Fix indexing with vscode + rustanalyzer

Seems like calling cargo to determine the workspace path every time
really slows down things when indexing polkadot-sdk(almost 10 minutes)
so let's cache the workspace manifest path an call cargo locate only
when building the cache entry, this gives us similar performance as
1.3.0 version of this crate.

Signed-off-by: Alexandru Gheorghe <[email protected]>

src/lib.rs

@@ -162,6 +162,7 @@ type Cache = BTreeMap<String, CacheEntry>;
 struct CacheEntry {
     manifest_ts: SystemTime,
     workspace_manifest_ts: SystemTime,
+    workspace_manifest_path: PathBuf,
     crate_names: CrateNames,
 }
 
@@ -186,17 +187,16 @@ pub fn crate_name(orig_name: &str) -> Result<FoundCrate, Error> {
     let manifest_dir = env::var("CARGO_MANIFEST_DIR").map_err(|_| Error::CargoManifestDirNotSet)?;
     let manifest_path = Path::new(&manifest_dir).join("Cargo.toml");
 
-    let workspace_manifest_path = workspace_manifest_path(&manifest_path)?;
-
     let manifest_ts = cargo_toml_timestamp(&manifest_path)?;
-    let workspace_manifest_ts = cargo_toml_timestamp(&workspace_manifest_path)?;
 
     static CACHE: Mutex<Cache> = Mutex::new(BTreeMap::new());
     let mut cache = CACHE.lock().unwrap();
 
     let crate_names = match cache.entry(manifest_dir) {
         btree_map::Entry::Occupied(entry) => {
             let cache_entry = entry.into_mut();
+            let workspace_manifest_path = cache_entry.workspace_manifest_path.as_path();
+            let workspace_manifest_ts = cargo_toml_timestamp(&workspace_manifest_path)?;
 
             // Timestamp changed, rebuild this cache entry.
             if manifest_ts != cache_entry.manifest_ts ||
@@ -213,6 +213,9 @@ pub fn crate_name(orig_name: &str) -> Result<FoundCrate, Error> {
             &cache_entry.crate_names
         },
         btree_map::Entry::Vacant(entry) => {
+            let workspace_manifest_path = workspace_manifest_path(&manifest_path)?;
+            let workspace_manifest_ts = cargo_toml_timestamp(&workspace_manifest_path)?;
+
             let cache_entry = entry.insert(read_cargo_toml(
                 &manifest_path,
                 &workspace_manifest_path,
@@ -273,7 +276,12 @@ fn read_cargo_toml(
 
     let crate_names = extract_crate_names(&manifest, workspace_dependencies)?;
 
-    Ok(CacheEntry { manifest_ts, workspace_manifest_ts, crate_names })
+    Ok(CacheEntry {
+        manifest_ts,
+        workspace_manifest_ts,
+        crate_names,
+        workspace_manifest_path: workspace_manifest_path.to_path_buf(),
+    })
 }
 
 /// Extract all `[workspace.dependencies]`.