[ci] Roll pinned nightly toolchain (#2588)

gherrit-pr-id: Ib303d771a1c1b63aaa4a5fd8b7db064ea77e7573

Author: google-pr-creation-bot

Cargo.toml

@@ -60,7 +60,7 @@ zerocopy-panic-in-const-and-vec-try-reserve-1-57-0 = "1.57.0"
 [package.metadata.ci]
 # The versions of the stable and nightly compiler toolchains to use in CI.
 pinned-stable = "1.87.0"
-pinned-nightly = "nightly-2025-06-09"
+pinned-nightly = "nightly-2025-06-12"
 
 [package.metadata.docs.rs]
 all-features = true

src/util/macros.rs

@@ -397,6 +397,9 @@ macro_rules! impl_or_verify {
     (@verify $trait:ident, $impl_block:tt) => {
         #[cfg(any(feature = "derive", test))]
         {
+            // On some toolchains, `Subtrait` triggers the `dead_code` lint
+            // because it is implemented but never used.
+            #[allow(dead_code)]
             trait Subtrait: $trait {}
             $impl_block
         };