Auto merge of #34956 - nikomatsakis:incr-comp-o-files, r=mw

Enable reuse of `.o` files if nothing has changed

This PR completes a first "spike" for incremental compilation by enabling us to reuse `.o` files when nothing has changed. When in incr. mode, we will save `.o` files into the temporary directory, then copy them back out again if they are still valid. The code is still a bit rough but it does seem to work. =)

r? @michaelwoerister

Fixes #34036
Fixes #34037
Fixes #34038

36 files changed, 1245 insertions, 197 deletions

diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 73b96651b05..c9247539990 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use std::fmt::Debug;
+use std::sync::Arc;
 
 macro_rules! try_opt {
     ($e:expr) => (
@@ -45,6 +46,10 @@ pub enum DepNode<D: Clone + Debug> {
     // in an extern crate.
     MetaData(D),
 
+    // Represents some artifact that we save to disk. Note that these
+    // do not have a def-id as part of their identifier.
+    WorkProduct(Arc<WorkProductId>),
+
     // Represents different phases in the compiler.
     CrateReader,
     CollectLanguageItems,
@@ -189,6 +194,11 @@ impl<D: Clone + Debug> DepNode<D> {
             TransCrate => Some(TransCrate),
             TransWriteMetadata => Some(TransWriteMetadata),
             LinkBinary => Some(LinkBinary),
+
+            // work product names do not need to be mapped, because
+            // they are always absolute.
+            WorkProduct(ref id) => Some(WorkProduct(id.clone())),
+
             Hir(ref d) => op(d).map(Hir),
             MetaData(ref d) => op(d).map(MetaData),
             CollectItem(ref d) => op(d).map(CollectItem),
@@ -229,3 +239,12 @@ impl<D: Clone + Debug> DepNode<D> {
         }
     }
 }
+
+/// A "work product" corresponds to a `.o` (or other) file that we
+/// save in between runs. These ids do not have a DefId but rather
+/// some independent path or string that persists between runs without
+/// the need to be mapped or unmapped. (This ensures we can serialize
+/// them even in the absence of a tcx.)
+#[derive(Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
+pub struct WorkProductId(pub String);
+
diff --git a/src/librustc/dep_graph/graph.rs b/src/librustc/dep_graph/graph.rs
index 741ad65c29f..bb027b11b45 100644
--- a/src/librustc/dep_graph/graph.rs
+++ b/src/librustc/dep_graph/graph.rs
@@ -9,22 +9,45 @@
 // except according to those terms.
 
 use hir::def_id::DefId;
+use rustc_data_structures::fnv::FnvHashMap;
+use session::config::OutputType;
+use std::cell::{Ref, RefCell};
 use std::rc::Rc;
+use std::sync::Arc;
 
-use super::dep_node::DepNode;
+use super::dep_node::{DepNode, WorkProductId};
 use super::query::DepGraphQuery;
 use super::raii;
 use super::thread::{DepGraphThreadData, DepMessage};
 
 #[derive(Clone)]
 pub struct DepGraph {
-    data: Rc<DepGraphThreadData>
+    data: Rc<DepGraphData>
+}
+
+struct DepGraphData {
+    /// We send messages to the thread to let it build up the dep-graph
+    /// from the current run.
+    thread: DepGraphThreadData,
+
+    /// When we load, there may be `.o` files, cached mir, or other such
+    /// things available to us. If we find that they are not dirty, we
+    /// load the path to the file storing those work-products here into
+    /// this map. We can later look for and extract that data.
+    previous_work_products: RefCell<FnvHashMap<Arc<WorkProductId>, WorkProduct>>,
+
+    /// Work-products that we generate in this run.
+    work_products: RefCell<FnvHashMap<Arc<WorkProductId>, WorkProduct>>,
 }
 
 impl DepGraph {
     pub fn new(enabled: bool) -> DepGraph {
         DepGraph {
-            data: Rc::new(DepGraphThreadData::new(enabled))
+            data: Rc::new(DepGraphData {
+                thread: DepGraphThreadData::new(enabled),
+                previous_work_products: RefCell::new(FnvHashMap()),
+                work_products: RefCell::new(FnvHashMap())
+            })
         }
     }
 
@@ -32,19 +55,19 @@ impl DepGraph {
     /// then the other methods on this `DepGraph` will have no net effect.
     #[inline]
     pub fn enabled(&self) -> bool {
-        self.data.enabled()
+        self.data.thread.enabled()
     }
 
     pub fn query(&self) -> DepGraphQuery<DefId> {
-        self.data.query()
+        self.data.thread.query()
     }
 
     pub fn in_ignore<'graph>(&'graph self) -> raii::IgnoreTask<'graph> {
-        raii::IgnoreTask::new(&self.data)
+        raii::IgnoreTask::new(&self.data.thread)
     }
 
     pub fn in_task<'graph>(&'graph self, key: DepNode<DefId>) -> raii::DepTask<'graph> {
-        raii::DepTask::new(&self.data, key)
+        raii::DepTask::new(&self.data.thread, key)
     }
 
     pub fn with_ignore<OP,R>(&self, op: OP) -> R
@@ -62,10 +85,84 @@ impl DepGraph {
     }
 
     pub fn read(&self, v: DepNode<DefId>) {
-        self.data.enqueue(DepMessage::Read(v));
+        self.data.thread.enqueue(DepMessage::Read(v));
     }
 
     pub fn write(&self, v: DepNode<DefId>) {
-        self.data.enqueue(DepMessage::Write(v));
+        self.data.thread.enqueue(DepMessage::Write(v));
+    }
+
+    /// Indicates that a previous work product exists for `v`. This is
+    /// invoked during initial start-up based on what nodes are clean
+    /// (and what files exist in the incr. directory).
+    pub fn insert_previous_work_product(&self, v: &Arc<WorkProductId>, data: WorkProduct) {
+        debug!("insert_previous_work_product({:?}, {:?})", v, data);
+        self.data.previous_work_products.borrow_mut()
+                                        .insert(v.clone(), data);
+    }
+
+    /// Indicates that we created the given work-product in this run
+    /// for `v`. This record will be preserved and loaded in the next
+    /// run.
+    pub fn insert_work_product(&self, v: &Arc<WorkProductId>, data: WorkProduct) {
+        debug!("insert_work_product({:?}, {:?})", v, data);
+        self.data.work_products.borrow_mut()
+                               .insert(v.clone(), data);
     }
+
+    /// Check whether a previous work product exists for `v` and, if
+    /// so, return the path that leads to it. Used to skip doing work.
+    pub fn previous_work_product(&self, v: &Arc<WorkProductId>) -> Option<WorkProduct> {
+        self.data.previous_work_products.borrow()
+                                        .get(v)
+                                        .cloned()
+    }
+
+    /// Access the map of work-products created during this run. Only
+    /// used during saving of the dep-graph.
+    pub fn work_products(&self) -> Ref<FnvHashMap<Arc<WorkProductId>, WorkProduct>> {
+        self.data.work_products.borrow()
+    }
+}
+
+/// A "work product" is an intermediate result that we save into the
+/// incremental directory for later re-use. The primary example are
+/// the object files that we save for each partition at code
+/// generation time.
+///
+/// Each work product is associated with a dep-node, representing the
+/// process that produced the work-product. If that dep-node is found
+/// to be dirty when we load up, then we will delete the work-product
+/// at load time. If the work-product is found to be clean, then we
+/// will keep a record in the `previous_work_products` list.
+///
+/// In addition, work products have an associated hash. This hash is
+/// an extra hash that can be used to decide if the work-product from
+/// a previous compilation can be re-used (in addition to the dirty
+/// edges check).
+///
+/// As the primary example, consider the object files we generate for
+/// each partition. In the first run, we create partitions based on
+/// the symbols that need to be compiled. For each partition P, we
+/// hash the symbols in P and create a `WorkProduct` record associated
+/// with `DepNode::TransPartition(P)`; the hash is the set of symbols
+/// in P.
+///
+/// The next time we compile, if the `DepNode::TransPartition(P)` is
+/// judged to be clean (which means none of the things we read to
+/// generate the partition were found to be dirty), it will be loaded
+/// into previous work products. We will then regenerate the set of
+/// symbols in the partition P and hash them (note that new symbols
+/// may be added -- for example, new monomorphizations -- even if
+/// nothing in P changed!). We will compare that hash against the
+/// previous hash. If it matches up, we can reuse the object file.
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
+pub struct WorkProduct {
+    /// Extra hash used to decide if work-product is still suitable;
+    /// note that this is *not* a hash of the work-product itself.
+    /// See documentation on `WorkProduct` type for an example.
+    pub input_hash: u64,
+
+    /// Saved files associated with this CGU
+    pub saved_files: Vec<(OutputType, String)>,
 }
diff --git a/src/librustc/dep_graph/mod.rs b/src/librustc/dep_graph/mod.rs
index e65f6bbcf7a..a499cb10f23 100644
--- a/src/librustc/dep_graph/mod.rs
+++ b/src/librustc/dep_graph/mod.rs
@@ -20,7 +20,9 @@ mod visit;
 
 pub use self::dep_tracking_map::{DepTrackingMap, DepTrackingMapConfig};
 pub use self::dep_node::DepNode;
+pub use self::dep_node::WorkProductId;
 pub use self::graph::DepGraph;
+pub use self::graph::WorkProduct;
 pub use self::query::DepGraphQuery;
 pub use self::visit::visit_all_items_in_krate;
 pub use self::raii::DepTask;
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index a0c2416d24c..690395399ef 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -61,7 +61,7 @@ pub enum DebugInfoLevel {
     FullDebugInfo,
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, RustcEncodable, RustcDecodable)]
 pub enum OutputType {
     Bitcode,
     Assembly,
@@ -105,6 +105,17 @@ impl OutputType {
             OutputType::DepInfo => "dep-info",
         }
     }
+
+    pub fn extension(&self) -> &'static str {
+        match *self {
+            OutputType::Bitcode => "bc",
+            OutputType::Assembly => "s",
+            OutputType::LlvmAssembly => "ll",
+            OutputType::Object => "o",
+            OutputType::DepInfo => "d",
+            OutputType::Exe => "",
+        }
+    }
 }
 
 #[derive(Clone)]
@@ -215,15 +226,7 @@ impl OutputFilenames {
                      flavor: OutputType,
                      codegen_unit_name: Option<&str>)
                      -> PathBuf {
-        let extension = match flavor {
-            OutputType::Bitcode => "bc",
-            OutputType::Assembly => "s",
-            OutputType::LlvmAssembly => "ll",
-            OutputType::Object => "o",
-            OutputType::DepInfo => "d",
-            OutputType::Exe => "",
-        };
-
+        let extension = flavor.extension();
         self.temp_path_ext(extension, codegen_unit_name)
     }
 
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 5901c42b525..cee18232ec9 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -80,7 +80,7 @@ pub struct Session {
     // forms a unique global identifier for the crate. It is used to allow
     // multiple crates with the same name to coexist. See the
     // trans::back::symbol_names module for more information.
-    pub crate_disambiguator: Cell<ast::Name>,
+    pub crate_disambiguator: RefCell<token::InternedString>,
     pub features: RefCell<feature_gate::Features>,
 
     /// The maximum recursion limit for potentially infinitely recursive
@@ -106,6 +106,9 @@ pub struct Session {
 }
 
 impl Session {
+    pub fn local_crate_disambiguator(&self) -> token::InternedString {
+        self.crate_disambiguator.borrow().clone()
+    }
     pub fn struct_span_warn<'a, S: Into<MultiSpan>>(&'a self,
                                                     sp: S,
                                                     msg: &str)
@@ -438,7 +441,7 @@ pub fn build_session_(sopts: config::Options,
         plugin_attributes: RefCell::new(Vec::new()),
         crate_types: RefCell::new(Vec::new()),
         dependency_formats: RefCell::new(FnvHashMap()),
-        crate_disambiguator: Cell::new(token::intern("")),
+        crate_disambiguator: RefCell::new(token::intern("").as_str()),
         features: RefCell::new(feature_gate::Features::new()),
         recursion_limit: Cell::new(64),
         next_node_id: Cell::new(1),
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 56938a7a838..5444dd94761 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -504,7 +504,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
     pub fn crate_disambiguator(self, cnum: ast::CrateNum) -> token::InternedString {
         if cnum == LOCAL_CRATE {
-            self.sess.crate_disambiguator.get().as_str()
+            self.sess.local_crate_disambiguator()
         } else {
             self.sess.cstore.crate_disambiguator(cnum)
         }
diff --git a/src/librustc/util/fs.rs b/src/librustc/util/fs.rs
index 4936e049ef2..f4e1c06090e 100644
--- a/src/librustc/util/fs.rs
+++ b/src/librustc/util/fs.rs
@@ -10,6 +10,8 @@
 
 use std::path::{self, Path, PathBuf};
 use std::ffi::OsString;
+use std::fs;
+use std::io;
 
 // Unfortunately, on windows, it looks like msvcrt.dll is silently translating
 // verbatim paths under the hood to non-verbatim paths! This manifests itself as
@@ -53,3 +55,15 @@ pub fn fix_windows_verbatim_for_gcc(p: &Path) -> PathBuf {
         _ => p.to_path_buf(),
     }
 }
+
+/// Copy `p` into `q`, preferring to use hard-linking if possible. If
+/// `q` already exists, it is removed first.
+pub fn link_or_copy<P: AsRef<Path>, Q: AsRef<Path>>(p: P, q: Q) -> io::Result<()> {
+    let p = p.as_ref();
+    let q = q.as_ref();
+    if q.exists() {
+        try!(fs::remove_file(&q));
+    }
+    fs::hard_link(p, q)
+        .or_else(|_| fs::copy(p, q).map(|_| ()))
+}
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index 9a94cc16bfe..f172f38b809 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -88,7 +88,7 @@ pub fn compile_input(sess: &Session,
     // We need nested scopes here, because the intermediate results can keep
     // large chunks of memory alive and we want to free them as soon as
     // possible to keep the peak memory usage low
-    let (outputs, trans) = {
+    let (outputs, trans, crate_name) = {
         let krate = match phase_1_parse_input(sess, cfg, input) {
             Ok(krate) => krate,
             Err(mut parse_error) => {
@@ -113,13 +113,13 @@ pub fn compile_input(sess: &Session,
         };
 
         let outputs = build_output_filenames(input, outdir, output, &krate.attrs, sess);
-        let id = link::find_crate_name(Some(sess), &krate.attrs, input);
+        let crate_name = link::find_crate_name(Some(sess), &krate.attrs, input);
         let ExpansionResult { expanded_crate, defs, analysis, resolutions, mut hir_forest } = {
             phase_2_configure_and_expand(
-                sess, &cstore, krate, &id, addl_plugins, control.make_glob_map,
+                sess, &cstore, krate, &crate_name, addl_plugins, control.make_glob_map,
                 |expanded_crate| {
                     let mut state = CompileState::state_after_expand(
-                        input, sess, outdir, output, &cstore, expanded_crate, &id,
+                        input, sess, outdir, output, &cstore, expanded_crate, &crate_name,
                     );
                     controller_entry_point!(after_expand, sess, state, Ok(()));
                     Ok(())
@@ -127,7 +127,7 @@ pub fn compile_input(sess: &Session,
             )?
         };
 
-        write_out_deps(sess, &outputs, &id);
+        write_out_deps(sess, &outputs, &crate_name);
 
         let arenas = ty::CtxtArenas::new();
 
@@ -151,7 +151,7 @@ pub fn compile_input(sess: &Session,
                                                                   &resolutions,
                                                                   &expanded_crate,
                                                                   &hir_map.krate(),
-                                                                  &id),
+                                                                  &crate_name),
                                     Ok(()));
         }
 
@@ -171,7 +171,7 @@ pub fn compile_input(sess: &Session,
                                     analysis,
                                     resolutions,
                                     &arenas,
-                                    &id,
+                                    &crate_name,
                                     |tcx, mir_map, analysis, result| {
             {
                 // Eventually, we will want to track plugins.
@@ -186,7 +186,7 @@ pub fn compile_input(sess: &Session,
                                                                    &analysis,
                                                                    mir_map.as_ref(),
                                                                    tcx,
-                                                                   &id);
+                                                                   &crate_name);
                 (control.after_analysis.callback)(&mut state);
 
                 if control.after_analysis.stop == Compilation::Stop {
@@ -212,11 +212,11 @@ pub fn compile_input(sess: &Session,
             // Discard interned strings as they are no longer required.
             token::clear_ident_interner();
 
-            Ok((outputs, trans))
+            Ok((outputs, trans, crate_name.clone()))
         })??
     };
 
-    let phase5_result = phase_5_run_llvm_passes(sess, &trans, &outputs);
+    let phase5_result = phase_5_run_llvm_passes(sess, &crate_name, &trans, &outputs);
 
     controller_entry_point!(after_llvm,
                             sess,
@@ -566,7 +566,8 @@ pub fn phase_2_configure_and_expand<'a, F>(sess: &Session,
     });
 
     *sess.crate_types.borrow_mut() = collect_crate_types(sess, &krate.attrs);
-    sess.crate_disambiguator.set(token::intern(&compute_crate_disambiguator(sess)));
+    *sess.crate_disambiguator.borrow_mut() =
+        token::intern(&compute_crate_disambiguator(sess)).as_str();
 
     time(time_passes, "recursion limit", || {
         middle::recursion_limit::update_recursion_limit(sess, &krate);
@@ -1019,6 +1020,7 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 /// Run LLVM itself, producing a bitcode file, assembly file or object file
 /// as a side effect.
 pub fn phase_5_run_llvm_passes(sess: &Session,
+                               crate_name: &str,
                                trans: &trans::CrateTranslation,
                                outputs: &OutputFilenames) -> CompileResult {
     if sess.opts.cg.no_integrated_as {
@@ -1040,6 +1042,10 @@ pub fn phase_5_run_llvm_passes(sess: &Session,
              || write::run_passes(sess, trans, &sess.opts.output_types, outputs));
     }
 
+    time(sess.time_passes(),
+         "serialize work products",
+         move || rustc_incremental::save_work_products(sess, crate_name));
+
     if sess.err_count() > 0 {
         Err(sess.err_count())
     } else {
@@ -1063,14 +1069,14 @@ fn escape_dep_filename(filename: &str) -> String {
     filename.replace(" ", "\\ ")
 }
 
-fn write_out_deps(sess: &Session, outputs: &OutputFilenames, id: &str) {
+fn write_out_deps(sess: &Session, outputs: &OutputFilenames, crate_name: &str) {
     let mut out_filenames = Vec::new();
     for output_type in sess.opts.output_types.keys() {
         let file = outputs.path(*output_type);
         match *output_type {
             OutputType::Exe => {
                 for output in sess.crate_types.borrow().iter() {
-                    let p = link::filename_for_input(sess, *output, id, outputs);
+                    let p = link::filename_for_input(sess, *output, crate_name, outputs);
                     out_filenames.push(p);
                 }
             }
diff --git a/src/librustc_incremental/calculate_svh.rs b/src/librustc_incremental/calculate_svh.rs
index cbc246ac2a1..7b1e0d2d0c8 100644
--- a/src/librustc_incremental/calculate_svh.rs
+++ b/src/librustc_incremental/calculate_svh.rs
@@ -36,7 +36,7 @@ impl<'a, 'tcx> SvhCalculate for TyCtxt<'a, 'tcx, 'tcx> {
         // to ensure it is not incorporating implementation artifacts into
         // the hash that are not otherwise visible.)
 
-        let crate_disambiguator = self.sess.crate_disambiguator.get();
+        let crate_disambiguator = self.sess.local_crate_disambiguator();
         let krate = self.map.krate();
 
         // FIXME: this should use SHA1, not SipHash. SipHash is not built to
@@ -47,10 +47,10 @@ impl<'a, 'tcx> SvhCalculate for TyCtxt<'a, 'tcx, 'tcx> {
         // FIXME(#32753) -- at (*) we `to_le` for endianness, but is
         // this enough, and does it matter anyway?
         "crate_disambiguator".hash(&mut state);
-        crate_disambiguator.as_str().len().to_le().hash(&mut state); // (*)
-        crate_disambiguator.as_str().hash(&mut state);
+        crate_disambiguator.len().to_le().hash(&mut state); // (*)
+        crate_disambiguator.hash(&mut state);
 
-        debug!("crate_disambiguator: {:?}", crate_disambiguator.as_str());
+        debug!("crate_disambiguator: {:?}", crate_disambiguator);
         debug!("state: {:?}", state);
 
         {
@@ -119,6 +119,7 @@ mod svh_visitor {
     use rustc::ty::TyCtxt;
     use rustc::hir;
     use rustc::hir::*;
+    use rustc::hir::map::DefPath;
     use rustc::hir::intravisit as visit;
     use rustc::hir::intravisit::{Visitor, FnKind};
 
@@ -135,6 +136,15 @@ mod svh_visitor {
                    -> Self {
             StrictVersionHashVisitor { st: st, tcx: tcx }
         }
+
+        fn hash_def_path(&mut self, path: &DefPath) {
+            self.tcx.crate_name(path.krate).hash(self.st);
+            self.tcx.crate_disambiguator(path.krate).hash(self.st);
+            for data in &path.data {
+                data.data.as_interned_str().hash(self.st);
+                data.disambiguator.hash(self.st);
+            }
+        }
     }
 
     // To off-load the bulk of the hash-computation on #[derive(Hash)],
@@ -289,19 +299,21 @@ mod svh_visitor {
 
     impl<'a, 'tcx> Visitor<'a> for StrictVersionHashVisitor<'a, 'tcx> {
         fn visit_nested_item(&mut self, item: ItemId) {
-            debug!("visit_nested_item: {:?} st={:?}", item, self.st);
-            let def_path = self.tcx.map.def_path_from_id(item.id);
-            def_path.hash(self.st);
+            let def_path = self.tcx.map.def_path_from_id(item.id).unwrap();
+            debug!("visit_nested_item: def_path={:?} st={:?}", def_path, self.st);
+            self.hash_def_path(&def_path);
         }
 
         fn visit_variant_data(&mut self, s: &'a VariantData, name: Name,
                               g: &'a Generics, _: NodeId, _: Span) {
+            debug!("visit_variant_data: st={:?}", self.st);
             SawStructDef(name.as_str()).hash(self.st);
             visit::walk_generics(self, g);
             visit::walk_struct_def(self, s)
         }
 
         fn visit_variant(&mut self, v: &'a Variant, g: &'a Generics, item_id: NodeId) {
+            debug!("visit_variant: st={:?}", self.st);
             SawVariant.hash(self.st);
             // walk_variant does not call walk_generics, so do it here.
             visit::walk_generics(self, g);
@@ -323,14 +335,17 @@ mod svh_visitor {
         // pattern, please move that method up above this comment.)
 
         fn visit_name(&mut self, _: Span, name: Name) {
+            debug!("visit_name: st={:?}", self.st);
             SawIdent(name.as_str()).hash(self.st);
         }
 
         fn visit_lifetime(&mut self, l: &'a Lifetime) {
+            debug!("visit_lifetime: st={:?}", self.st);
             SawLifetime(l.name.as_str()).hash(self.st);
         }
 
         fn visit_lifetime_def(&mut self, l: &'a LifetimeDef) {
+            debug!("visit_lifetime_def: st={:?}", self.st);
             SawLifetimeDef(l.lifetime.name.as_str()).hash(self.st);
         }
 
@@ -340,14 +355,18 @@ mod svh_visitor {
         // that a change to a crate body will require downstream
         // crates to be recompiled.
         fn visit_expr(&mut self, ex: &'a Expr) {
+            debug!("visit_expr: st={:?}", self.st);
             SawExpr(saw_expr(&ex.node)).hash(self.st); visit::walk_expr(self, ex)
         }
 
         fn visit_stmt(&mut self, s: &'a Stmt) {
+            debug!("visit_stmt: st={:?}", self.st);
             SawStmt(saw_stmt(&s.node)).hash(self.st); visit::walk_stmt(self, s)
         }
 
         fn visit_foreign_item(&mut self, i: &'a ForeignItem) {
+            debug!("visit_foreign_item: st={:?}", self.st);
+
             // FIXME (#14132) ideally we would incorporate privacy (or
             // perhaps reachability) somewhere here, so foreign items
             // that do not leak into downstream crates would not be
@@ -357,6 +376,7 @@ mod svh_visitor {
 
         fn visit_item(&mut self, i: &'a Item) {
             debug!("visit_item: {:?} st={:?}", i, self.st);
+
             // FIXME (#14132) ideally would incorporate reachability
             // analysis somewhere here, so items that never leak into
             // downstream crates (e.g. via monomorphisation or
@@ -365,55 +385,68 @@ mod svh_visitor {
         }
 
         fn visit_mod(&mut self, m: &'a Mod, _s: Span, _n: NodeId) {
+            debug!("visit_mod: st={:?}", self.st);
             SawMod.hash(self.st); visit::walk_mod(self, m)
         }
 
         fn visit_decl(&mut self, d: &'a Decl) {
+            debug!("visit_decl: st={:?}", self.st);
             SawDecl.hash(self.st); visit::walk_decl(self, d)
         }
 
         fn visit_ty(&mut self, t: &'a Ty) {
+            debug!("visit_ty: st={:?}", self.st);
             SawTy.hash(self.st); visit::walk_ty(self, t)
         }
 
         fn visit_generics(&mut self, g: &'a Generics) {
+            debug!("visit_generics: st={:?}", self.st);
             SawGenerics.hash(self.st); visit::walk_generics(self, g)
         }
 
         fn visit_fn(&mut self, fk: FnKind<'a>, fd: &'a FnDecl,
                     b: &'a Block, s: Span, _: NodeId) {
+            debug!("visit_fn: st={:?}", self.st);
             SawFn.hash(self.st); visit::walk_fn(self, fk, fd, b, s)
         }
 
         fn visit_trait_item(&mut self, ti: &'a TraitItem) {
+            debug!("visit_trait_item: st={:?}", self.st);
             SawTraitItem.hash(self.st); visit::walk_trait_item(self, ti)
         }
 
         fn visit_impl_item(&mut self, ii: &'a ImplItem) {
+            debug!("visit_impl_item: st={:?}", self.st);
             SawImplItem.hash(self.st); visit::walk_impl_item(self, ii)
         }
 
         fn visit_struct_field(&mut self, s: &'a StructField) {
+            debug!("visit_struct_field: st={:?}", self.st);
             SawStructField.hash(self.st); visit::walk_struct_field(self, s)
         }
 
         fn visit_path(&mut self, path: &'a Path, _: ast::NodeId) {
+            debug!("visit_path: st={:?}", self.st);
             SawPath.hash(self.st); visit::walk_path(self, path)
         }
 
         fn visit_block(&mut self, b: &'a Block) {
+            debug!("visit_block: st={:?}", self.st);
             SawBlock.hash(self.st); visit::walk_block(self, b)
         }
 
         fn visit_pat(&mut self, p: &'a Pat) {
+            debug!("visit_pat: st={:?}", self.st);
             SawPat.hash(self.st); visit::walk_pat(self, p)
         }
 
         fn visit_local(&mut self, l: &'a Local) {
+            debug!("visit_local: st={:?}", self.st);
             SawLocal.hash(self.st); visit::walk_local(self, l)
         }
 
         fn visit_arm(&mut self, a: &'a Arm) {
+            debug!("visit_arm: st={:?}", self.st);
             SawArm.hash(self.st); visit::walk_arm(self, a)
         }
     }
diff --git a/src/librustc_incremental/lib.rs b/src/librustc_incremental/lib.rs
index ed31e0ba510..0d11b0794fe 100644
--- a/src/librustc_incremental/lib.rs
+++ b/src/librustc_incremental/lib.rs
@@ -19,6 +19,7 @@
       html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![cfg_attr(not(stage0), deny(warnings))]
 
+#![feature(question_mark)]
 #![feature(rustc_private)]
 #![feature(staged_api)]
 
@@ -40,3 +41,6 @@ pub use assert_dep_graph::assert_dep_graph;
 pub use calculate_svh::SvhCalculate;
 pub use persist::load_dep_graph;
 pub use persist::save_dep_graph;
+pub use persist::save_trans_partition;
+pub use persist::save_work_products;
+pub use persist::in_incr_comp_dir;
diff --git a/src/librustc_incremental/persist/data.rs b/src/librustc_incremental/persist/data.rs
index f57ab19a525..95e9a16f29b 100644
--- a/src/librustc_incremental/persist/data.rs
+++ b/src/librustc_incremental/persist/data.rs
@@ -10,8 +10,9 @@
 
 //! The data that we will serialize and deserialize.
 
-use rustc::dep_graph::DepNode;
+use rustc::dep_graph::{DepNode, WorkProduct, WorkProductId};
 use rustc::hir::def_id::DefIndex;
+use std::sync::Arc;
 
 use super::directory::DefPathIndex;
 
@@ -55,6 +56,15 @@ pub struct SerializedHash {
     pub hash: u64,
 }
 
+#[derive(Debug, RustcEncodable, RustcDecodable)]
+pub struct SerializedWorkProduct {
+    /// node that produced the work-product
+    pub id: Arc<WorkProductId>,
+
+    /// work-product data itself
+    pub work_product: WorkProduct,
+}
+
 /// Data for use when downstream crates get recompiled.
 #[derive(Debug, RustcEncodable, RustcDecodable)]
 pub struct SerializedMetadataHashes {
diff --git a/src/librustc_incremental/persist/load.rs b/src/librustc_incremental/persist/load.rs
index 0ac1018462e..36b6c79c40f 100644
--- a/src/librustc_incremental/persist/load.rs
+++ b/src/librustc_incremental/persist/load.rs
@@ -14,12 +14,13 @@ use rbml::Error;
 use rbml::opaque::Decoder;
 use rustc::dep_graph::DepNode;
 use rustc::hir::def_id::DefId;
+use rustc::session::Session;
 use rustc::ty::TyCtxt;
 use rustc_data_structures::fnv::FnvHashSet;
 use rustc_serialize::Decodable as RustcDecodable;
 use std::io::Read;
-use std::fs::File;
-use std::path::Path;
+use std::fs::{self, File};
+use std::path::{Path};
 
 use super::data::*;
 use super::directory::*;
@@ -38,18 +39,43 @@ type CleanEdges = Vec<(DepNode<DefId>, DepNode<DefId>)>;
 /// actually it doesn't matter all that much.) See `README.md` for
 /// more general overview.
 pub fn load_dep_graph<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
+    if tcx.sess.opts.incremental.is_none() {
+        return;
+    }
+
     let _ignore = tcx.dep_graph.in_ignore();
+    load_dep_graph_if_exists(tcx);
+    dirty_clean::check_dirty_clean_annotations(tcx);
+}
 
-    if let Some(dep_graph) = dep_graph_path(tcx) {
-        // FIXME(#32754) lock file?
-        load_dep_graph_if_exists(tcx, &dep_graph);
-        dirty_clean::check_dirty_clean_annotations(tcx);
+fn load_dep_graph_if_exists<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
+    let dep_graph_path = dep_graph_path(tcx).unwrap();
+    let dep_graph_data = match load_data(tcx.sess, &dep_graph_path) {
+        Some(p) => p,
+        None => return // no file
+    };
+
+    let work_products_path = tcx_work_products_path(tcx).unwrap();
+    let work_products_data = match load_data(tcx.sess, &work_products_path) {
+        Some(p) => p,
+        None => return // no file
+    };
+
+    match decode_dep_graph(tcx, &dep_graph_data, &work_products_data) {
+        Ok(()) => return,
+        Err(err) => {
+            tcx.sess.warn(
+                &format!("decoding error in dep-graph from `{}` and `{}`: {}",
+                         dep_graph_path.display(),
+                         work_products_path.display(),
+                         err));
+        }
     }
 }
 
-pub fn load_dep_graph_if_exists<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, path: &Path) {
+fn load_data(sess: &Session, path: &Path) -> Option<Vec<u8>> {
     if !path.exists() {
-        return;
+        return None;
     }
 
     let mut data = vec![];
@@ -57,31 +83,30 @@ pub fn load_dep_graph_if_exists<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, path: &Pa
         File::open(path)
         .and_then(|mut file| file.read_to_end(&mut data))
     {
-        Ok(_) => { }
+        Ok(_) => {
+            Some(data)
+        }
         Err(err) => {
-            tcx.sess.err(
+            sess.err(
                 &format!("could not load dep-graph from `{}`: {}",
                          path.display(), err));
-            return;
+            None
         }
     }
 
-    match decode_dep_graph(tcx, &data) {
-        Ok(dirty) => dirty,
-        Err(err) => {
-            bug!("decoding error in dep-graph from `{}`: {}", path.display(), err);
-        }
-    }
 }
 
+/// Decode the dep graph and load the edges/nodes that are still clean
+/// into `tcx.dep_graph`.
 pub fn decode_dep_graph<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                                  data: &[u8])
+                                  dep_graph_data: &[u8],
+                                  work_products_data: &[u8])
                                   -> Result<(), Error>
 {
     // Deserialize the directory and dep-graph.
-    let mut decoder = Decoder::new(data, 0);
-    let directory = try!(DefIdDirectory::decode(&mut decoder));
-    let serialized_dep_graph = try!(SerializedDepGraph::decode(&mut decoder));
+    let mut dep_graph_decoder = Decoder::new(dep_graph_data, 0);
+    let directory = try!(DefIdDirectory::decode(&mut dep_graph_decoder));
+    let serialized_dep_graph = try!(SerializedDepGraph::decode(&mut dep_graph_decoder));
 
     debug!("decode_dep_graph: directory = {:#?}", directory);
     debug!("decode_dep_graph: serialized_dep_graph = {:#?}", serialized_dep_graph);
@@ -121,12 +146,18 @@ pub fn decode_dep_graph<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // Add nodes and edges that are not dirty into our main graph.
     let dep_graph = tcx.dep_graph.clone();
     for (source, target) in clean_edges.into_iter().chain(clean_nodes) {
-        let _task = dep_graph.in_task(target.clone());
-        dep_graph.read(source.clone());
-
         debug!("decode_dep_graph: clean edge: {:?} -> {:?}", source, target);
+
+        let _task = dep_graph.in_task(target);
+        dep_graph.read(source);
     }
 
+    // Add in work-products that are still clean, and delete those that are
+    // dirty.
+    let mut work_product_decoder = Decoder::new(work_products_data, 0);
+    let work_products = try!(<Vec<SerializedWorkProduct>>::decode(&mut work_product_decoder));
+    reconcile_work_products(tcx, work_products, &dirty_nodes);
+
     Ok(())
 }
 
@@ -141,9 +172,9 @@ fn initial_dirty_nodes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         match hash.node.map_def(|&i| retraced.def_id(i)) {
             Some(dep_node) => {
                 let current_hash = hcx.hash(&dep_node).unwrap();
-                debug!("initial_dirty_nodes: hash of {:?} is {:?}, was {:?}",
-                       dep_node, current_hash, hash.hash);
                 if current_hash != hash.hash {
+                    debug!("initial_dirty_nodes: {:?} is dirty as hash is {:?}, was {:?}",
+                           dep_node, current_hash, hash.hash);
                     dirty_nodes.insert(dep_node);
                 }
             }
@@ -177,6 +208,8 @@ fn compute_clean_edges(serialized_edges: &[(SerializedEdge)],
                 clean_edges.push((source, target))
             } else {
                 // source removed, target must be dirty
+                debug!("compute_clean_edges: {:?} dirty because {:?} no longer exists",
+                       target, serialized_source);
                 dirty_nodes.insert(target);
             }
         } else {
@@ -213,3 +246,51 @@ fn compute_clean_edges(serialized_edges: &[(SerializedEdge)],
 
     clean_edges
 }
+
+/// Go through the list of work-products produced in the previous run.
+/// Delete any whose nodes have been found to be dirty or which are
+/// otherwise no longer applicable.
+fn reconcile_work_products<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                     work_products: Vec<SerializedWorkProduct>,
+                                     dirty_nodes: &DirtyNodes) {
+    debug!("reconcile_work_products({:?})", work_products);
+    for swp in work_products {
+        let dep_node = DepNode::WorkProduct(swp.id.clone());
+        if dirty_nodes.contains(&dep_node) {
+            debug!("reconcile_work_products: dep-node for {:?} is dirty", swp);
+            delete_dirty_work_product(tcx, swp);
+        } else {
+            let all_files_exist =
+                swp.work_product
+                   .saved_files
+                   .iter()
+                   .all(|&(_, ref file_name)| {
+                       let path = in_incr_comp_dir(tcx.sess, &file_name).unwrap();
+                       path.exists()
+                   });
+            if all_files_exist {
+                debug!("reconcile_work_products: all files for {:?} exist", swp);
+                tcx.dep_graph.insert_previous_work_product(&swp.id, swp.work_product);
+            } else {
+                debug!("reconcile_work_products: some file for {:?} does not exist", swp);
+                delete_dirty_work_product(tcx, swp);
+            }
+        }
+    }
+}
+
+fn delete_dirty_work_product(tcx: TyCtxt,
+                             swp: SerializedWorkProduct) {
+    debug!("delete_dirty_work_product({:?})", swp);
+    for &(_, ref file_name) in &swp.work_product.saved_files {
+        let path = in_incr_comp_dir(tcx.sess, file_name).unwrap();
+        match fs::remove_file(&path) {
+            Ok(()) => { }
+            Err(err) => {
+                tcx.sess.warn(
+                    &format!("file-system error deleting outdated file `{}`: {}",
+                             path.display(), err));
+            }
+        }
+    }
+}
diff --git a/src/librustc_incremental/persist/mod.rs b/src/librustc_incremental/persist/mod.rs
index 72ccc29c97b..1157f494ce6 100644
--- a/src/librustc_incremental/persist/mod.rs
+++ b/src/librustc_incremental/persist/mod.rs
@@ -19,6 +19,10 @@ mod hash;
 mod load;
 mod save;
 mod util;
+mod work_product;
 
 pub use self::load::load_dep_graph;
 pub use self::save::save_dep_graph;
+pub use self::save::save_work_products;
+pub use self::work_product::save_trans_partition;
+pub use self::util::in_incr_comp_dir;
diff --git a/src/librustc_incremental/persist/save.rs b/src/librustc_incremental/persist/save.rs
index 99f4d4f3072..305250d5962 100644
--- a/src/librustc_incremental/persist/save.rs
+++ b/src/librustc_incremental/persist/save.rs
@@ -11,6 +11,7 @@
 use rbml::opaque::Encoder;
 use rustc::dep_graph::DepNode;
 use rustc::middle::cstore::LOCAL_CRATE;
+use rustc::session::Session;
 use rustc::ty::TyCtxt;
 use rustc_serialize::{Encodable as RustcEncodable};
 use std::hash::{Hasher, SipHasher};
@@ -24,19 +25,26 @@ use super::hash::*;
 use super::util::*;
 
 pub fn save_dep_graph<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
+    debug!("save_dep_graph()");
     let _ignore = tcx.dep_graph.in_ignore();
+    let sess = tcx.sess;
     let mut hcx = HashContext::new(tcx);
-    save_in(&mut hcx, dep_graph_path(tcx), encode_dep_graph);
-    save_in(&mut hcx, metadata_hash_path(tcx, LOCAL_CRATE), encode_metadata_hashes);
+    save_in(sess, dep_graph_path(tcx), |e| encode_dep_graph(&mut hcx, e));
+    save_in(sess, metadata_hash_path(tcx, LOCAL_CRATE), |e| encode_metadata_hashes(&mut hcx, e));
 }
 
-fn save_in<'a, 'tcx, F>(hcx: &mut HashContext<'a, 'tcx>,
-                        opt_path_buf: Option<PathBuf>,
-                        encode: F)
-    where F: FnOnce(&mut HashContext<'a, 'tcx>, &mut Encoder) -> io::Result<()>
-{
-    let tcx = hcx.tcx;
+pub fn save_work_products(sess: &Session, local_crate_name: &str) {
+    debug!("save_work_products()");
+    let _ignore = sess.dep_graph.in_ignore();
+    let path = sess_work_products_path(sess, local_crate_name);
+    save_in(sess, path, |e| encode_work_products(sess, e));
+}
 
+fn save_in<F>(sess: &Session,
+              opt_path_buf: Option<PathBuf>,
+              encode: F)
+    where F: FnOnce(&mut Encoder) -> io::Result<()>
+{
     let path_buf = match opt_path_buf {
         Some(p) => p,
         None => return
@@ -49,7 +57,7 @@ fn save_in<'a, 'tcx, F>(hcx: &mut HashContext<'a, 'tcx>,
         match fs::remove_file(&path_buf) {
             Ok(()) => { }
             Err(err) => {
-                tcx.sess.err(
+                sess.err(
                     &format!("unable to delete old dep-graph at `{}`: {}",
                              path_buf.display(), err));
                 return;
@@ -59,10 +67,10 @@ fn save_in<'a, 'tcx, F>(hcx: &mut HashContext<'a, 'tcx>,
 
     // generate the data in a memory buffer
     let mut wr = Cursor::new(Vec::new());
-    match encode(hcx, &mut Encoder::new(&mut wr)) {
+    match encode(&mut Encoder::new(&mut wr)) {
         Ok(()) => { }
         Err(err) => {
-            tcx.sess.err(
+            sess.err(
                 &format!("could not encode dep-graph to `{}`: {}",
                          path_buf.display(), err));
             return;
@@ -77,7 +85,7 @@ fn save_in<'a, 'tcx, F>(hcx: &mut HashContext<'a, 'tcx>,
     {
         Ok(_) => { }
         Err(err) => {
-            tcx.sess.err(
+            sess.err(
                 &format!("failed to write dep-graph to `{}`: {}",
                          path_buf.display(), err));
             return;
@@ -192,3 +200,22 @@ pub fn encode_metadata_hashes<'a, 'tcx>(hcx: &mut HashContext<'a, 'tcx>,
 
     Ok(())
 }
+
+pub fn encode_work_products(sess: &Session,
+                            encoder: &mut Encoder)
+                            -> io::Result<()>
+{
+    let work_products: Vec<_> =
+        sess.dep_graph.work_products()
+                     .iter()
+                     .map(|(id, work_product)| {
+                         SerializedWorkProduct {
+                             id: id.clone(),
+                             work_product: work_product.clone(),
+                         }
+                     })
+                     .collect();
+
+    work_products.encode(encoder)
+}
+
diff --git a/src/librustc_incremental/persist/util.rs b/src/librustc_incremental/persist/util.rs
index a77a9607e77..f1e81fdb266 100644
--- a/src/librustc_incremental/persist/util.rs
+++ b/src/librustc_incremental/persist/util.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use rustc::middle::cstore::LOCAL_CRATE;
+use rustc::session::Session;
 use rustc::ty::TyCtxt;
 
 use std::fs;
@@ -17,33 +18,56 @@ use std::path::{Path, PathBuf};
 use syntax::ast;
 
 pub fn dep_graph_path(tcx: TyCtxt) -> Option<PathBuf> {
-    path(tcx, LOCAL_CRATE, "local")
+    tcx_path(tcx, LOCAL_CRATE, "local")
 }
 
 pub fn metadata_hash_path(tcx: TyCtxt, cnum: ast::CrateNum) -> Option<PathBuf> {
-    path(tcx, cnum, "metadata")
+    tcx_path(tcx, cnum, "metadata")
 }
 
-fn path(tcx: TyCtxt, cnum: ast::CrateNum, suffix: &str) -> Option<PathBuf> {
+pub fn tcx_work_products_path(tcx: TyCtxt) -> Option<PathBuf> {
+    let crate_name = tcx.crate_name(LOCAL_CRATE);
+    sess_work_products_path(tcx.sess, &crate_name)
+}
+
+pub fn sess_work_products_path(sess: &Session,
+                               local_crate_name: &str)
+                               -> Option<PathBuf> {
+    let crate_disambiguator = sess.local_crate_disambiguator();
+    path(sess, local_crate_name, &crate_disambiguator, "work-products")
+}
+
+pub fn in_incr_comp_dir(sess: &Session, file_name: &str) -> Option<PathBuf> {
+    sess.opts.incremental.as_ref().map(|incr_dir| incr_dir.join(file_name))
+}
+
+fn tcx_path(tcx: TyCtxt,
+            cnum: ast::CrateNum,
+            middle: &str)
+            -> Option<PathBuf> {
+    path(tcx.sess, &tcx.crate_name(cnum), &tcx.crate_disambiguator(cnum), middle)
+}
+
+fn path(sess: &Session,
+        crate_name: &str,
+        crate_disambiguator: &str,
+        middle: &str)
+        -> Option<PathBuf> {
     // For now, just save/load dep-graph from
     // directory/dep_graph.rbml
-    tcx.sess.opts.incremental.as_ref().and_then(|incr_dir| {
+    sess.opts.incremental.as_ref().and_then(|incr_dir| {
         match create_dir_racy(&incr_dir) {
             Ok(()) => {}
             Err(err) => {
-                tcx.sess.err(
+                sess.err(
                     &format!("could not create the directory `{}`: {}",
                              incr_dir.display(), err));
                 return None;
             }
         }
 
-        let crate_name = tcx.crate_name(cnum);
-        let crate_disambiguator = tcx.crate_disambiguator(cnum);
-        let file_name = format!("{}-{}.{}.bin",
-                                crate_name,
-                                crate_disambiguator,
-                                suffix);
+        let file_name = format!("{}-{}.{}.bin", crate_name, crate_disambiguator, middle);
+
         Some(incr_dir.join(file_name))
     })
 }
diff --git a/src/librustc_incremental/persist/work_product.rs b/src/librustc_incremental/persist/work_product.rs
new file mode 100644
index 00000000000..c106ea8f262
--- /dev/null
+++ b/src/librustc_incremental/persist/work_product.rs
@@ -0,0 +1,63 @@
+// Copyright 2012-2015 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.
+
+//! This module contains files for saving intermediate work-products.
+
+use persist::util::*;
+use rustc::dep_graph::{WorkProduct, WorkProductId};
+use rustc::session::Session;
+use rustc::session::config::OutputType;
+use rustc::util::fs::link_or_copy;
+use std::path::PathBuf;
+use std::sync::Arc;
+
+pub fn save_trans_partition(sess: &Session,
+                            cgu_name: &str,
+                            partition_hash: u64,
+                            files: &[(OutputType, PathBuf)]) {
+    debug!("save_trans_partition({:?},{},{:?})",
+           cgu_name,
+           partition_hash,
+           files);
+    if sess.opts.incremental.is_none() {
+        return;
+    }
+    let work_product_id = Arc::new(WorkProductId(cgu_name.to_string()));
+
+    let saved_files: Option<Vec<_>> =
+        files.iter()
+             .map(|&(kind, ref path)| {
+                 let file_name = format!("cgu-{}.{}", cgu_name, kind.extension());
+                 let path_in_incr_dir = in_incr_comp_dir(sess, &file_name).unwrap();
+                 match link_or_copy(path, &path_in_incr_dir) {
+                     Ok(_) => Some((kind, file_name)),
+                     Err(err) => {
+                         sess.warn(&format!("error copying object file `{}` \
+                                             to incremental directory as `{}`: {}",
+                                            path.display(),
+                                            path_in_incr_dir.display(),
+                                            err));
+                         None
+                     }
+                 }
+             })
+             .collect();
+    let saved_files = match saved_files {
+        Some(v) => v,
+        None => return,
+    };
+
+    let work_product = WorkProduct {
+        input_hash: partition_hash,
+        saved_files: saved_files,
+    };
+
+    sess.dep_graph.insert_work_product(&work_product_id, work_product);
+}
diff --git a/src/librustc_metadata/creader.rs b/src/librustc_metadata/creader.rs
index d4443c6d09d..0b60fc386a7 100644
--- a/src/librustc_metadata/creader.rs
+++ b/src/librustc_metadata/creader.rs
@@ -243,7 +243,7 @@ impl<'a> CrateReader<'a> {
 
         // Check for (potential) conflicts with the local crate
         if self.local_crate_name == crate_name &&
-           self.sess.crate_disambiguator.get().as_str() == disambiguator {
+           self.sess.local_crate_disambiguator() == disambiguator {
             span_fatal!(self.sess, span, E0519,
                         "the current crate is indistinguishable from one of its \
                          dependencies: it has the same crate-name `{}` and was \
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 73142594235..c896263de94 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1893,7 +1893,7 @@ fn encode_metadata_inner(rbml_w: &mut Encoder,
     encode_crate_name(rbml_w, &ecx.link_meta.crate_name);
     encode_crate_triple(rbml_w, &ecx.tcx.sess.opts.target_triple);
     encode_hash(rbml_w, &ecx.link_meta.crate_hash);
-    encode_crate_disambiguator(rbml_w, &ecx.tcx.sess.crate_disambiguator.get().as_str());
+    encode_crate_disambiguator(rbml_w, &ecx.tcx.sess.local_crate_disambiguator());
     encode_dylib_dependency_formats(rbml_w, &ecx);
     encode_panic_strategy(rbml_w, &ecx);
 
diff --git a/src/librustc_trans/assert_module_sources.rs b/src/librustc_trans/assert_module_sources.rs
new file mode 100644
index 00000000000..e0532e7476f
--- /dev/null
+++ b/src/librustc_trans/assert_module_sources.rs
@@ -0,0 +1,149 @@
+// Copyright 2012-2015 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.
+
+//! This pass is only used for UNIT TESTS related to incremental
+//! compilation. It tests whether a particular `.o` file will be re-used
+//! from a previous compilation or whether it must be regenerated.
+//!
+//! The user adds annotations to the crate of the following form:
+//!
+//! ```
+//! #![rustc_partition_reused(module="spike", cfg="rpass2")]
+//! #![rustc_partition_translated(module="spike-x", cfg="rpass2")]
+//! ```
+//!
+//! The first indicates (in the cfg `rpass2`) that `spike.o` will be
+//! reused, the second that `spike-x.o` will be recreated. If these
+//! annotations are inaccurate, errors are reported.
+//!
+//! The reason that we use `cfg=...` and not `#[cfg_attr]` is so that
+//! the HIR doesn't change as a result of the annotations, which might
+//! perturb the reuse results.
+
+use rustc::ty::TyCtxt;
+use syntax::ast;
+use syntax::attr::AttrMetaMethods;
+use syntax::parse::token::InternedString;
+
+use {ModuleSource, ModuleTranslation};
+
+const PARTITION_REUSED: &'static str = "rustc_partition_reused";
+const PARTITION_TRANSLATED: &'static str = "rustc_partition_translated";
+
+const MODULE: &'static str = "module";
+const CFG: &'static str = "cfg";
+
+#[derive(Debug, PartialEq)]
+enum Disposition { Reused, Translated }
+
+pub fn assert_module_sources<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                       modules: &[ModuleTranslation]) {
+    let _ignore = tcx.dep_graph.in_ignore();
+
+    if tcx.sess.opts.incremental.is_none() {
+        return;
+    }
+
+    let ams = AssertModuleSource { tcx: tcx, modules: modules };
+    for attr in &tcx.map.krate().attrs {
+        ams.check_attr(attr);
+    }
+}
+
+struct AssertModuleSource<'a, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    modules: &'a [ModuleTranslation],
+}
+
+impl<'a, 'tcx> AssertModuleSource<'a, 'tcx> {
+    fn check_attr(&self, attr: &ast::Attribute) {
+        let disposition = if attr.check_name(PARTITION_REUSED) {
+            Disposition::Reused
+        } else if attr.check_name(PARTITION_TRANSLATED) {
+            Disposition::Translated
+        } else {
+            return;
+        };
+
+        if !self.check_config(attr) {
+            debug!("check_attr: config does not match, ignoring attr");
+            return;
+        }
+
+        let mname = self.field(attr, MODULE);
+        let mtrans = self.modules.iter().find(|mtrans| &mtrans.name[..] == &mname[..]);
+        let mtrans = match mtrans {
+            Some(m) => m,
+            None => {
+                debug!("module name `{}` not found amongst:", mname);
+                for mtrans in self.modules {
+                    debug!("module named `{}` with disposition {:?}",
+                           mtrans.name,
+                           self.disposition(mtrans));
+                }
+
+                self.tcx.sess.span_err(
+                    attr.span,
+                    &format!("no module named `{}`", mname));
+                return;
+            }
+        };
+
+        let mtrans_disposition = self.disposition(mtrans);
+        if disposition != mtrans_disposition {
+            self.tcx.sess.span_err(
+                attr.span,
+                &format!("expected module named `{}` to be {:?} but is {:?}",
+                         mname,
+                         disposition,
+                         mtrans_disposition));
+        }
+    }
+
+    fn disposition(&self, mtrans: &ModuleTranslation) -> Disposition {
+        match mtrans.source {
+            ModuleSource::Preexisting(_) => Disposition::Reused,
+            ModuleSource::Translated(_) => Disposition::Translated,
+        }
+    }
+
+    fn field(&self, attr: &ast::Attribute, name: &str) -> InternedString {
+        for item in attr.meta_item_list().unwrap_or(&[]) {
+            if item.check_name(name) {
+                if let Some(value) = item.value_str() {
+                    return value;
+                } else {
+                    self.tcx.sess.span_fatal(
+                        item.span,
+                        &format!("associated value expected for `{}`", name));
+                }
+            }
+        }
+
+        self.tcx.sess.span_fatal(
+            attr.span,
+            &format!("no field `{}`", name));
+    }
+
+    /// Scan for a `cfg="foo"` attribute and check whether we have a
+    /// cfg flag called `foo`.
+    fn check_config(&self, attr: &ast::Attribute) -> bool {
+        let config = &self.tcx.map.krate().config;
+        let value = self.field(attr, CFG);
+        debug!("check_config(config={:?}, value={:?})", config, value);
+        if config.iter().any(|c| c.check_name(&value[..])) {
+            debug!("check_config: matched");
+            return true;
+        }
+        debug!("check_config: no match found");
+        return false;
+    }
+
+}
diff --git a/src/librustc_trans/back/write.rs b/src/librustc_trans/back/write.rs
index 33cffa8a480..4b9d5dd9e8d 100644
--- a/src/librustc_trans/back/write.rs
+++ b/src/librustc_trans/back/write.rs
@@ -10,15 +10,17 @@
 
 use back::lto;
 use back::link::{get_linker, remove};
+use rustc_incremental::save_trans_partition;
 use session::config::{OutputFilenames, Passes, SomePasses, AllPasses};
 use session::Session;
 use session::config::{self, OutputType};
 use llvm;
 use llvm::{ModuleRef, TargetMachineRef, PassManagerRef, DiagnosticInfoRef, ContextRef};
 use llvm::SMDiagnosticRef;
-use {CrateTranslation, ModuleTranslation};
+use {CrateTranslation, ModuleLlvm, ModuleSource, ModuleTranslation};
 use util::common::time;
 use util::common::path2cstr;
+use util::fs::link_or_copy;
 use errors::{self, Handler, Level, DiagnosticBuilder};
 use errors::emitter::Emitter;
 use syntax_pos::MultiSpan;
@@ -335,6 +337,8 @@ struct CodegenContext<'a> {
     remark: Passes,
     // Worker thread number
     worker: usize,
+    // Directory where incremental data is stored (if any)
+    incremental: Option<PathBuf>,
 }
 
 impl<'a> CodegenContext<'a> {
@@ -345,6 +349,7 @@ impl<'a> CodegenContext<'a> {
             plugin_passes: sess.plugin_llvm_passes.borrow().clone(),
             remark: sess.opts.cg.remark.clone(),
             worker: 0,
+            incremental: sess.opts.incremental.clone(),
         }
     }
 }
@@ -422,10 +427,11 @@ unsafe extern "C" fn diagnostic_handler(info: DiagnosticInfoRef, user: *mut c_vo
 // Unsafe due to LLVM calls.
 unsafe fn optimize_and_codegen(cgcx: &CodegenContext,
                                mtrans: ModuleTranslation,
+                               mllvm: ModuleLlvm,
                                config: ModuleConfig,
                                output_names: OutputFilenames) {
-    let llmod = mtrans.llmod;
-    let llcx = mtrans.llcx;
+    let llmod = mllvm.llmod;
+    let llcx = mllvm.llcx;
     let tm = config.tm;
 
     // llcx doesn't outlive this function, so we can put this on the stack.
@@ -609,7 +615,7 @@ unsafe fn optimize_and_codegen(cgcx: &CodegenContext,
 
     if copy_bc_to_obj {
         debug!("copying bitcode {:?} to obj {:?}", bc_out, obj_out);
-        if let Err(e) = fs::copy(&bc_out, &obj_out) {
+        if let Err(e) = link_or_copy(&bc_out, &obj_out) {
             cgcx.handler.err(&format!("failed to copy bitcode to object file: {}", e));
         }
     }
@@ -628,8 +634,14 @@ unsafe fn optimize_and_codegen(cgcx: &CodegenContext,
 pub fn cleanup_llvm(trans: &CrateTranslation) {
     for module in trans.modules.iter() {
         unsafe {
-            llvm::LLVMDisposeModule(module.llmod);
-            llvm::LLVMContextDispose(module.llcx);
+            match module.source {
+                ModuleSource::Translated(llvm) => {
+                    llvm::LLVMDisposeModule(llvm.llmod);
+                    llvm::LLVMContextDispose(llvm.llcx);
+                }
+                ModuleSource::Preexisting(_) => {
+                }
+            }
         }
     }
 }
@@ -743,6 +755,23 @@ pub fn run_passes(sess: &Session,
         run_work_multithreaded(sess, work_items, num_workers);
     }
 
+    // If in incr. comp. mode, preserve the `.o` files for potential re-use
+    for mtrans in trans.modules.iter() {
+        let mut files = vec![];
+
+        if modules_config.emit_obj {
+            let path = crate_output.temp_path(OutputType::Object, Some(&mtrans.name));
+            files.push((OutputType::Object, path));
+        }
+
+        if modules_config.emit_bc {
+            let path = crate_output.temp_path(OutputType::Bitcode, Some(&mtrans.name));
+            files.push((OutputType::Bitcode, path));
+        }
+
+        save_trans_partition(sess, &mtrans.name, mtrans.symbol_name_hash, &files);
+    }
+
     // All codegen is finished.
     unsafe {
         llvm::LLVMRustDisposeTargetMachine(tm);
@@ -916,10 +945,37 @@ fn build_work_item(sess: &Session,
 fn execute_work_item(cgcx: &CodegenContext,
                      work_item: WorkItem) {
     unsafe {
-        optimize_and_codegen(cgcx,
-                             work_item.mtrans,
-                             work_item.config,
-                             work_item.output_names);
+        match work_item.mtrans.source {
+            ModuleSource::Translated(mllvm) => {
+                debug!("llvm-optimizing {:?}", work_item.mtrans.name);
+                optimize_and_codegen(cgcx,
+                                     work_item.mtrans,
+                                     mllvm,
+                                     work_item.config,
+                                     work_item.output_names);
+            }
+            ModuleSource::Preexisting(wp) => {
+                let incremental = cgcx.incremental.as_ref().unwrap();
+                let name = &work_item.mtrans.name;
+                for (kind, saved_file) in wp.saved_files {
+                    let obj_out = work_item.output_names.temp_path(kind, Some(name));
+                    let source_file = incremental.join(&saved_file);
+                    debug!("copying pre-existing module `{}` from {:?} to {}",
+                           work_item.mtrans.name,
+                           source_file,
+                           obj_out.display());
+                    match link_or_copy(&source_file, &obj_out) {
+                        Ok(()) => { }
+                        Err(err) => {
+                            cgcx.handler.err(&format!("unable to copy {} to {}: {}",
+                                                      source_file.display(),
+                                                      obj_out.display(),
+                                                      err));
+                        }
+                    }
+                }
+            }
+        }
     }
 }
 
@@ -955,6 +1011,8 @@ fn run_work_multithreaded(sess: &Session,
         let mut tx = Some(tx);
         futures.push(rx);
 
+        let incremental = sess.opts.incremental.clone();
+
         thread::Builder::new().name(format!("codegen-{}", i)).spawn(move || {
             let diag_handler = Handler::with_emitter(true, false, box diag_emitter);
 
@@ -966,6 +1024,7 @@ fn run_work_multithreaded(sess: &Session,
                 plugin_passes: plugin_passes,
                 remark: remark,
                 worker: i,
+                incremental: incremental,
             };
 
             loop {
diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs
index ea8c248d023..5a19ddff746 100644
--- a/src/librustc_trans/base.rs
+++ b/src/librustc_trans/base.rs
@@ -26,8 +26,11 @@
 #![allow(non_camel_case_types)]
 
 use super::CrateTranslation;
+use super::ModuleLlvm;
+use super::ModuleSource;
 use super::ModuleTranslation;
 
+use assert_module_sources;
 use back::link;
 use back::linker::LinkerInfo;
 use llvm::{BasicBlockRef, Linkage, ValueRef, Vector, get_param};
@@ -40,7 +43,7 @@ use rustc::ty::subst::{self, Substs};
 use rustc::traits;
 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc::ty::adjustment::CustomCoerceUnsized;
-use rustc::dep_graph::DepNode;
+use rustc::dep_graph::{DepNode, WorkProduct};
 use rustc::hir::map as hir_map;
 use rustc::util::common::time;
 use rustc::mir::mir_map::MirMap;
@@ -2133,7 +2136,7 @@ pub fn maybe_create_entry_wrapper(ccx: &CrateContext) {
 
     let instance = Instance::mono(ccx.shared(), main_def_id);
 
-    if !ccx.codegen_unit().items.contains_key(&TransItem::Fn(instance)) {
+    if !ccx.codegen_unit().contains_item(&TransItem::Fn(instance)) {
         // We want to create the wrapper in the same codegen unit as Rust's main
         // function.
         return;
@@ -2257,12 +2260,20 @@ fn write_metadata(cx: &SharedCrateContext,
 
 /// Find any symbols that are defined in one compilation unit, but not declared
 /// in any other compilation unit.  Give these symbols internal linkage.
-fn internalize_symbols<'a, 'tcx>(ccxs: &CrateContextList<'a, 'tcx>,
+fn internalize_symbols<'a, 'tcx>(sess: &Session,
+                                 ccxs: &CrateContextList<'a, 'tcx>,
                                  symbol_map: &SymbolMap<'tcx>,
                                  reachable: &FnvHashSet<&str>) {
     let scx = ccxs.shared();
     let tcx = scx.tcx();
 
+    // In incr. comp. mode, we can't necessarily see all refs since we
+    // don't generate LLVM IR for reused modules, so skip this
+    // step. Later we should get smarter.
+    if sess.opts.debugging_opts.incremental.is_some() {
+        return;
+    }
+
     // 'unsafe' because we are holding on to CStr's from the LLVM module within
     // this block.
     unsafe {
@@ -2270,7 +2281,7 @@ fn internalize_symbols<'a, 'tcx>(ccxs: &CrateContextList<'a, 'tcx>,
 
         // Collect all symbols that need to stay externally visible because they
         // are referenced via a declaration in some other codegen unit.
-        for ccx in ccxs.iter() {
+        for ccx in ccxs.iter_need_trans() {
             for val in iter_globals(ccx.llmod()).chain(iter_functions(ccx.llmod())) {
                 let linkage = llvm::LLVMGetLinkage(val);
                 // We only care about external declarations (not definitions)
@@ -2315,7 +2326,7 @@ fn internalize_symbols<'a, 'tcx>(ccxs: &CrateContextList<'a, 'tcx>,
         // Examine each external definition.  If the definition is not used in
         // any other compilation unit, and is not reachable from other crates,
         // then give it internal linkage.
-        for ccx in ccxs.iter() {
+        for ccx in ccxs.iter_need_trans() {
             for val in iter_globals(ccx.llmod()).chain(iter_functions(ccx.llmod())) {
                 let linkage = llvm::LLVMGetLinkage(val);
 
@@ -2362,7 +2373,7 @@ fn create_imps(cx: &CrateContextList) {
         "\x01__imp_"
     };
     unsafe {
-        for ccx in cx.iter() {
+        for ccx in cx.iter_need_trans() {
             let exported: Vec<_> = iter_globals(ccx.llmod())
                                        .filter(|&val| {
                                            llvm::LLVMGetLinkage(val) ==
@@ -2514,8 +2525,11 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
     let metadata_module = ModuleTranslation {
         name: "metadata".to_string(),
-        llcx: shared_ccx.metadata_llcx(),
-        llmod: shared_ccx.metadata_llmod(),
+        symbol_name_hash: 0, // we always rebuild metadata, at least for now
+        source: ModuleSource::Translated(ModuleLlvm {
+            llcx: shared_ccx.metadata_llcx(),
+            llmod: shared_ccx.metadata_llmod(),
+        }),
     };
     let no_builtins = attr::contains_name(&krate.attrs, "no_builtins");
 
@@ -2525,17 +2539,34 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
     let symbol_map = Rc::new(symbol_map);
 
+    let previous_work_products = trans_reuse_previous_work_products(tcx,
+                                                                    &codegen_units,
+                                                                    &symbol_map);
+
     let crate_context_list = CrateContextList::new(&shared_ccx,
                                                    codegen_units,
+                                                   previous_work_products,
                                                    symbol_map.clone());
-    let modules = crate_context_list.iter()
-        .map(|ccx| ModuleTranslation {
-            name: String::from(&ccx.codegen_unit().name[..]),
-            llcx: ccx.llcx(),
-            llmod: ccx.llmod()
+    let modules: Vec<_> = crate_context_list.iter_all()
+        .map(|ccx| {
+            let source = match ccx.previous_work_product() {
+                Some(buf) => ModuleSource::Preexisting(buf.clone()),
+                None => ModuleSource::Translated(ModuleLlvm {
+                    llcx: ccx.llcx(),
+                    llmod: ccx.llmod(),
+                }),
+            };
+
+            ModuleTranslation {
+                name: String::from(ccx.codegen_unit().name()),
+                symbol_name_hash: ccx.codegen_unit().compute_symbol_name_hash(tcx, &symbol_map),
+                source: source,
+            }
         })
         .collect();
 
+    assert_module_sources::assert_module_sources(tcx, &modules);
+
     // Skip crate items and just output metadata in -Z no-trans mode.
     if tcx.sess.opts.no_trans {
         let linker_info = LinkerInfo::new(&shared_ccx, &[]);
@@ -2551,41 +2582,44 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 
     // Instantiate translation items without filling out definitions yet...
-    for ccx in crate_context_list.iter() {
-        let trans_items = ccx.codegen_unit()
-                             .items_in_deterministic_order(tcx, &symbol_map);
+    for ccx in crate_context_list.iter_need_trans() {
+        let cgu = ccx.codegen_unit();
+        let trans_items = cgu.items_in_deterministic_order(tcx, &symbol_map);
 
-        for (trans_item, linkage) in trans_items {
-            trans_item.predefine(&ccx, linkage);
-        }
+        tcx.dep_graph.with_task(cgu.work_product_dep_node(), || {
+            for (trans_item, linkage) in trans_items {
+                trans_item.predefine(&ccx, linkage);
+            }
+        });
     }
 
     // ... and now that we have everything pre-defined, fill out those definitions.
-    for ccx in crate_context_list.iter() {
-        let trans_items = ccx.codegen_unit()
-                             .items_in_deterministic_order(tcx, &symbol_map);
-
-        for (trans_item, _) in trans_items {
-           trans_item.define(&ccx);
-        }
+    for ccx in crate_context_list.iter_need_trans() {
+        let cgu = ccx.codegen_unit();
+        let trans_items = cgu.items_in_deterministic_order(tcx, &symbol_map);
+        tcx.dep_graph.with_task(cgu.work_product_dep_node(), || {
+            for (trans_item, _) in trans_items {
+                trans_item.define(&ccx);
+            }
 
-        // If this codegen unit contains the main function, also create the
-        // wrapper here
-        maybe_create_entry_wrapper(&ccx);
+            // If this codegen unit contains the main function, also create the
+            // wrapper here
+            maybe_create_entry_wrapper(&ccx);
 
-        // Run replace-all-uses-with for statics that need it
-        for &(old_g, new_g) in ccx.statics_to_rauw().borrow().iter() {
-            unsafe {
-                let bitcast = llvm::LLVMConstPointerCast(new_g, llvm::LLVMTypeOf(old_g));
-                llvm::LLVMReplaceAllUsesWith(old_g, bitcast);
-                llvm::LLVMDeleteGlobal(old_g);
+            // Run replace-all-uses-with for statics that need it
+            for &(old_g, new_g) in ccx.statics_to_rauw().borrow().iter() {
+                unsafe {
+                    let bitcast = llvm::LLVMConstPointerCast(new_g, llvm::LLVMTypeOf(old_g));
+                    llvm::LLVMReplaceAllUsesWith(old_g, bitcast);
+                    llvm::LLVMDeleteGlobal(old_g);
+                }
             }
-        }
 
-        // Finalize debuginfo
-        if ccx.sess().opts.debuginfo != NoDebugInfo {
-            debuginfo::finalize(&ccx);
-        }
+            // Finalize debuginfo
+            if ccx.sess().opts.debuginfo != NoDebugInfo {
+                debuginfo::finalize(&ccx);
+            }
+        });
     }
 
     symbol_names_test::report_symbol_names(&shared_ccx);
@@ -2654,7 +2688,8 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 
     time(shared_ccx.sess().time_passes(), "internalize symbols", || {
-        internalize_symbols(&crate_context_list,
+        internalize_symbols(sess,
+                            &crate_context_list,
                             &symbol_map,
                             &reachable_symbols.iter()
                                               .map(|s| &s[..])
@@ -2679,6 +2714,38 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 }
 
+/// For each CGU, identify if we can reuse an existing object file (or
+/// maybe other context).
+fn trans_reuse_previous_work_products(tcx: TyCtxt,
+                                      codegen_units: &[CodegenUnit],
+                                      symbol_map: &SymbolMap)
+                                      -> Vec<Option<WorkProduct>> {
+    debug!("trans_reuse_previous_work_products()");
+    codegen_units
+        .iter()
+        .map(|cgu| {
+            let id = cgu.work_product_id();
+
+            let hash = cgu.compute_symbol_name_hash(tcx, symbol_map);
+
+            debug!("trans_reuse_previous_work_products: id={:?} hash={}", id, hash);
+
+            if let Some(work_product) = tcx.dep_graph.previous_work_product(&id) {
+                if work_product.input_hash == hash {
+                    debug!("trans_reuse_previous_work_products: reusing {:?}", work_product);
+                    return Some(work_product);
+                } else {
+                    debug!("trans_reuse_previous_work_products: \
+                            not reusing {:?} because hash changed to {:?}",
+                           work_product, hash);
+                }
+            }
+
+            None
+        })
+        .collect()
+}
+
 fn collect_and_partition_translation_items<'a, 'tcx>(scx: &SharedCrateContext<'a, 'tcx>)
                                                      -> (Vec<CodegenUnit<'tcx>>, SymbolMap<'tcx>) {
     let time_passes = scx.sess().time_passes();
@@ -2739,10 +2806,10 @@ fn collect_and_partition_translation_items<'a, 'tcx>(scx: &SharedCrateContext<'a
         let mut item_to_cgus = HashMap::new();
 
         for cgu in &codegen_units {
-            for (&trans_item, &linkage) in &cgu.items {
+            for (&trans_item, &linkage) in cgu.items() {
                 item_to_cgus.entry(trans_item)
                             .or_insert(Vec::new())
-                            .push((cgu.name.clone(), linkage));
+                            .push((cgu.name().clone(), linkage));
             }
         }
 
diff --git a/src/librustc_trans/consts.rs b/src/librustc_trans/consts.rs
index f662ba75cc6..571d2731fb2 100644
--- a/src/librustc_trans/consts.rs
+++ b/src/librustc_trans/consts.rs
@@ -1029,7 +1029,7 @@ pub fn get_static<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, def_id: DefId)
                 assert!(!ccx.external_srcs().borrow().contains_key(&id));
 
                 let defined_in_current_codegen_unit = ccx.codegen_unit()
-                                                         .items
+                                                         .items()
                                                          .contains_key(&TransItem::Static(id));
                 if defined_in_current_codegen_unit {
                     if declare::get_declared_value(ccx, sym).is_none() {
diff --git a/src/librustc_trans/context.rs b/src/librustc_trans/context.rs
index 88903726d64..a8f8474e940 100644
--- a/src/librustc_trans/context.rs
+++ b/src/librustc_trans/context.rs
@@ -10,7 +10,7 @@
 
 use llvm;
 use llvm::{ContextRef, ModuleRef, ValueRef, BuilderRef};
-use rustc::dep_graph::{DepNode, DepTrackingMap, DepTrackingMapConfig};
+use rustc::dep_graph::{DepNode, DepTrackingMap, DepTrackingMapConfig, WorkProduct};
 use middle::cstore::LinkMeta;
 use rustc::hir::def::ExportMap;
 use rustc::hir::def_id::DefId;
@@ -95,6 +95,7 @@ pub struct SharedCrateContext<'a, 'tcx: 'a> {
 pub struct LocalCrateContext<'tcx> {
     llmod: ModuleRef,
     llcx: ContextRef,
+    previous_work_product: Option<WorkProduct>,
     tn: TypeNames, // FIXME: This seems to be largely unused.
     codegen_unit: CodegenUnit<'tcx>,
     needs_unwind_cleanup_cache: RefCell<FnvHashMap<Ty<'tcx>, bool>>,
@@ -198,24 +199,39 @@ pub struct CrateContextList<'a, 'tcx: 'a> {
 }
 
 impl<'a, 'tcx: 'a> CrateContextList<'a, 'tcx> {
-
     pub fn new(shared_ccx: &'a SharedCrateContext<'a, 'tcx>,
                codegen_units: Vec<CodegenUnit<'tcx>>,
+               previous_work_products: Vec<Option<WorkProduct>>,
                symbol_map: Rc<SymbolMap<'tcx>>)
                -> CrateContextList<'a, 'tcx> {
         CrateContextList {
             shared: shared_ccx,
-            local_ccxs: codegen_units.into_iter().map(|codegen_unit| {
-                LocalCrateContext::new(shared_ccx, codegen_unit, symbol_map.clone())
+            local_ccxs: codegen_units.into_iter().zip(previous_work_products).map(|(cgu, wp)| {
+                LocalCrateContext::new(shared_ccx, cgu, wp, symbol_map.clone())
             }).collect()
         }
     }
 
-    pub fn iter<'b>(&'b self) -> CrateContextIterator<'b, 'tcx> {
+    /// Iterate over all crate contexts, whether or not they need
+    /// translation.  That is, whether or not a `.o` file is available
+    /// for re-use from a previous incr. comp.).
+    pub fn iter_all<'b>(&'b self) -> CrateContextIterator<'b, 'tcx> {
         CrateContextIterator {
             shared: self.shared,
             index: 0,
-            local_ccxs: &self.local_ccxs[..]
+            local_ccxs: &self.local_ccxs[..],
+            filter_to_previous_work_product_unavail: false,
+        }
+    }
+
+    /// Iterator over all CCX that need translation (cannot reuse results from
+    /// previous incr. comp.).
+    pub fn iter_need_trans<'b>(&'b self) -> CrateContextIterator<'b, 'tcx> {
+        CrateContextIterator {
+            shared: self.shared,
+            index: 0,
+            local_ccxs: &self.local_ccxs[..],
+            filter_to_previous_work_product_unavail: true,
         }
     }
 
@@ -239,24 +255,38 @@ pub struct CrateContextIterator<'a, 'tcx: 'a> {
     shared: &'a SharedCrateContext<'a, 'tcx>,
     local_ccxs: &'a [LocalCrateContext<'tcx>],
     index: usize,
+
+    /// if true, only return results where `previous_work_product` is none
+    filter_to_previous_work_product_unavail: bool,
 }
 
 impl<'a, 'tcx> Iterator for CrateContextIterator<'a,'tcx> {
     type Item = CrateContext<'a, 'tcx>;
 
     fn next(&mut self) -> Option<CrateContext<'a, 'tcx>> {
-        if self.index >= self.local_ccxs.len() {
-            return None;
-        }
+        loop {
+            if self.index >= self.local_ccxs.len() {
+                return None;
+            }
 
-        let index = self.index;
-        self.index += 1;
+            let index = self.index;
+            self.index += 1;
 
-        Some(CrateContext {
-            shared: self.shared,
-            index: index,
-            local_ccxs: self.local_ccxs,
-        })
+            let ccx = CrateContext {
+                shared: self.shared,
+                index: index,
+                local_ccxs: self.local_ccxs,
+            };
+
+            if
+                self.filter_to_previous_work_product_unavail &&
+                ccx.previous_work_product().is_some()
+            {
+                continue;
+            }
+
+            return Some(ccx);
+        }
     }
 }
 
@@ -510,6 +540,7 @@ impl<'b, 'tcx> SharedCrateContext<'b, 'tcx> {
 impl<'tcx> LocalCrateContext<'tcx> {
     fn new<'a>(shared: &SharedCrateContext<'a, 'tcx>,
                codegen_unit: CodegenUnit<'tcx>,
+               previous_work_product: Option<WorkProduct>,
                symbol_map: Rc<SymbolMap<'tcx>>)
            -> LocalCrateContext<'tcx> {
         unsafe {
@@ -521,7 +552,7 @@ impl<'tcx> LocalCrateContext<'tcx> {
             // crashes if the module identifier is same as other symbols
             // such as a function name in the module.
             // 1. http://llvm.org/bugs/show_bug.cgi?id=11479
-            let llmod_id = format!("{}.rs", codegen_unit.name);
+            let llmod_id = format!("{}.rs", codegen_unit.name());
 
             let (llcx, llmod) = create_context_and_module(&shared.tcx.sess,
                                                           &llmod_id[..]);
@@ -535,6 +566,7 @@ impl<'tcx> LocalCrateContext<'tcx> {
             let local_ccx = LocalCrateContext {
                 llmod: llmod,
                 llcx: llcx,
+                previous_work_product: previous_work_product,
                 codegen_unit: codegen_unit,
                 tn: TypeNames::new(),
                 needs_unwind_cleanup_cache: RefCell::new(FnvHashMap()),
@@ -694,6 +726,10 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> {
         self.local().llcx
     }
 
+    pub fn previous_work_product(&self) -> Option<&WorkProduct> {
+        self.local().previous_work_product.as_ref()
+    }
+
     pub fn codegen_unit(&self) -> &CodegenUnit<'tcx> {
         &self.local().codegen_unit
     }
diff --git a/src/librustc_trans/glue.rs b/src/librustc_trans/glue.rs
index ef7d0ea165d..6bc48546dfa 100644
--- a/src/librustc_trans/glue.rs
+++ b/src/librustc_trans/glue.rs
@@ -239,7 +239,7 @@ fn get_drop_glue_core<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                     Falling back to on-demand instantiation.",
                     g,
                     TransItem::DropGlue(g).to_raw_string(),
-                    ccx.codegen_unit().name);
+                    ccx.codegen_unit().name());
 
             ccx.stats().n_fallback_instantiations.set(ccx.stats()
                                                          .n_fallback_instantiations
diff --git a/src/librustc_trans/lib.rs b/src/librustc_trans/lib.rs
index fa0a1fdc375..81a1dbeb7fe 100644
--- a/src/librustc_trans/lib.rs
+++ b/src/librustc_trans/lib.rs
@@ -37,6 +37,8 @@
 #![feature(unicode)]
 #![feature(question_mark)]
 
+use rustc::dep_graph::WorkProduct;
+
 extern crate arena;
 extern crate flate;
 extern crate getopts;
@@ -86,6 +88,7 @@ mod macros;
 mod abi;
 mod adt;
 mod asm;
+mod assert_module_sources;
 mod attributes;
 mod base;
 mod basic_block;
@@ -132,7 +135,27 @@ mod value;
 
 #[derive(Clone)]
 pub struct ModuleTranslation {
+    /// The name of the module. When the crate may be saved between
+    /// compilations, incremental compilation requires that name be
+    /// unique amongst **all** crates.  Therefore, it should contain
+    /// something unique to this crate (e.g., a module path) as well
+    /// as the crate name and disambiguator.
     pub name: String,
+    pub symbol_name_hash: u64,
+    pub source: ModuleSource,
+}
+
+#[derive(Clone)]
+pub enum ModuleSource {
+    /// Copy the `.o` files or whatever from the incr. comp. directory.
+    Preexisting(WorkProduct),
+
+    /// Rebuild from this LLVM module.
+    Translated(ModuleLlvm),
+}
+
+#[derive(Copy, Clone)]
+pub struct ModuleLlvm {
     pub llcx: llvm::ContextRef,
     pub llmod: llvm::ModuleRef,
 }
diff --git a/src/librustc_trans/monomorphize.rs b/src/librustc_trans/monomorphize.rs
index 00c0e911035..96a05f11bfd 100644
--- a/src/librustc_trans/monomorphize.rs
+++ b/src/librustc_trans/monomorphize.rs
@@ -52,7 +52,7 @@ pub fn monomorphic_fn<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
         debug!("leaving monomorphic fn {:?}", instance);
         return (val, mono_ty);
     } else {
-        assert!(!ccx.codegen_unit().items.contains_key(&TransItem::Fn(instance)));
+        assert!(!ccx.codegen_unit().contains_item(&TransItem::Fn(instance)));
     }
 
     debug!("monomorphic_fn({:?})", instance);
diff --git a/src/librustc_trans/partitioning.rs b/src/librustc_trans/partitioning.rs
index 8073359ede8..ade6e8abeb3 100644
--- a/src/librustc_trans/partitioning.rs
+++ b/src/librustc_trans/partitioning.rs
@@ -119,12 +119,15 @@
 use collector::InliningMap;
 use llvm;
 use monomorphize;
+use rustc::dep_graph::{DepNode, WorkProductId};
 use rustc::hir::def_id::DefId;
 use rustc::hir::map::DefPathData;
 use rustc::session::config::NUMBERED_CODEGEN_UNIT_MARKER;
 use rustc::ty::TyCtxt;
 use rustc::ty::item_path::characteristic_def_id_of_type;
 use std::cmp::Ordering;
+use std::hash::{Hash, Hasher, SipHasher};
+use std::sync::Arc;
 use symbol_map::SymbolMap;
 use syntax::ast::NodeId;
 use syntax::parse::token::{self, InternedString};
@@ -140,11 +143,59 @@ pub enum PartitioningStrategy {
 }
 
 pub struct CodegenUnit<'tcx> {
-    pub name: InternedString,
-    pub items: FnvHashMap<TransItem<'tcx>, llvm::Linkage>,
+    /// A name for this CGU. Incremental compilation requires that
+    /// name be unique amongst **all** crates.  Therefore, it should
+    /// contain something unique to this crate (e.g., a module path)
+    /// as well as the crate name and disambiguator.
+    name: InternedString,
+
+    items: FnvHashMap<TransItem<'tcx>, llvm::Linkage>,
 }
 
 impl<'tcx> CodegenUnit<'tcx> {
+    pub fn new(name: InternedString,
+               items: FnvHashMap<TransItem<'tcx>, llvm::Linkage>)
+               -> Self {
+        CodegenUnit {
+            name: name,
+            items: items,
+        }
+    }
+
+    pub fn empty(name: InternedString) -> Self {
+        Self::new(name, FnvHashMap())
+    }
+
+    pub fn contains_item(&self, item: &TransItem<'tcx>) -> bool {
+        self.items.contains_key(item)
+    }
+
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    pub fn items(&self) -> &FnvHashMap<TransItem<'tcx>, llvm::Linkage> {
+        &self.items
+    }
+
+    pub fn work_product_id(&self) -> Arc<WorkProductId> {
+        Arc::new(WorkProductId(self.name().to_string()))
+    }
+
+    pub fn work_product_dep_node(&self) -> DepNode<DefId> {
+        DepNode::WorkProduct(self.work_product_id())
+    }
+
+    pub fn compute_symbol_name_hash(&self, tcx: TyCtxt, symbol_map: &SymbolMap) -> u64 {
+        let mut state = SipHasher::new();
+        let all_items = self.items_in_deterministic_order(tcx, symbol_map);
+        for (item, _) in all_items {
+            let symbol_name = symbol_map.get(item).unwrap();
+            symbol_name.hash(&mut state);
+        }
+        state.finish()
+    }
+
     pub fn items_in_deterministic_order(&self,
                                         tcx: TyCtxt,
                                         symbol_map: &SymbolMap)
@@ -277,10 +328,7 @@ fn place_root_translation_items<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             };
 
             let make_codegen_unit = || {
-                CodegenUnit {
-                    name: codegen_unit_name.clone(),
-                    items: FnvHashMap(),
-                }
+                CodegenUnit::empty(codegen_unit_name.clone())
             };
 
             let mut codegen_unit = codegen_units.entry(codegen_unit_name.clone())
@@ -319,10 +367,7 @@ fn place_root_translation_items<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     if codegen_units.is_empty() {
         let codegen_unit_name = InternedString::new(FALLBACK_CODEGEN_UNIT);
         codegen_units.entry(codegen_unit_name.clone())
-                     .or_insert_with(|| CodegenUnit {
-                         name: codegen_unit_name.clone(),
-                         items: FnvHashMap(),
-                     });
+                     .or_insert_with(|| CodegenUnit::empty(codegen_unit_name.clone()));
     }
 
     PreInliningPartitioning {
@@ -362,10 +407,8 @@ fn merge_codegen_units<'tcx>(initial_partitioning: &mut PreInliningPartitioning<
     // we reach the target count
     while codegen_units.len() < target_cgu_count {
         let index = codegen_units.len();
-        codegen_units.push(CodegenUnit {
-            name: numbered_codegen_unit_name(crate_name, index),
-            items: FnvHashMap()
-        });
+        codegen_units.push(
+            CodegenUnit::empty(numbered_codegen_unit_name(crate_name, index)));
     }
 }
 
@@ -381,10 +424,8 @@ fn place_inlined_translation_items<'tcx>(initial_partitioning: PreInliningPartit
             follow_inlining(*root, inlining_map, &mut reachable);
         }
 
-        let mut new_codegen_unit = CodegenUnit {
-            name: codegen_unit.name.clone(),
-            items: FnvHashMap(),
-        };
+        let mut new_codegen_unit =
+            CodegenUnit::empty(codegen_unit.name.clone());
 
         // Add all translation items that are not already there
         for trans_item in reachable {
@@ -560,10 +601,9 @@ fn single_codegen_unit<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         items.insert(trans_item, linkage);
     }
 
-    CodegenUnit {
-        name: numbered_codegen_unit_name(&tcx.crate_name[..], 0),
-        items: items
-    }
+    CodegenUnit::new(
+        numbered_codegen_unit_name(&tcx.crate_name[..], 0),
+        items)
 }
 
 fn numbered_codegen_unit_name(crate_name: &str, index: usize) -> InternedString {
diff --git a/src/librustc_trans/trans_item.rs b/src/librustc_trans/trans_item.rs
index fc95d208f32..fc2758e50f2 100644
--- a/src/librustc_trans/trans_item.rs
+++ b/src/librustc_trans/trans_item.rs
@@ -23,12 +23,12 @@ use glue::DropGlueKind;
 use llvm;
 use monomorphize::{self, Instance};
 use inline;
+use rustc::dep_graph::DepNode;
 use rustc::hir;
 use rustc::hir::map as hir_map;
 use rustc::hir::def_id::DefId;
 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc::ty::subst;
-use rustc::dep_graph::DepNode;
 use rustc_const_eval::fatal_const_eval_err;
 use std::hash::{Hash, Hasher};
 use syntax::ast::{self, NodeId};
@@ -68,16 +68,27 @@ impl<'tcx> Hash for TransItem<'tcx> {
 impl<'a, 'tcx> TransItem<'tcx> {
 
     pub fn define(&self, ccx: &CrateContext<'a, 'tcx>) {
-
         debug!("BEGIN IMPLEMENTING '{} ({})' in cgu {}",
                   self.to_string(ccx.tcx()),
                   self.to_raw_string(),
-                  ccx.codegen_unit().name);
+                  ccx.codegen_unit().name());
+
+        // (*) This code executes in the context of a dep-node for the
+        // entire CGU. In some cases, we introduce dep-nodes for
+        // particular items that we are translating (these nodes will
+        // have read edges coming into the CGU node). These smaller
+        // nodes are not needed for correctness -- we always
+        // invalidate an entire CGU at a time -- but they enable
+        // finer-grained testing, since you can write tests that check
+        // that the incoming edges to a particular fn are from a
+        // particular set.
 
         self.register_reads(ccx);
 
         match *self {
             TransItem::Static(node_id) => {
+                let def_id = ccx.tcx().map.local_def_id(node_id);
+                let _task = ccx.tcx().dep_graph.in_task(DepNode::TransCrateItem(def_id)); // (*)
                 let item = ccx.tcx().map.expect_item(node_id);
                 if let hir::ItemStatic(_, m, ref expr) = item.node {
                     match consts::trans_static(&ccx, m, expr, item.id, &item.attrs) {
@@ -93,6 +104,9 @@ impl<'a, 'tcx> TransItem<'tcx> {
                 }
             }
             TransItem::Fn(instance) => {
+                let _task = ccx.tcx().dep_graph.in_task(
+                    DepNode::TransCrateItem(instance.def)); // (*)
+
                 base::trans_instance(&ccx, instance);
             }
             TransItem::DropGlue(dg) => {
@@ -103,7 +117,7 @@ impl<'a, 'tcx> TransItem<'tcx> {
         debug!("END IMPLEMENTING '{} ({})' in cgu {}",
                self.to_string(ccx.tcx()),
                self.to_raw_string(),
-               ccx.codegen_unit().name);
+               ccx.codegen_unit().name());
     }
 
     /// If necessary, creates a subtask for trans'ing a particular item and registers reads on
@@ -152,7 +166,7 @@ impl<'a, 'tcx> TransItem<'tcx> {
         debug!("BEGIN PREDEFINING '{} ({})' in cgu {}",
                self.to_string(ccx.tcx()),
                self.to_raw_string(),
-               ccx.codegen_unit().name);
+               ccx.codegen_unit().name());
 
         let symbol_name = ccx.symbol_map()
                              .get_or_compute(ccx.shared(), *self);
@@ -174,7 +188,7 @@ impl<'a, 'tcx> TransItem<'tcx> {
         debug!("END PREDEFINING '{} ({})' in cgu {}",
                self.to_string(ccx.tcx()),
                self.to_raw_string(),
-               ccx.codegen_unit().name);
+               ccx.codegen_unit().name());
     }
 
     fn predefine_static(ccx: &CrateContext<'a, 'tcx>,
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 27485ee65fc..f80f25a2e77 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -481,6 +481,16 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
                                         is just used for rustc unit tests \
                                         and will never be stable",
                                        cfg_fn!(rustc_attrs))),
+    ("rustc_partition_reused", Whitelisted, Gated("rustc_attrs",
+                                                  "this attribute \
+                                                   is just used for rustc unit tests \
+                                                   and will never be stable",
+                                                  cfg_fn!(rustc_attrs))),
+    ("rustc_partition_translated", Whitelisted, Gated("rustc_attrs",
+                                                      "this attribute \
+                                                       is just used for rustc unit tests \
+                                                       and will never be stable",
+                                                      cfg_fn!(rustc_attrs))),
     ("rustc_symbol_name", Whitelisted, Gated("rustc_attrs",
                                              "internal rustc attributes will never be stable",
                                              cfg_fn!(rustc_attrs))),
diff --git a/src/test/incremental/rlib_cross_crate/auxiliary/a.rs b/src/test/incremental/rlib_cross_crate/auxiliary/a.rs
new file mode 100644
index 00000000000..ff5fd634714
--- /dev/null
+++ b/src/test/incremental/rlib_cross_crate/auxiliary/a.rs
@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// no-prefer-dynamic
+
+#![crate_type="rlib"]
+
+#[cfg(rpass1)]
+pub type X = u32;
+
+#[cfg(rpass2)]
+pub type X = i32;
+
+// this version doesn't actually change anything:
+#[cfg(rpass3)]
+pub type X = i32;
+
+pub type Y = char;
diff --git a/src/test/incremental/rlib_cross_crate/b.rs b/src/test/incremental/rlib_cross_crate/b.rs
new file mode 100644
index 00000000000..55398370425
--- /dev/null
+++ b/src/test/incremental/rlib_cross_crate/b.rs
@@ -0,0 +1,38 @@
+// Copyright 2014 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.
+
+// Same test as `type_alias_cross_crate`, but with
+// `no-prefer-dynamic`, ensuring that we test what happens when we
+// build rlibs (before we were only testing dylibs, which meant we
+// didn't realize we had to preserve a `bc` file as well).
+
+// aux-build:a.rs
+// revisions:rpass1 rpass2 rpass3
+// no-prefer-dynamic
+
+
+#![feature(rustc_attrs)]
+
+extern crate a;
+
+#[rustc_dirty(label="TypeckItemBody", cfg="rpass2")]
+#[rustc_clean(label="TypeckItemBody", cfg="rpass3")]
+pub fn use_X() -> u32 {
+    let x: a::X = 22;
+    x as u32
+}
+
+#[rustc_clean(label="TypeckItemBody", cfg="rpass2")]
+#[rustc_clean(label="TypeckItemBody", cfg="rpass3")]
+pub fn use_Y() {
+    let x: a::Y = 'c';
+}
+
+pub fn main() { }
diff --git a/src/test/incremental/spike-neg1.rs b/src/test/incremental/spike-neg1.rs
new file mode 100644
index 00000000000..b00c68a184e
--- /dev/null
+++ b/src/test/incremental/spike-neg1.rs
@@ -0,0 +1,62 @@
+// Copyright 2014 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.
+
+// A variant of the first "spike" test that serves to test the
+// `rustc_partition_reused` and `rustc_partition_translated` tests.
+// Here we change and say that the `x` module will be reused (when in
+// fact it will not), and then indicate that the test itself
+// should-fail (because an error will be reported, and hence the
+// revision rpass2 will not compile, despite being named rpass).
+
+// revisions:rpass1 rpass2
+// should-fail
+
+#![feature(rustc_attrs)]
+
+#![rustc_partition_reused(module="spike_neg1", cfg="rpass2")]
+#![rustc_partition_reused(module="spike_neg1-x", cfg="rpass2")] // this is wrong!
+#![rustc_partition_reused(module="spike_neg1-y", cfg="rpass2")]
+
+mod x {
+    pub struct X {
+        x: u32, y: u32,
+    }
+
+    #[cfg(rpass1)]
+    fn make() -> X {
+        X { x: 22, y: 0 }
+    }
+
+    #[cfg(rpass2)]
+    fn make() -> X {
+        X { x: 11, y: 11 }
+    }
+
+    pub fn new() -> X {
+        make()
+    }
+
+    pub fn sum(x: &X) -> u32 {
+        x.x + x.y
+    }
+}
+
+mod y {
+    use x;
+
+    pub fn assert_sum() -> bool {
+        let x = x::new();
+        x::sum(&x) == 22
+    }
+}
+
+pub fn main() {
+    y::assert_sum();
+}
diff --git a/src/test/incremental/spike-neg2.rs b/src/test/incremental/spike-neg2.rs
new file mode 100644
index 00000000000..472d11d7f90
--- /dev/null
+++ b/src/test/incremental/spike-neg2.rs
@@ -0,0 +1,62 @@
+// Copyright 2014 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.
+
+// A variant of the first "spike" test that serves to test the
+// `rustc_partition_reused` and `rustc_partition_translated` tests.
+// Here we change and say that the `y` module will be translated (when
+// in fact it will not), and then indicate that the test itself
+// should-fail (because an error will be reported, and hence the
+// revision rpass2 will not compile, despite being named rpass).
+
+// revisions:rpass1 rpass2
+// should-fail
+
+#![feature(rustc_attrs)]
+
+#![rustc_partition_reused(module="spike_neg2", cfg="rpass2")]
+#![rustc_partition_translated(module="spike_neg2-x", cfg="rpass2")]
+#![rustc_partition_translated(module="spike_neg2-y", cfg="rpass2")] // this is wrong!
+
+mod x {
+    pub struct X {
+        x: u32, y: u32,
+    }
+
+    #[cfg(rpass1)]
+    fn make() -> X {
+        X { x: 22, y: 0 }
+    }
+
+    #[cfg(rpass2)]
+    fn make() -> X {
+        X { x: 11, y: 11 }
+    }
+
+    pub fn new() -> X {
+        make()
+    }
+
+    pub fn sum(x: &X) -> u32 {
+        x.x + x.y
+    }
+}
+
+mod y {
+    use x;
+
+    pub fn assert_sum() -> bool {
+        let x = x::new();
+        x::sum(&x) == 22
+    }
+}
+
+pub fn main() {
+    y::assert_sum();
+}
diff --git a/src/test/incremental/spike.rs b/src/test/incremental/spike.rs
new file mode 100644
index 00000000000..68af20d4191
--- /dev/null
+++ b/src/test/incremental/spike.rs
@@ -0,0 +1,63 @@
+// Copyright 2014 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.
+
+// A first "spike" for incremental compilation: here, we change the
+// content of the `make` function, and we find that we can reuse the
+// `y` module entirely (but not the `x` module).
+
+// revisions:rpass1 rpass2
+
+#![feature(rustc_attrs)]
+
+#![rustc_partition_reused(module="spike", cfg="rpass2")]
+#![rustc_partition_translated(module="spike-x", cfg="rpass2")]
+#![rustc_partition_reused(module="spike-y", cfg="rpass2")]
+
+mod x {
+    pub struct X {
+        x: u32, y: u32,
+    }
+
+    #[cfg(rpass1)]
+    fn make() -> X {
+        X { x: 22, y: 0 }
+    }
+
+    #[cfg(rpass2)]
+    fn make() -> X {
+        X { x: 11, y: 11 }
+    }
+
+    #[rustc_dirty(label="TypeckItemBody", cfg="rpass2")]
+    #[rustc_clean(label="ItemSignature", cfg="rpass2")]
+    pub fn new() -> X {
+        make()
+    }
+
+    #[rustc_clean(label="TypeckItemBody", cfg="rpass2")]
+    #[rustc_clean(label="ItemSignature", cfg="rpass2")]
+    pub fn sum(x: &X) -> u32 {
+        x.x + x.y
+    }
+}
+
+mod y {
+    use x;
+
+    #[rustc_clean(label="TypeckItemBody", cfg="rpass2")]
+    pub fn assert_sum() -> bool {
+        let x = x::new();
+        x::sum(&x) == 22
+    }
+}
+
+pub fn main() {
+    y::assert_sum();
+}
diff --git a/src/test/run-make/execution-engine/test.rs b/src/test/run-make/execution-engine/test.rs
index 2e90b518432..b58295d47f2 100644
--- a/src/test/run-make/execution-engine/test.rs
+++ b/src/test/run-make/execution-engine/test.rs
@@ -20,6 +20,7 @@ extern crate rustc_metadata;
 extern crate rustc_resolve;
 extern crate rustc_errors;
 extern crate rustc_errors as errors;
+extern crate rustc_trans;
 #[macro_use] extern crate syntax;
 
 use std::ffi::{CStr, CString};
@@ -37,6 +38,7 @@ use rustc::session::build_session;
 use rustc_driver::{driver, abort_on_err};
 use rustc_resolve::MakeGlobMap;
 use rustc_metadata::cstore::CStore;
+use rustc_trans::ModuleSource;
 use libc::c_void;
 
 use rustc_errors::registry::Registry;
@@ -261,7 +263,10 @@ fn compile_program(input: &str, sysroot: PathBuf)
                 .filter_map(|(_, p)| p).collect();
 
             assert_eq!(trans.modules.len(), 1);
-            let llmod = trans.modules[0].llmod;
+            let llmod = match trans.modules[0].source {
+                ModuleSource::Preexisting(_) => unimplemented!(),
+                ModuleSource::Translated(llvm) => llvm.llmod,
+            };
 
             // Workaround because raw pointers do not impl Send
             let modp = llmod as usize;
diff --git a/src/test/run-make/llvm-phase/test.rs b/src/test/run-make/llvm-phase/test.rs
index 402b5ed8355..19e410fef53 100644
--- a/src/test/run-make/llvm-phase/test.rs
+++ b/src/test/run-make/llvm-phase/test.rs
@@ -13,11 +13,13 @@
 extern crate rustc;
 extern crate rustc_driver;
 extern crate rustc_llvm;
+extern crate rustc_trans;
 #[macro_use] extern crate syntax;
 extern crate getopts;
 
 use rustc_driver::{CompilerCalls, Compilation};
 use rustc_driver::driver::CompileController;
+use rustc_trans::ModuleSource;
 use rustc::session::Session;
 use syntax::codemap::FileLoader;
 use std::io;
@@ -51,7 +53,10 @@ impl<'a> CompilerCalls<'a> for JitCalls {
             state.session.abort_if_errors();
             let trans = state.trans.unwrap();
             assert_eq!(trans.modules.len(), 1);
-            let rs_llmod = trans.modules[0].llmod;
+            let rs_llmod = match trans.modules[0].source {
+                ModuleSource::Preexisting(_) => unimplemented!(),
+                ModuleSource::Translated(llvm) => llvm.llmod,
+            };
             unsafe { rustc_llvm::LLVMDumpModule(rs_llmod) };
         });
         cc