coord: ranged1d: fix irrefutable if let warning

[10ne1]

I keep getting this annoying warning with rust 1.83:

warning: irrefutable if let pattern
--> plotters/src/coord/ranged1d/types/numeric.rs:29:20
|
29 | if let Ok(index) = Self::ValueType::try_from(index) {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
365 | impl_discrete_trait!(RangedCoordusize);
| -------------------------------------- in this macro invocation
|
= note: this pattern will always match, so the if let is useless
= help: consider replacing the if let with a let
= note: #[warn(irrefutable_let_patterns)] on by default
= note: this warning originates in the macro impl_discrete_trait (in Nightly builds, run with -Z macro-backtrace for more info)

This is because the conversion of usize to ValueType always passes, so we don't need the try_from().

The problem with this operation is not the conversion which always succeeds, instead it is the potential overflows, so we use checked_add() which returns None if the addition fails.

[booti386]

Thanks for your PR.
However, i will complete your analysis. The macro impl_discrete_trait that you patched is used several times at the bottom of the file, to implement DiscreteRanged for all the different RangedCoord* types.
The specific call that produces the warning that you're citing is impl_discrete_trait!(RangedCoordusize);, because we convert usize to usize, which always succeed. However it is not always true for the other calls, e.g. impl_discrete_trait!(RangedCoordu32); when usize is bigger than u32 on the target architecture.
So I suggest you modify your patch slightly (and don't bother adding me as a co-author, I don't care):

            fn from_index(&self, index: usize) -> Option<Self::ValueType> {
                Self::ValueType::try_from(index)
                    .ok()
                    .and_then(|index| self.0.checked_add(index))
            }

[10ne1]

Thanks @booti386 I've updated the PR with your suggestion.

@AaronErhardt Can we please land this? It's been waiting for some while now. The failing checks seem to be unrelated.