bug: incorrect/confusing inline diff from rust?

[sourcefrog]

I have some changes in Rust source code that are incorrectly (or at least confusingly) represented in difftastic's inline view. It's omitting some unchanged context lines which tends to make them look as if they're being deleted.

It looks like the backup struct member is being replaced, but it's not. A regular GNU diff makes this clearer:

--- conserve-before.rs	2025-07-05 10:26:23.983726899 -0700
+++ conserve-after.rs	2025-07-05 10:26:12.390702623 -0700
@@ -105,15 +105,28 @@
     Delete {
         /// Archive to delete from.
         archive: String,
+
         /// Backup to delete, as an id like 'b1'. May be repeated with commas.
-        #[arg(long, short, value_delimiter = ',', required(true))]
+        #[arg(
+            long,
+            short,
+            value_delimiter = ',',
+            required_unless_present = "expiry_days"
+        )]
         backup: Vec<BandId>,
+
+        /// Delete backups that are older than the specified number of days.
+        #[arg(long, required_unless_present = "backup")]
+        expiry_days: Option<u32>,
+

Here's the difft output as text:

; difft --display=inline conserve-before.rs conserve-after.rs 
conserve-after.rs --- 1/4 --- Rust
105        Delete {
106            /// Archive to delete from.
107            archive: String,
108            /// Backup to delete, as an id like 'b1'. May be repeated with commas.
109            #[arg(long, short, value_delimiter = ',', required(true))]
   110         #[arg(
   114             required_unless_present = "expiry_days"
   118         /// Delete backups that are older than the specified number of days.
   119         #[arg(long, required_unless_present = "backup")]
   120         expiry_days: Option<u32>,
   121 
   122         /// Don't actually delete, just check what could be deleted.
   123         #[arg(long)]
   124         dry_run: bool,

The side-by-side view makes it clear the backup field isn't actually deleted:

; difft  conserve-before.rs conserve-after.rs
conserve-after.rs --- 1/4 --- Rust
107         archive: String,                                                107         archive: String,
...                                                                         108 
108         /// Backup to delete, as an id like 'b1'. May be repeated with  109         /// Backup to delete, as an id like 'b1'. May be repeated with 
... commas.                                                                 ... commas.
109         #[arg(long, short, value_delimiter = ',', required(true))]      110         #[arg(
...                                                                         111             long,
...                                                                         112             short,
...                                                                         113             value_delimiter = ',',
...                                                                         114             required_unless_present = "expiry_days"
...                                                                         115         )]
110         backup: Vec<BandId>,                                            116         backup: Vec<BandId>,
...                                                                         117 
...                                                                         118         /// Delete backups that are older than the specified number of 
...                                                                         ... days.
...                                                                         119         #[arg(long, required_unless_present = "backup")]
...                                                                         120         expiry_days: Option<u32>,
...                                                                         121 
111         /// Don't actually delete, just check what could be deleted.    122         /// Don't actually delete, just check what could be deleted.
112         #[arg(long)]                                                    123         #[arg(long)]

Here are the files (with .txt added to allow them to be attached.)

conserve-before.rs.txt
conserve-after.rs.txt

; difft --version
Difftastic 0.64.0

Revision:  484708c 2025-06-15
Toolchain: 1.74.1
System:    linux x86_64