shuffle ownership of `external_traits`

constraints:

- clean/inline.rs needs this map to fill in traits when inlining
- fold.rs needs this map to allow passes to fold trait items
- html/render.rs needs this map to seed the Cache.traits map of all
  known traits

The first two are the real problem, since `DocFolder` only operates on
`clean::Crate` but `clean/inline.rs` only sees the `DocContext`. The
introduction of early passes means that these two now exist at the same
time, so they need to share ownership of the map. Even better, the use
of `Crate` in a rustc thread pool means that it needs to be Sync, so it
can't use `Lrc<Lock>` to manually activate thread-safety.

`parking_lot` is reused from elsewhere in the tree to allow use of its
`ReentrantMutex`, as the relevant parts of rustdoc are still
single-threaded and this allows for easier use in that context.

src/Cargo.lock

@@ -2443,6 +2443,7 @@ name = "rustdoc"
 version = "0.0.0"
 dependencies = [
  "minifier 0.0.19 (registry+https://github.com/rust-lang/crates.io-index)",
+ "parking_lot 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "pulldown-cmark 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "tempfile 3.0.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]

src/librustdoc/Cargo.toml

@@ -11,3 +11,4 @@ path = "lib.rs"
 pulldown-cmark = { version = "0.1.2", default-features = false }
 minifier = "0.0.19"
 tempfile = "3"
+parking_lot = "0.6.4"

src/librustdoc/clean/inline.rs

@@ -539,10 +539,13 @@ pub fn record_extern_trait(cx: &DocContext, did: DefId) {
         return;
     }
 
-    if cx.external_traits.borrow().contains_key(&did) ||
-        cx.active_extern_traits.borrow().contains(&did)
     {
-        return;
+        let external_traits = cx.external_traits.lock();
+        if external_traits.borrow().contains_key(&did) ||
+            cx.active_extern_traits.borrow().contains(&did)
+        {
+            return;
+        }
     }
 
     cx.active_extern_traits.borrow_mut().push(did);
@@ -550,6 +553,9 @@ pub fn record_extern_trait(cx: &DocContext, did: DefId) {
     debug!("record_extern_trait: {:?}", did);
     let trait_ = build_external_trait(cx, did);
 
-    cx.external_traits.borrow_mut().insert(did, trait_);
+    {
+        let external_traits = cx.external_traits.lock();
+        external_traits.borrow_mut().insert(did, trait_);
+    }
     cx.active_extern_traits.borrow_mut().remove_item(&did);
 }

src/librustdoc/clean/mod.rs

@@ -55,6 +55,8 @@ use std::cell::RefCell;
 use std::sync::Arc;
 use std::u32;
 
+use parking_lot::ReentrantMutex;
+
 use core::{self, DocContext};
 use doctree;
 use visit_ast;
@@ -136,7 +138,7 @@ pub struct Crate {
     pub primitives: Vec<(DefId, PrimitiveType, Attributes)>,
     // These are later on moved into `CACHEKEY`, leaving the map empty.
     // Only here so that they can be filtered through the rustdoc passes.
-    pub external_traits: FxHashMap<DefId, Trait>,
+    pub external_traits: Arc<ReentrantMutex<RefCell<FxHashMap<DefId, Trait>>>>,
     pub masked_crates: FxHashSet<CrateNum>,
 }
 
@@ -214,7 +216,7 @@ impl<'a, 'tcx, 'rcx, 'cstore> Clean<Crate> for visit_ast::RustdocVisitor<'a, 'tc
             module: Some(module),
             externs,
             primitives,
-            external_traits: Default::default(),
+            external_traits: cx.external_traits.clone(),
             masked_crates,
         }
     }

src/librustdoc/core.rs

@@ -36,11 +36,13 @@ use syntax::symbol::keywords;
 use syntax_pos::DUMMY_SP;
 use errors;
 use errors::emitter::{Emitter, EmitterWriter};
+use parking_lot::ReentrantMutex;
 
 use std::cell::RefCell;
 use std::mem;
 use rustc_data_structures::sync::{self, Lrc};
 use std::rc::Rc;
+use std::sync::Arc;
 use std::path::PathBuf;
 
 use visit_ast::RustdocVisitor;
@@ -66,7 +68,7 @@ pub struct DocContext<'a, 'tcx: 'a, 'rcx: 'a, 'cstore: 'rcx> {
     /// Later on moved into `html::render::CACHE_KEY`
     pub renderinfo: RefCell<RenderInfo>,
     /// Later on moved through `clean::Crate` into `html::render::CACHE_KEY`
-    pub external_traits: RefCell<FxHashMap<DefId, clean::Trait>>,
+    pub external_traits: Arc<ReentrantMutex<RefCell<FxHashMap<DefId, clean::Trait>>>>,
     /// Used while populating `external_traits` to ensure we don't process the same trait twice at
     /// the same time.
     pub active_extern_traits: RefCell<Vec<DefId>>,
@@ -599,8 +601,6 @@ pub fn run_core(search_paths: SearchPaths,
 
             ctxt.sess().abort_if_errors();
 
-            krate.external_traits = ctxt.external_traits.into_inner();
-
             (krate, ctxt.renderinfo.into_inner(), passes)
         }), &sess)
     })

src/librustdoc/fold.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::mem;
-
 use clean::*;
 
 pub struct StripItem(pub Item);
@@ -116,11 +114,14 @@ pub trait DocFolder : Sized {
     fn fold_crate(&mut self, mut c: Crate) -> Crate {
         c.module = c.module.take().and_then(|module| self.fold_item(module));
 
-        let traits = mem::replace(&mut c.external_traits, Default::default());
-        c.external_traits.extend(traits.into_iter().map(|(k, mut v)| {
-            v.items = v.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
-            (k, v)
-        }));
+        {
+            let guard = c.external_traits.lock();
+            let traits = guard.replace(Default::default());
+            guard.borrow_mut().extend(traits.into_iter().map(|(k, mut v)| {
+                v.items = v.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
+                (k, v)
+            }));
+        }
         c
     }
 }

src/librustdoc/html/render.rs

@@ -606,7 +606,7 @@ pub fn run(mut krate: clean::Crate,
         crate_version: krate.version.take(),
         orphan_impl_items: Vec::new(),
         orphan_trait_impls: Vec::new(),
-        traits: mem::replace(&mut krate.external_traits, FxHashMap()),
+        traits: krate.external_traits.lock().replace(FxHashMap()),
         deref_trait_did,
         deref_mut_trait_did,
         owned_box_did,

src/librustdoc/lib.rs

@@ -49,6 +49,7 @@ extern crate rustc_errors as errors;
 extern crate pulldown_cmark;
 extern crate tempfile;
 extern crate minifier;
+extern crate parking_lot;
 
 extern crate serialize as rustc_serialize; // used by deriving
 

src/librustdoc/passes/strip_hidden.rs

@@ -24,13 +24,9 @@ pub const STRIP_HIDDEN: Pass =
                 "strips all doc(hidden) items from the output");
 
 /// Strip items marked `#[doc(hidden)]`
-pub fn strip_hidden(mut krate: clean::Crate, cx: &DocContext) -> clean::Crate {
+pub fn strip_hidden(krate: clean::Crate, _: &DocContext) -> clean::Crate {
     let mut retained = DefIdSet();
 
-    // as an early pass, the external traits haven't been swapped in, so we need to do that ahead
-    // of time
-    mem::swap(&mut krate.external_traits, &mut cx.external_traits.borrow_mut());
-
     // strip all #[doc(hidden)] items
     let krate = {
         let mut stripper = Stripper{ retained: &mut retained, update_retained: true };
@@ -39,8 +35,7 @@ pub fn strip_hidden(mut krate: clean::Crate, cx: &DocContext) -> clean::Crate {
 
     // strip all impls referencing stripped items
     let mut stripper = ImplStripper { retained: &retained };
-    let mut krate = stripper.fold_crate(krate);
-    mem::swap(&mut krate.external_traits, &mut cx.external_traits.borrow_mut());
+    let krate = stripper.fold_crate(krate);
 
     krate
 }