rust2rpm: fall back to latest pre-release if there is no stable release

This situation can occur in two ways:
- either the crate has published only pre-releases so far, or
- all previously published stable releases were yanked

rust2rpm/__main__.py

@@ -53,6 +53,10 @@ TARGET_DEPENDENCY_LINE = re.compile(
 )
 
 
+class NoVersionsError(Exception):
+    pass
+
+
 def sortify(func):
     """Return a sorted list from a generator"""
 
@@ -122,22 +126,34 @@ def local_crate(crate, version):
     return crate, cratename, version
 
 
-def query_newest_version(crate):
+def query_newest_version(crate) -> str:
     url = requests.compat.urljoin(API_URL, f"crates/{crate}/versions")
     req = requests.get(url, headers={"User-Agent": "rust2rpm"})
     req.raise_for_status()
     versions = req.json()["versions"]
-    for struct in versions:
-        version = struct["num"]
-        if struct["yanked"]:
-            log.info(f"Ignoring yanked version {version!r}.")
-        elif re.search("alpha|beta|rc|pre", version):
-            log.info(f"Ignoring pre-release version {version!r}.")
-        else:
-            log.success(f"Using latest stable version {version!r}.")
+
+    is_stable = lambda s: not re.search("alpha|beta|rc|pre", s["num"])
+    is_not_yanked = lambda s: not s["yanked"]
+
+    # return the most recent, non-yanked stable version
+
+    is_not_yanked_and_stable = lambda s: is_stable(s) and is_not_yanked(s)
+    not_yanked_and_stable = [*filter(is_not_yanked_and_stable, versions)]
+    if len(not_yanked_and_stable) > 0:
+        version = not_yanked_and_stable[0]["num"]
         return version
 
-    raise ValueError("Couldn't find any release versions. Specify a version explicitly.")
+    # there are no non-yanked stable versions:
+    # fall back to the latest pre-release
+
+    not_yanked = [*filter(is_not_yanked, versions)]
+    if len(not_yanked) > 0:
+        version = not_yanked[0]["num"]
+        log.warn(f"No stable versions available. Falling back to the latest pre-release version {version!r}.")
+        return version
+
+    # there are no non-yanked versions: fatal
+    raise NoVersionsError()
 
 
 def download(crate, version):
@@ -637,7 +653,11 @@ def main():
         if args.crate is None:
             parser.error("crate/path argument missing and autodetection failed")
 
+    try:
         crate, diffs, metadata, doc_files, license_files = make_diff_metadata(args, args.crate, args.version)
+    except NoVersionsError:
+        log.error(f"No versions are available for crate {args.crate!r}.")
+        sys.exit(1)
 
     pkg_name = package_name_suffixed(metadata.name, args.suffix)