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

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.
+"));
+}