Oldest valid date initialised only once!

**NOTE**: This uses `LazyLock` which was stabilised recently in Rust 1.80.
This means we're effectively pushing the MSRV (minimum supported Rust
version) to 1.80+ which is relatively fresh ([July 2024]).

I'm not too keen on [the alternatives]:
1. No nothing: Create a new datetime every time the `validate_date()` is
   called
   - possibly slow/unnecesary, although maybe not a tragedy
2. Use a dependency just for this (either `lazy_static` or `once_cell`)
  - just seems wrong to use a crate when this is now in the standard library

I think `LazyLock` is just too good to pass on and we should keep it simple
and just use it. From local testing we require 1.74+ anyway so we're "just"
pushing the MSRV 6 Rust releases ahead. And update Rust with `rustup update`
is trivial.

[July 2024]: https://blog.rust-lang.org/2024/07/25/Rust-1.80.0.html
[the alternatives]: rust-lang-nursery/lazy-static.rs#214

src/lib.rs

@@ -1,8 +1,10 @@
 //! API for retrieving data from the Carbon Intensity API
 //! <https://api.carbonintensity.org.uk/>
 
-use chrono::{Datelike, Duration, Local, NaiveDate, NaiveDateTime, NaiveTime};
 use futures::future;
+use std::sync::LazyLock;
+
+use chrono::{Datelike, Duration, Local, NaiveDate, NaiveDateTime, NaiveTime};
 use reqwest::{Client, StatusCode};
 use serde::{de::DeserializeOwned, Deserialize, Serialize};
 use thiserror::Error;
@@ -13,6 +15,14 @@ mod target;
 pub use region::Region;
 pub use target::Target;
 
+// oldest entry available for 2018-05-10 23:30:00
+static OLDEST_VALID_DATE: LazyLock<NaiveDateTime> = LazyLock::new(|| {
+    NaiveDate::from_ymd_opt(2018, 5, 10)
+        .unwrap()
+        .and_hms_opt(23, 30, 0)
+        .unwrap()
+});
+
 /// An error communicating with the Carbon Intensity API.
 #[derive(Debug, Error)]
 pub enum ApiError {
@@ -231,14 +241,8 @@ fn to_tuples(data: Vec<Data>) -> Result<Vec<IntensityForDate>> {
 ///
 /// Otherwise an `Ok` value with the input datetime is returned
 fn validate_date(date: NaiveDateTime) -> Result<NaiveDateTime> {
-    // oldest entry available for 2018-05-10 23:30:00
-    let oldest_date = NaiveDate::from_ymd_opt(2018, 5, 10)
-        .unwrap()
-        .and_hms_opt(23, 30, 0)
-        .unwrap();
-
     // check if date is too old
-    if date < oldest_date {
+    if date < *OLDEST_VALID_DATE {
         return Err(ApiError::DateTooOld);
     }