refactor(core): use #![feature(const_result_drop)]

This unstable library feature, which covers the `const fn` version of
`Result::{ok, error, and, or, unwrap_or}`, was added by
[rust-lang/rust#92385][1].

[1]: rust-lang/rust#92385

Author: yvt

doc/toolchain_limitations.md

@@ -436,20 +436,6 @@ const _: () = Ok::<(), ()>(()).expect("");
 ```
 
 
-### `[tag:const_result_ok]` `Result::ok` is not `const fn`
-
-*Upstream PR:* [rust-lang/rust#92385](https://github.com/rust-lang/rust/pull/92385) will unstably add this
-
-```rust
-Ok::<(), ()>(()).ok();
-```
-
-```rust,compile_fail,E0015
-// error[E0015]: cannot call non-const fn `Result::<(), ()>::ok` in constants
-const _: () = { Ok::<(), ()>(()).ok(); };
-```
-
-
 ### `[tag:const_result_map]` `Result::map[_err]` is not `const fn`
 
 ```rust

src/r3_core/src/lib.rs

@@ -24,6 +24,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(maybe_uninit_slice)]
 #![feature(const_nonnull_new)]
+#![feature(const_result_drop)]
 #![feature(const_impl_trait)]
 #![feature(const_option_ext)]
 #![feature(const_ptr_offset)]

src/r3_core/src/time/time.rs

@@ -140,9 +140,11 @@ impl Time {
     /// `None` if the result overflows the representable range of `Duration`.
     #[inline]
     pub const fn duration_since(self, reference: Self) -> Option<Duration> {
-        Some(Duration::from_micros(result_ok(
-            (self.micros as i128 - reference.micros as i128).try_into(),
-        )?))
+        Some(Duration::from_micros(
+            (self.micros as i128 - reference.micros as i128)
+                .try_into()
+                .ok()?,
+        ))
     }
 
     /// Advance the time by `duration` and return the result.
@@ -278,12 +280,3 @@ impl TryFrom<Time> for chrono_0p4::DateTime<chrono_0p4::Utc> {
 }
 
 // TODO: Add more tests
-
-/// Polyfill for `[ref:const_result_ok]`
-#[inline]
-const fn result_ok<T, E: ~const Drop>(x: Result<T, E>) -> Option<T> {
-    match x {
-        Ok(x) => Some(x),
-        Err(_x) => None,
-    }
-}