Fix default Steps without paths.

Some Steps are by-default run but don't have any paths associated with
them. We need to have at least one PathSet per each Step, though, so we
add an empty one on calls to `never()`.

src/bootstrap/builder.rs

@@ -95,7 +95,7 @@ pub struct RunConfig<'a> {
     pub builder: &'a Builder<'a>,
     pub host: Interned<String>,
     pub target: Interned<String>,
-    pub path: &'a Path,
+    pub path: PathBuf,
 }
 
 struct StepDescription {
@@ -114,6 +114,10 @@ struct PathSet {
 }
 
 impl PathSet {
+    fn empty() -> PathSet {
+        PathSet { set: BTreeSet::new() }
+    }
+
     fn one<P: Into<PathBuf>>(path: P) -> PathSet {
         let mut set = BTreeSet::new();
         set.insert(path.into());
@@ -124,8 +128,8 @@ impl PathSet {
         self.set.iter().any(|p| p.ends_with(needle))
     }
 
-    fn path(&self) -> &Path {
-        self.set.iter().next().unwrap()
+    fn path(&self, builder: &Builder) -> PathBuf {
+        self.set.iter().next().unwrap_or(&builder.build.src).to_path_buf()
     }
 }
 
@@ -174,7 +178,7 @@ impl StepDescription {
             for target in targets {
                 let run = RunConfig {
                     builder,
-                    path: pathset.path(),
+                    path: pathset.path(builder),
                     host: *host,
                     target: *target,
                 };
@@ -278,7 +282,8 @@ impl<'a> ShouldRun<'a> {
     }
 
     // allows being more explicit about why should_run in Step returns the value passed to it
-    pub fn never(self) -> ShouldRun<'a> {
+    pub fn never(mut self) -> ShouldRun<'a> {
+        self.paths.insert(PathSet::empty());
         self
     }