fix: improve error descriptions and provide details when decoding of crate versions fails.

It was suggested in this comment:
rust-lang/docs.rs#1807 (comment) ,
and if marker references aren't as expected it's possible to diff lines
that are not actually crate versions.

Author: Byron

Cargo.toml

@@ -19,6 +19,7 @@ git-repository = { version = "0.23.1", default-features = false, features = ["ma
 similar = { version = "2.2.0", features = ["bytes"] }
 serde = { version = "1", features = ["std", "derive"] }
 serde_json = "1"
+bstr = "0.2.17"
 thiserror = "1.0.32"
 
 [dependencies.git2]

src/index/diff/delegate.rs

@@ -41,7 +41,7 @@ impl<'repo> Delegate<'repo> {
             Addition { entry_mode, oid } => {
                 if let Some(obj) = entry_data(self.repo, entry_mode, oid)? {
                     for line in (&obj.data).lines() {
-                        let version: CrateVersion = serde_json::from_slice(line)?;
+                        let version = version_from_json_line(line)?;
                         self.changes.push(if version.yanked {
                             Change::Yanked(version)
                         } else {
@@ -70,8 +70,7 @@ impl<'repo> Delegate<'repo> {
                     for change in diff.iter_all_changes() {
                         match change.tag() {
                             ChangeTag::Delete | ChangeTag::Insert => {
-                                let version =
-                                    serde_json::from_slice::<CrateVersion>(change.value())?;
+                                let version = version_from_json_line(change.value())?;
                                 if change.tag() == ChangeTag::Insert {
                                     self.changes.push(if version.yanked {
                                         Change::Yanked(version)
@@ -129,3 +128,10 @@ impl git::diff::tree::Visit for Delegate<'_> {
         }
     }
 }
+
+fn version_from_json_line(line: &[u8]) -> Result<CrateVersion, Error> {
+    serde_json::from_slice(line).map_err(|err| Error::VersionDecode {
+        source: err,
+        line: line.into(),
+    })
+}

src/index/diff/mod.rs

@@ -11,20 +11,23 @@ use delegate::Delegate;
 #[derive(Debug, thiserror::Error)]
 #[allow(missing_docs)]
 pub enum Error {
-    #[error(transparent)]
+    #[error("Failed to fetch crates.io index repository")]
     Fetch(#[from] git2::Error),
-    #[error(transparent)]
+    #[error("Couldn't update marker reference")]
     ReferenceEdit(#[from] git::reference::edit::Error),
-    #[error(transparent)]
+    #[error("Failed to parse rev-spec to determine which revisions to diff")]
     RevParse(#[from] git::revision::spec::parse::Error),
-    #[error(transparent)]
+    #[error("Couldn't find blob that showed up when diffing trees")]
     FindObject(#[from] git::object::find::existing::Error),
-    #[error(transparent)]
+    #[error("Couldn't get the tree of a commit for diffing purposes")]
     PeelToTree(#[from] git::object::peel::to_kind::Error),
-    #[error(transparent)]
+    #[error("Failed to diff two trees to find changed crates")]
     Diff(#[from] git::diff::tree::changes::Error),
-    #[error(transparent)]
-    VersionDecode(#[from] serde_json::Error),
+    #[error("Failed to decode {line:?} as crate version")]
+    VersionDecode {
+        source: serde_json::Error,
+        line: bstr::BString,
+    },
 }
 
 /// Find changes without modifying the underling repository