Auto merge of #1873 - alexcrichton:fix-transitive-activation, r=brson

When activating the feature `foo/bar` you're actually activating both the
feature `foo` and the `bar` feature of the relevant package. Cargo previously
forgot to activate the `foo` feature, and this commit fixes that up.

Closes #1871

Author: bors

Diff for: src/cargo/core/resolver/mod.rs

@@ -556,6 +556,7 @@ fn build_features(s: &Summary, method: Method)
         match parts.next() {
             Some(feat) => {
                 let package = feat_or_package;
+                used.insert(package.to_string());
                 deps.entry(package.to_string())
                     .or_insert(Vec::new())
                     .push(feat.to_string());

Diff for: tests/test_cargo_features.rs

@@ -769,3 +769,41 @@ test!(optional_and_dev_dep {
 {compiling} test v0.1.0 ([..])
 ", compiling = COMPILING)));
 });
+
+test!(activating_feature_activates_dep {
+    let p = project("foo")
+        .file("Cargo.toml", r#"
+            [package]
+            name    = "test"
+            version = "0.1.0"
+            authors = []
+
+            [dependencies]
+            foo = { path = "foo", optional = true }
+
+            [features]
+            a = ["foo/a"]
+        "#)
+        .file("src/lib.rs", "
+            extern crate foo;
+            pub fn bar() {
+                foo::bar();
+            }
+        ")
+        .file("foo/Cargo.toml", r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+            authors = []
+
+            [features]
+            a = []
+        "#)
+        .file("foo/src/lib.rs", r#"
+            #[cfg(feature = "a")]
+            pub fn bar() {}
+        "#);
+
+    assert_that(p.cargo_process("build").arg("--features").arg("a").arg("-v"),
+                execs().with_status(0));
+});