Rollup merge of #41734 - nikomatsakis:incr-comp-refactor-variance, r=pnkfelix

Refactor variance and remove last `[pub]` map

This PR refactors variance to work in a more red-green friendly way. Because red-green doesn't exist yet, it has to be a bit hacky. The basic idea is this:

- We compute a big map with the variance for all items in the crate; when you request variances for a particular item, we read it from the crate
- We now hard-code that traits are invariant (which they are, for deep reasons, not gonna' change)
- When building constraints, we compute the transitive closure of all things within the crate that depend on what using `TransitiveRelation`
    - this lets us gin up the correct dependencies when requesting variance of a single item

Ah damn, just remembered, one TODO:

- [x] Update the variance README -- ah, I guess the README updates I did are sufficient

r? @michaelwoerister

1 files changed, 3 insertions, 3 deletions

diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 125026b799c..796cb8c4d65 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -240,8 +240,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 }
 
 impl<'a, 'b: 'a, 'tcx: 'b> EntryBuilder<'a, 'b, 'tcx> {
-    fn encode_item_variances(&mut self, def_id: DefId) -> LazySeq<ty::Variance> {
-        debug!("EntryBuilder::encode_item_variances({:?})", def_id);
+    fn encode_variances_of(&mut self, def_id: DefId) -> LazySeq<ty::Variance> {
+        debug!("EntryBuilder::encode_variances_of({:?})", def_id);
         let tcx = self.tcx;
         self.lazy_seq_from_slice(&tcx.variances_of(def_id))
     }
@@ -824,7 +824,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> EntryBuilder<'a, 'b, 'tcx> {
                 hir::ItemEnum(..) |
                 hir::ItemStruct(..) |
                 hir::ItemUnion(..) |
-                hir::ItemTrait(..) => self.encode_item_variances(def_id),
+                hir::ItemTrait(..) => self.encode_variances_of(def_id),
                 _ => LazySeq::empty(),
             },
             generics: match item.node {