FIX: Don't try to generate Gargo.lock on empty workspaces.

There was a panic! when the command `cargo update` was
executed in a workspace like this:

    mkdir ws
    cd ws
    echo '[workspace]' > Cargo.toml
    cargo new p1
    cargo new p2
    cargo update

The problem is that cargo tries to generate the Cargo.lock file
even if there aren't any members on the workspace.

This fix checks the existence of members in the workspace before trying
to do anything so at least we report an error instead of throwing a
panic!

src/cargo/core/workspace.rs

@@ -101,7 +101,7 @@ impl<'cfg> Workspace<'cfg> {
         Ok(ws)
     }
 
-    /// Creates a "tempoarary workspace" from one package which only contains
+    /// Creates a "temporary workspace" from one package which only contains
     /// that package.
     ///
     /// This constructor will not touch the filesystem and only creates an
@@ -464,6 +464,12 @@ impl<'cfg> Packages<'cfg> {
     }
 }
 
+impl<'a, 'cfg> Members<'a, 'cfg> {
+    pub fn is_empty(self) -> bool {
+        self.count() == 0
+    }
+}
+
 impl<'a, 'cfg> Iterator for Members<'a, 'cfg> {
     type Item = &'a Package;
 

src/cargo/ops/cargo_generate_lockfile.rs

@@ -31,6 +31,10 @@ pub fn update_lockfile(ws: &Workspace, opts: &UpdateOptions)
         bail!("cannot specify both aggressive and precise simultaneously")
     }
 
+    if ws.members().is_empty() {
+        bail!("you can't generate a lockfile for an empty workspace.")
+    }
+
     let previous_resolve = match try!(ops::load_pkg_lockfile(ws)) {
         Some(resolve) => resolve,
         None => return generate_lockfile(ws),

tests/workspaces.rs

@@ -940,3 +940,25 @@ fn lockfile_can_specify_nonexistant_members() {
 
     assert_that(p.cargo("build").cwd(p.root().join("a")), execs().with_status(0));
 }
+
+#[test]
+fn you_cannot_generate_lockfile_for_empty_workspaces() {
+    let p = project("foo")
+        .file("Cargo.toml", r#"
+            [workspace]
+        "#)
+        .file("bar/Cargo.toml", r#"
+            [project]
+            name = "foo"
+            version = "0.1.0"
+            authors = []
+        "#)
+        .file("bar/src/main.rs", "fn main() {}");
+    p.build();
+
+    assert_that(p.cargo("update"),
+                execs().with_status(101)
+                       .with_stderr("\
+error: you can't generate a lockfile for an empty workspace.
+"));
+}