span: allow ambiguous_negative_literals

This was bad timing. The lang team just stabilized (in nightly) a new
deny-by-default lint, named `ambiguous_negative_literals`, which
triggers an error for things like `-1.hour()`. While such things can be
confusingly ambiguous in some cases, in Jiff, `-1.hour()`, `(-1).hour()`
and `-(1.hour())` are all, very intentionally, precisely equivalent.

For now we just `allow` the lint. If the lint stays, we'll likely want
to recommend allowing it in the Jiff docs.

See: rust-lang/rust#121364

src/lib.rs

@@ -575,6 +575,8 @@ specifiers and other APIs.
 #![warn(missing_debug_implementations)]
 // Document ALL THE THINGS!
 #![deny(missing_docs)]
+// See: https://github.com/rust-lang/rust/pull/121364
+#![allow(ambiguous_negative_literals)]
 
 // It should be possible to support other pointer widths, but this library
 // hasn't been tested nor thought about much in contexts with pointers less

tests/lib.rs

@@ -1 +1,4 @@
+// See: https://github.com/rust-lang/rust/pull/121364
+#![allow(ambiguous_negative_literals)]
+
 mod tc39_262;