Rollup merge of #150064 - Ayush1325:uefi-io-repr-comment, r=bjorn3

std: io: error: Add comment for UEFI unpacked repr use

The following commit adds the comment explaining the rational why UEFI uses unpacked representation on 64-bit platforms as opposed to bit-packed representation used in all other 64-bit platforms.

r? `@bjorn3`

Author: JonathanBrouwer

library/std/src/io/error.rs

@@ -1,6 +1,13 @@
 #[cfg(test)]
 mod tests;
 
+// On 64-bit platforms, `io::Error` may use a bit-packed representation to
+// reduce size. However, this representation assumes that error codes are
+// always 32-bit wide.
+//
+// This assumption is invalid on 64-bit UEFI, where error codes are 64-bit.
+// Therefore, the packed representation is explicitly disabled for UEFI
+// targets, and the unpacked representation must be used instead.
 #[cfg(all(target_pointer_width = "64", not(target_os = "uefi")))]
 mod repr_bitpacked;
 #[cfg(all(target_pointer_width = "64", not(target_os = "uefi")))]