minor refactoring

- untangle `CacheFile::{rewind_chunk, abort_upload}`
- change `file::Uploading::rewind` signature to a more "builder"esque pattern

src/cachefile.rs

@@ -169,23 +169,22 @@ impl CacheFile {
     }
 
     pub fn rewind_chunk(mut self) -> Option<Self> {
-        self.uploading = Some(
-            self.uploading
+        let upl = self
+            .uploading
             .take()
-                .expect("rewind_chunk called while not uploading")
-                .rewind()?,
-        );
+            .expect("rewind_chunk called while not uploading");
 
+        self.uploading = Some(upl.rewind()?);
         Some(self)
     }
 
     pub fn abort_upload(&mut self) {
-        self.files.push_front(
-            self.uploading
+        let upl = self
+            .uploading
             .take()
-                .expect("abort_upload called while not uploading")
-                .abort(),
-        );
+            .expect("abort_upload called while not uploading");
+
+        self.files.push_front(upl.abort());
     }
 
     pub fn share_notify(&self, client: &impl Client) -> crate::Result<()> {

src/file/uploading.rs

@@ -46,13 +46,11 @@ impl Uploading {
         self.offset
     }
 
-    pub fn rewind(self) -> Option<Self> {
+    pub fn rewind(mut self) -> Option<Self> {
         if let Some(last_offset) = self.last_offset {
-            Some(Self {
-                last_offset: None,
-                offset: last_offset,
-                ..self
-            })
+            self.last_offset = None;
+            self.offset = last_offset;
+            Some(self)
         } else {
             warn!("attempted to rewind twice");
             None