Rollup merge of #94288 - Mark-Simulacrum:ser-opt, r=nnethercote

Cleanup a few Decoder methods

This is just some simple follow up to #93839.

r? `@nnethercote`

compiler/rustc_data_structures/src/fingerprint.rs

@@ -153,9 +153,7 @@ impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
 impl<D: rustc_serialize::Decoder> Decodable<D> for Fingerprint {
     #[inline]
     fn decode(d: &mut D) -> Self {
-        let mut bytes = [0u8; 16];
-        d.read_raw_bytes_into(&mut bytes);
-        Fingerprint::from_le_bytes(bytes)
+        Fingerprint::from_le_bytes(d.read_raw_bytes(16).try_into().unwrap())
     }
 }