Guess crate name based on %crate or directory name

This makes is much nicer to work with dist-git: the same command
without modifications can be used for different crates.

rust2rpm/__main__.py

@@ -6,6 +6,7 @@ from datetime import datetime, timezone
 import difflib
 import functools
 import itertools
+import glob
 import os
 import pathlib
 import re
@@ -356,6 +357,44 @@ def detect_rpmautospec(default_target, spec_file):
     return any(re.match(autochangelog_re, line) for line in text.splitlines())
 
 
+def guess_crate_name():
+    """Guess crate name from directory name and/or spec file name
+
+    If a spec file is present, we use the %crate variable. This is the best
+    option. But if we're in a new directory, and don't have a spec file yet,
+    let's assume that this is a dist-git directory and extract the crate name.
+    For compat packages, the directory name will contain a version suffix,
+    but a compat package would almost always be created from an existing
+    dist-git directory, hence there'd be a spec file, so we can ignore this.
+    """
+    specs = glob.glob('*.spec')
+    if len(specs) > 1:
+        print('Multiple spec files found, cannot guess crate name')
+        return None
+    if len(specs) == 1:
+        crate = None
+        for line in open(specs[0]):
+            if m := re.match(r'^%(?:global|define)\s+crate\s+(\S+)\s+', line):
+                if crate:
+                    print(f'{specs[0]}: Found duplicated %crate define, cannot guess crate name')
+                    return None
+                crate = m.group(1)
+                if '%' in crate:
+                    print(f'{specs[0]}: Crate name appears to use a macro, and evaluation is not implemented')
+                    return None
+        if crate:
+            print(f'{specs[0]}: Found crate name {crate!r}')
+        else:
+            print(f'{specs[0]}: %crate define not found, cannot guess crate name')
+        return crate
+
+    dirname = os.path.basename(os.getcwd())
+    if m := re.match('^rust-([a-z+0-9_-]+)$', dirname):
+        print(f'Using crate name {m.group(1)!r} based on the directory name')
+        return m.group(1)
+
+    return None
+
 @contextlib.contextmanager
 def exit_on_common_errors():
     """Suppress tracebacks on common "expected" exceptions"""
@@ -432,7 +471,9 @@ def main():
         return
 
     if args.crate is None:
-        parser.error("required crate/path argument missing")
+        args.crate = guess_crate_name()
+        if args.crate is None:
+            parser.error("crate/path argument missing and autodetection failed")
 
     crate, diff, metadata, doc_files, license_files = make_diff_metadata(args, args.crate, args.version)