rust-manifest: encode flags as `Option<NonZeroU8>`

rust-manifest: encode flags as `Option<NonZeroU8>`

This makes the compiler use the niche optimization for all flags: since 0 is
not a valid representation of any flags, we can use 0 as a replacement for
`None`, which reduces memory footprint and could yield a little performance
improvement over many iterations.

--- a/rust/hg-core/src/revlog/manifest.rs	Mon Sep 30 17:46:52 2024 +0200
+++ b/rust/hg-core/src/revlog/manifest.rs	Mon Sep 30 12:08:49 2024 +0200
@@ -1,3 +1,5 @@
+use std::num::NonZeroU8;
+
 use crate::errors::HgError;
 use crate::revlog::{Node, NodePrefix};
 use crate::revlog::{Revlog, RevlogError};
@@ -178,7 +180,7 @@
     pub hex_node_id: &'manifest [u8],
 
     /// `Some` values are b'x', b'l', or 't'
-    pub flags: Option<u8>,
+    pub flags: Option<NonZeroU8>,
 }
 
 impl<'a> ManifestEntry<'a> {
@@ -198,7 +200,7 @@
         Self {
             path: HgPath::new(path),
             hex_node_id,
-            flags,
+            flags: flags.map(|f| f.try_into().expect("invalid flag")),
         }
     }
 

--- a/rust/rhg/src/commands/status.rs	Mon Sep 30 17:46:52 2024 +0200
+++ b/rust/rhg/src/commands/status.rs	Mon Sep 30 12:08:49 2024 +0200
@@ -776,13 +776,13 @@
 
     let entry_flags = if check_exec {
         entry.flags
-    } else if entry.flags == Some(b'x') {
+    } else if entry.flags.map(|f| f.into()) == Some(b'x') {
         None
     } else {
         entry.flags
     };
 
-    if entry_flags != fs_flags {
+    if entry_flags.map(|f| f.into()) != fs_flags {
         return Ok(UnsureOutcome::Modified);
     }
     let filelog = hg::filelog::Filelog::open_vfs(