Fix --build-plan with dev-dependencies

Regressed in rust-lang#6005 it looks like the build plan requires all packages to
be downloaded rather than just those coming out of `unit_dependenices`,
so let's make sure to download everything!

Closes rust-lang#6082

src/cargo/core/compiler/context/mod.rs

@@ -167,7 +167,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         queue.execute(&mut self, &mut plan)?;
 
         if build_plan {
-            plan.set_inputs(self.inputs()?);
+            plan.set_inputs(self.build_plan_inputs()?);
             plan.output_plan();
         }
 
@@ -512,10 +512,14 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
 
     /// Return the list of filenames read by cargo to generate the BuildContext
     /// (all Cargo.toml, etc).
-    pub fn inputs(&self) -> CargoResult<Vec<PathBuf>> {
+    pub fn build_plan_inputs(&self) -> CargoResult<Vec<PathBuf>> {
         let mut inputs = Vec::new();
-        for id in self.bcx.packages.package_ids() {
-            let pkg = self.get_package(id)?;
+        // Note that we're using the `package_cache`, which should have been
+        // populated by `build_unit_dependencies`, and only those packages are
+        // considered as all the inputs.
+        //
+        // (notably we skip dev-deps here if they aren't present)
+        for pkg in self.package_cache.values() {
             inputs.push(pkg.manifest_path().to_path_buf());
         }
         inputs.sort();

tests/testsuite/build_plan.rs

@@ -1,3 +1,4 @@
+use support::registry::Package;
 use support::{basic_bin_manifest, basic_manifest, main_file, project};
 
 #[test]
@@ -180,3 +181,28 @@ fn cargo_build_plan_build_script() {
     "#,
         ).run();
 }
+
+#[test]
+fn build_plan_with_dev_dep() {
+    Package::new("bar", "0.1.0").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.5.0"
+                authors = []
+
+                [dev-dependencies]
+                bar = "*"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build --build-plan -Zunstable-options")
+        .masquerade_as_nightly_cargo()
+        .run();
+}