disuse

Description

The way to notify the implementation which return value is disuse.

Usage

When an algorithm can calculate multiple useful values simultaneously, you will write like the following code. ```rust pub fn some_algorithm(x: f64) -> (f64, f64, f64) { let mut r1 = 0f64; let mut r2 = 0f64; let mut r3 = 0f64;

for _ in 0..1000 { // ... some eavy calculations here ...
    r1 += x;
    r2 += x * 2.0;
    r3 += x * 3.0;
}
(r1, r2, r3)

} But your users are not always need all return values. rust

pub fn some_algorithm(x: f64) -> (f64, f64, f64) {

let mut r1 = 0f64; let mut r2 = 0f64; let mut r3 = 0f64;

#

for _ in 0..1000 { // ... heavy calculations here ...

r1 += x;

r2 += x * 2.0;

r3 += x * 3.0;

}

(r1, r2, r3)

}

// Yes, we can ignore the return values, but calculations done. let (a, , _) = somealgorithm(4.0); Above example code can be rewrite using [Disuse] as follows, rust use disuse::Disuse;

pub fn some_algorithm(x: f64) -> (R1, R2, R3) where R1: From<(f64,)>, R2: From<(f64,)>, R3: From<(f64,)> { let mut r1 = 0f64; let mut r2 = 0f64; let mut r3 = 0f64;

for _ in 0..1000 { // ... heavy calculations here ... r1 += x; r2 += x * 2.0; // When user call this function like below, r3 += x * 3.0; // we can expect the compiler eliminate this two line, right? } ((r1,).into(), (r2,).into(), (r3,).into()) }

let ((a,), , _): ((f64,), Disuse, Disuse) = somealgorithm(4.0); ``` Oops, just a little ugly...(a little?)

If the unit type (()) implements [From] trait like as follows, rust impl<T> From<T> for () { fn from(_: T) -> () { () } } above example code can be write more smart. ```rust pub fn some_algorithm(x: f64) -> (R1, R2, R3) where R1: From, R2: From, R3: From { let mut r1 = 0f64; let mut r2 = 0f64; let mut r3 = 0f64;

for _ in 0..1000 { // ... heavy calculations here ... r1 += x; r2 += x * 2.0; // When user call this function like below, r3 += x * 3.0; // we can expect the compiler eliminate this two line, right? } (r1.into(), r2.into(), r3.into()) }

let (a, , _): (f64, (), ()) = somealgorithm(4.0); ``` It's just unrealizable dream...