#[must_use] is missing on many methods

[0xForerunner]

#[must_use] tells the compiler that the output of a particular function must be used, and that not using it is a bug. A perfect example of where this belongs is:

    // This is a great place for #[must_use]
    pub fn saturating_sub(self, other: Self) -> Self {
        Self(self.0.saturating_sub(other.0))
    }

This rust code is currently valid and gives no warnings

let a = Uint256::one();
let mut b = Uint256::from(2u128);
b += a;
b.saturating_sub(a);
println("{}", b.to_string());

A user might wrongly assume that saturating_sub is modifying b, when it is not. (Ask me how I know haha). This fix should be pretty easy to implement!

[0xForerunner]

We wasted 8 hours searching for a bug caused by this in my code base today haha. Sad times.

[webmaster128]

Good idea. The standard library does this too:

Do you mind adding such a block to the relevant methods in a PR?

        #[must_use = "this returns the result of the operation, \
                      without modifying the original"]

[webmaster128]

Same for Timestamp::plus_seconds, ::plus_nanos, ::minus_seconds, ::minus_nanos

[webmaster128]

Done in #1678 on the 1.2 branch