formatter: make error type of with_label() callback generic

This will help eliminate push/pop_label() calls from show_diff_*().

cli/src/cli_util.rs

@@ -1971,7 +1971,7 @@ pub fn print_conflicted_paths(
                     }
                 };
             }
-            Ok(())
+            io::Result::Ok(())
         })?;
         writeln!(formatter)?;
     }

cli/src/command_error.rs

@@ -738,7 +738,6 @@ fn print_error_hints(ui: &Ui, hints: &[ErrorHint]) -> io::Result<()> {
             match hint {
                 ErrorHint::PlainText(message) => {
                     writeln!(formatter, "{message}")?;
-                    Ok(())
                 }
                 ErrorHint::Formatted(recorded) => {
                     recorded.replay(formatter)?;
@@ -747,9 +746,9 @@ fn print_error_hints(ui: &Ui, hints: &[ErrorHint]) -> io::Result<()> {
                     if !recorded.data().ends_with(b"\n") {
                         writeln!(formatter)?;
                     }
-                    Ok(())
                 }
             }
+            io::Result::Ok(())
         })?;
     }
     Ok(())

cli/src/commands/operation/diff.rs

@@ -136,8 +136,7 @@ pub fn cmd_op_diff(
                 &to_op.metadata().description
             }
         )?;
-        writeln!(formatter)?;
-        Ok(())
+        writeln!(formatter)
     })?;
 
     show_op_diff(

cli/src/diff_util.rs

@@ -984,7 +984,7 @@ fn show_unified_diff_hunks(
                             .with_label("token", |formatter| formatter.write_all(content))?,
                     }
                 }
-                Ok(())
+                io::Result::Ok(())
             })?;
             let (_, content) = tokens.last().expect("hunk line must not be empty");
             if !content.ends_with(b"\n") {
@@ -1036,7 +1036,7 @@ pub fn show_git_diff(
                     }
                     (None, None) => panic!("either left or right part should be present"),
                 }
-                Ok(())
+                io::Result::Ok(())
             })?;
 
             if left_part.content.contents == right_part.content.contents {
@@ -1061,7 +1061,7 @@ pub fn show_git_diff(
                 formatter.with_label("file_header", |formatter| {
                     writeln!(formatter, "--- {left_path}")?;
                     writeln!(formatter, "+++ {right_path}")?;
-                    Ok(())
+                    io::Result::Ok(())
                 })?;
                 show_unified_diff_hunks(
                     formatter,
@@ -1210,7 +1210,7 @@ pub fn show_diff_stat(
             total_removed,
             if total_removed == 1 { "" } else { "s" },
         )?;
-        Ok(())
+        io::Result::Ok(())
     })?;
     Ok(())
 }

cli/src/formatter.rs

@@ -42,15 +42,15 @@ impl dyn Formatter + '_ {
         }
     }
 
-    pub fn with_label(
+    pub fn with_label<E: From<io::Error>>(
         &mut self,
         label: &str,
-        write_inner: impl FnOnce(&mut dyn Formatter) -> io::Result<()>,
-    ) -> io::Result<()> {
+        write_inner: impl FnOnce(&mut dyn Formatter) -> Result<(), E>,
+    ) -> Result<(), E> {
         self.push_label(label)?;
         // Call `pop_label()` whether or not `write_inner()` fails, but don't let
         // its error replace the one from `write_inner()`.
-        write_inner(self).and(self.pop_label())
+        write_inner(self).and(self.pop_label().map_err(Into::into))
     }
 }