Add some timing info to rustdoc

There are various improvements, but the main one is to time each pass
that rustdoc performs (`rustdoc::passes`).

Before, these were the top five timings for `cargo doc` on the cargo
repository:

```
+---------------------------------+-----------+-----------------+----------+------------+
| Item                            | Self time | % of total time | Time     | Item count |
+---------------------------------+-----------+-----------------+----------+------------+
| <unknown>                       | 854.70ms  | 20.888          | 2.47s    | 744823     |
+---------------------------------+-----------+-----------------+----------+------------+
| expand_crate                    | 795.29ms  | 19.436          | 848.00ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| metadata_decode_entry           | 256.73ms  | 6.274           | 279.49ms | 518344     |
+---------------------------------+-----------+-----------------+----------+------------+
| resolve_crate                   | 240.56ms  | 5.879           | 242.86ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| hir_lowering                    | 146.79ms  | 3.587           | 146.79ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
```

Now the timings are:

```
+---------------------------------+-----------+-----------------+----------+------------+
| Item                            | Self time | % of total time | Time     | Item count |
+---------------------------------+-----------+-----------------+----------+------------+
| <unknown>                       | 1.40s     | 22.662          | 3.73s    | 771430     |
+---------------------------------+-----------+-----------------+----------+------------+
| collect-trait-impls             | 1.34s     | 21.672          | 2.87s    | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| expand_crate                    | 1.21s     | 19.577          | 1.28s    | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| build extern trait impl         | 704.66ms  | 11.427          | 1.07s    | 21893      |
+---------------------------------+-----------+-----------------+----------+------------+
| metadata_decode_entry           | 354.84ms  | 5.754           | 391.81ms | 544919     |
+---------------------------------+-----------+-----------------+----------+------------+
```

src/librustdoc/core.rs

@@ -407,7 +407,9 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                 let hir = tcx.hir();
                 let body = hir.body(hir.body_owned_by(hir.local_def_id_to_hir_id(def_id)));
                 debug!("visiting body for {:?}", def_id);
+                tcx.sess.time("emit ignored resolution errors", || {
                 EmitIgnoredResolutionErrors::new(tcx).visit_body(body);
+                });
                 (rustc_interface::DEFAULT_QUERY_PROVIDERS.typeck)(tcx, def_id)
             };
         }),
@@ -430,6 +432,7 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                 // intra-doc-links
                 resolver.borrow_mut().access(|resolver| {
                     for extern_name in &extern_names {
+                        sess.time("load extern crate", || {
                         resolver
                             .resolve_str_path_error(
                                 DUMMY_SP,
@@ -440,6 +443,7 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                             .unwrap_or_else(|()| {
                                 panic!("Unable to resolve external crate {}", extern_name)
                             });
+                        });
                     }
                 });
 
@@ -480,7 +484,9 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                     tcx.ensure().check_mod_attrs(local_def_id);
                 }
 
-                let access_levels = tcx.privacy_access_levels(LOCAL_CRATE);
+                let access_levels = tcx
+                    .sess
+                    .time("privacy_access_levels", || tcx.privacy_access_levels(LOCAL_CRATE));
                 // Convert from a HirId set to a DefId set since we don't always have easy access
                 // to the map from defid -> hirid
                 let access_levels = AccessLevels {
@@ -519,7 +525,7 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                 };
                 debug!("crate: {:?}", tcx.hir().krate());
 
-                let mut krate = clean::krate(&mut ctxt);
+                let mut krate = tcx.sess.time("clean crate", || clean::krate(&mut ctxt));
 
                 if let Some(ref m) = krate.module {
                     if let None | Some("") = m.doc_value() {
@@ -618,7 +624,7 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
                     };
                     if run {
                         debug!("running pass {}", p.pass.name);
-                        krate = (p.pass.run)(krate, &ctxt);
+                        krate = ctxt.tcx.sess.time(p.pass.name, || (p.pass.run)(krate, &ctxt));
                     }
                 }
 

src/librustdoc/passes/collect_trait_impls.rs

@@ -29,7 +29,9 @@ pub fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
 
     for &cnum in cx.tcx.crates().iter() {
         for &(did, _) in cx.tcx.all_trait_implementations(cnum).iter() {
+            cx.tcx.sess.time("build extern trait impl", || {
                 inline::build_impl(cx, did, None, &mut new_items);
+            });
         }
     }
 
@@ -87,7 +89,9 @@ pub fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
     for &trait_did in cx.tcx.all_traits(LOCAL_CRATE).iter() {
         for &impl_node in cx.tcx.hir().trait_impls(trait_did) {
             let impl_did = cx.tcx.hir().local_def_id(impl_node);
+            cx.tcx.sess.time("build local trait impl", || {
                 inline::build_impl(cx, impl_did.to_def_id(), None, &mut new_items);
+            });
         }
     }