Auto merge of #45867 - michaelwoerister:check-ich-stability, r=nikomatsakis

incr.comp.: Verify stability of incr. comp. hashes and clean up various other things.

The main contribution of this PR is that it adds the `-Z incremental-verify-ich` functionality. Normally, when the red-green tracking system determines that a certain query result has not changed, it does not re-compute the incr. comp. hash (ICH) for that query result because that hash is already known. `-Z incremental-verify-ich` tells the compiler to re-hash the query result and compare the new hash against the cached hash. This is a rather thorough way of
- testing hashing implementation stability,
- finding missing `[input]` annotations on `DepNodes`, and
- finding missing read-edges,

since both a missed read and a missing `[input]` annotation can lead to something being marked as green instead of red and thus will have a different hash than it should have.

Case in point, implementing this verification logic and activating it for all `src/test/incremental` tests has revealed several such oversights, all of which are fixed in this PR.

r? @nikomatsakis

src/librustc/middle/cstore.rs

@@ -273,7 +273,7 @@ pub trait CrateStore {
     fn item_children_untracked(&self, did: DefId, sess: &Session) -> Vec<def::Export>;
     fn load_macro_untracked(&self, did: DefId, sess: &Session) -> LoadedMacro;
     fn extern_mod_stmt_cnum_untracked(&self, emod_id: ast::NodeId) -> Option<CrateNum>;
-    fn item_generics_cloned_untracked(&self, def: DefId) -> ty::Generics;
+    fn item_generics_cloned_untracked(&self, def: DefId, sess: &Session) -> ty::Generics;
     fn associated_item_cloned_untracked(&self, def: DefId) -> ty::AssociatedItem;
     fn postorder_cnums_untracked(&self) -> Vec<CrateNum>;
 
@@ -327,7 +327,7 @@ impl CrateStore for DummyCrateStore {
         { bug!("crate_data_as_rc_any") }
     // item info
     fn visibility_untracked(&self, def: DefId) -> ty::Visibility { bug!("visibility") }
-    fn item_generics_cloned_untracked(&self, def: DefId) -> ty::Generics
+    fn item_generics_cloned_untracked(&self, def: DefId, sess: &Session) -> ty::Generics
         { bug!("item_generics_cloned") }
 
     // trait/impl-item info

src/librustc/middle/resolve_lifetime.rs

@@ -1001,8 +1001,12 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                 &map.object_lifetime_defaults[&id]
             } else {
                 let cstore = self.cstore;
+                let sess = self.sess;
                 self.xcrate_object_lifetime_defaults.entry(def_id).or_insert_with(|| {
-                    cstore.item_generics_cloned_untracked(def_id).types.into_iter().map(|def| {
+                    cstore.item_generics_cloned_untracked(def_id, sess)
+                          .types
+                          .into_iter()
+                          .map(|def| {
                         def.object_lifetime_default
                     }).collect()
                 })