optional dependencies are also features

References: https://github.com/rust-lang/cargo/issues/4911
Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

rust2rpm/metadata.py

@@ -1,5 +1,6 @@
 __all__ = ["Dependency", "Metadata"]
 
+import itertools
 import json
 import subprocess
 import sys
@@ -162,7 +163,11 @@ class Metadata(object):
         self.targets = [Target(tgt["kind"][0], tgt["name"]) for tgt in md["targets"]]
 
         # Provides
-        provides = Dependency(self.name, version, features=md["features"], provides=True)
+        # All optional depdencies are also features
+        # https://github.com/rust-lang/cargo/issues/4911
+        features = itertools.chain((x["name"] for x in md["dependencies"] if x["optional"]),
+                                   md["features"])
+        provides = Dependency(self.name, version, features=features, provides=True)
         self.provides = str(provides).split(" and ")
 
         # Dependencies

test.py

@@ -75,6 +75,30 @@ def cargo_toml(request):
       "crate(hello/color) = 1.2.3"],
      []),
 
+    # Provides for optional dependencies
+    ("""
+     [package]
+     name = "hello"
+     version = "1.2.3"
+
+     [dependencies]
+     non_optional = "1"
+     serde = { version = "1", optional = true }
+     rand = { version = "0.4", optional = true }
+
+     [features]
+     std = []
+     v1 = ["rand"]
+     """,
+     ["crate(hello) = 1.2.3",
+      "crate(hello/rand) = 1.2.3",
+      "crate(hello/serde) = 1.2.3",
+      "crate(hello/std) = 1.2.3",
+      "crate(hello/v1) = 1.2.3"],
+     ["(crate(non_optional) >= 1.0.0 with crate(non_optional) < 2.0.0)",
+      "(crate(rand) >= 0.4.0 with crate(rand) < 0.5.0)",
+      "(crate(serde) >= 1.0.0 with crate(serde) < 2.0.0)"]),
+
     # Caret requirements
     ("""
      [package]