diff --git a/src/bootstrap/build/clean.rs b/src/bootstrap/build/clean.rs
new file mode 100644
index 0000000000000..8f78fed001cf1
--- /dev/null
+++ b/src/bootstrap/build/clean.rs
@@ -0,0 +1,36 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::fs;
+use std::path::Path;
+
+use build::Build;
+
+pub fn clean(build: &Build) {
+    for host in build.config.host.iter() {
+
+        let out = build.out.join(host);
+
+        rm_rf(build, &out.join("compiler-rt"));
+
+        for stage in 0..4 {
+            rm_rf(build, &out.join(format!("stage{}", stage)));
+            rm_rf(build, &out.join(format!("stage{}-std", stage)));
+            rm_rf(build, &out.join(format!("stage{}-rustc", stage)));
+        }
+    }
+}
+
+fn rm_rf(build: &Build, path: &Path) {
+    if path.exists() {
+        build.verbose(&format!("removing `{}`", path.display()));
+        t!(fs::remove_dir_all(path));
+    }
+}
diff --git a/src/bootstrap/build/flags.rs b/src/bootstrap/build/flags.rs
index cd538bb0a905a..d91dfe0903d11 100644
--- a/src/bootstrap/build/flags.rs
+++ b/src/bootstrap/build/flags.rs
@@ -26,6 +26,7 @@ pub struct Flags {
     pub src: Option<PathBuf>,
     pub jobs: Option<u32>,
     pub args: Vec<String>,
+    pub clean: bool,
 }
 
 pub struct Filter {
@@ -44,6 +45,7 @@ impl Flags {
         opts.optopt("", "stage", "stage to build", "N");
         opts.optopt("", "src", "path to repo root", "DIR");
         opts.optopt("j", "jobs", "number of jobs to run in parallel", "JOBS");
+        opts.optflag("", "clean", "clean output directory");
         opts.optflag("h", "help", "print this help message");
 
         let usage = |n| -> ! {
@@ -75,6 +77,7 @@ impl Flags {
 
         Flags {
             verbose: m.opt_present("v"),
+            clean: m.opt_present("clean"),
             stage: m.opt_str("stage").map(|j| j.parse().unwrap()),
             build: m.opt_str("build").unwrap(),
             host: Filter { values: m.opt_strs("host") },
diff --git a/src/bootstrap/build/job.rs b/src/bootstrap/build/job.rs
index 49e027ffda596..a4e53bc45fcfb 100644
--- a/src/bootstrap/build/job.rs
+++ b/src/bootstrap/build/job.rs
@@ -64,9 +64,20 @@ pub unsafe fn setup() {
                                     mem::size_of_val(&info) as DWORD);
     assert!(r != 0, "{}", io::Error::last_os_error());
 
-    // Assign our process to this job object
+    // Assign our process to this job object. Note that if this fails, one very
+    // likely reason is that we are ourselves already in a job object! This can
+    // happen on the build bots that we've got for Windows, or if just anyone
+    // else is instrumenting the build. In this case we just bail out
+    // immediately and assume that they take care of it.
+    //
+    // Also note that nested jobs (why this might fail) are supported in recent
+    // versions of Windows, but the version of Windows that our bots are running
+    // at least don't support nested job objects.
     let r = AssignProcessToJobObject(job, GetCurrentProcess());
-    assert!(r != 0, "{}", io::Error::last_os_error());
+    if r == 0 {
+        CloseHandle(job);
+        return
+    }
 
     // If we've got a parent process (e.g. the python script that called us)
     // then move ownership of this job object up to them. That way if the python
diff --git a/src/bootstrap/build/mod.rs b/src/bootstrap/build/mod.rs
index 6f962aae92334..92043ee3f253d 100644
--- a/src/bootstrap/build/mod.rs
+++ b/src/bootstrap/build/mod.rs
@@ -30,6 +30,7 @@ macro_rules! t {
 
 mod cc;
 mod channel;
+mod clean;
 mod compile;
 mod config;
 mod flags;
@@ -122,6 +123,10 @@ impl Build {
         #[cfg(not(windows))] fn setup_job() {}
         setup_job();
 
+        if self.flags.clean {
+            return clean::clean(self);
+        }
+
         cc::find(self);
         sanity::check(self);
         channel::collect(self);
diff --git a/src/bootstrap/mk/Makefile.in b/src/bootstrap/mk/Makefile.in
index 1157346d774ab..bd29fd8612d4e 100644
--- a/src/bootstrap/mk/Makefile.in
+++ b/src/bootstrap/mk/Makefile.in
@@ -21,3 +21,6 @@ BOOTSTRAP := $(CFG_PYTHON) $(CFG_SRC_DIR)src/bootstrap/bootstrap.py $(BOOTSTRAP_
 
 all:
 	$(Q)$(BOOTSTRAP)
+
+clean:
+	$(Q)$(BOOTSTRAP) --clean