blanket implentation using `TryFrom`, but can also be manually implemented as desired. `.try_into()` cannot be used in intermediate expressions, because it is impossible for the compiler’s type engine to select a unique `TryInto<T>` implementation. This means that expressions like `v.try_into().use()` will never compile. Users can replace `.try_into()` with `.try_conv::<Dest>()` in order to inform the compiler of the type of the expression after the conversion, and make compilation succeed. `TryConv` cannot be used in trait bounds, because the trait itself is not generic. All `Sized` types implement `TryConv` by default, so specifying that a type must be `TryConv` adds no information to the solver. # Examples ## Conversion as methods Conversion with `.try_into()` will fail to compile, even with the type annotation: ```rust,ignore let s: String = "static".try_into().unwrap().clone(); // ^^^^^^^^^^^^^^^^^^^ cannot infer type for `T` // note: type must be known at this point ``` while the equivalent code with `.try_conv::<_>` does compile: ```rust # use wyz::conv::TryConv; let s = "static".try_conv::<String>().unwrap().clone(); ``` ## Conversion as traits Bounding a type with `TryConv` will not compile, because the trait itself gives no information: ```rust,ignore # use wyz::conv::TryConv; fn lift<T: TryConv>(src: T) -> String { src.try_conv::<String>().clone() // ^^^^^^^^ the trait `From<T>` is not implemented for `String` // help: consider adding a `where String: From<T>` bound // note: required because of the requirements on the impl of `Into<String>` for `T` // note: required because of the requirements on the impl of `TryFrom<T>` for `String` } ``` This can be fixed by adding the clause `where String: TryFrom<T>`, or by using the bound `TryInto`: ```rust # use std::convert::TryInto; # use wyz::conv::TryConv; fn lift<T: TryInto<String>>(src: T) -> String { src.try_conv::<String>().ok().unwrap().clone() } ``` The `TryInto<T>` trait bound makes available both the `TryInto::<T>::try_into` method and the `TryConv::try_conv::<T>` method. [`TryFrom`]: https://doc.rust-lang.org/std/convert/trait.TryFrom.html [`TryInto`]: https://doc.rust-lang.org/std/convert/trait.TryInto.html **/ pub trait TryConv: Sized { /// Attempts to convert `self` into a target type. /// /// This method runs `<Self as TryInto<T>>::try_into` on `self` to produce /// the desired output. The only difference between using /// `TryConv::try_conv` and `TryInto::try_into` is where the target type is /// placed in the name; `.try_conv()` can be used in intermediate positions /// of an expression, while `.try_into()` cannot. /// /// # Examples /// /// ```rust /// use wyz::conv::TryConv; /// /// let t = "hello".try_conv::<String>().unwrap(); /// ``` fn try_conv<T: Sized>(self) -> Result<T, Self::Error> where Self: TryInto<T> { <Self as TryInto<T>>::try_into(self) } } impl<T: Sized> TryConv for T { }