Introduce new variance inference pass that replaces (and generalizes) old

region-parameterization/variance inference. We now compute variance for
type parameters but do not make use of it (most of the way towards rust-lang#3598).

Author: nikomatsakis

src/librustc/metadata/common.rs

@@ -89,7 +89,7 @@ pub static tag_path_elt_name: uint = 0x43u;
 pub static tag_item_field: uint = 0x44u;
 pub static tag_struct_mut: uint = 0x45u;
 
-pub static tag_region_param: uint = 0x46u;
+pub static tag_item_variances: uint = 0x46;
 pub static tag_mod_impl_trait: uint = 0x47u;
 /*
   trait items contain tag_item_trait_method elements,

src/librustc/metadata/csearch.rs

@@ -14,7 +14,6 @@
 use metadata::common::*;
 use metadata::cstore;
 use metadata::decoder;
-use metadata;
 use middle::ty;
 use middle::typeck;
 
@@ -144,6 +143,12 @@ pub fn get_trait_method_def_ids(cstore: @mut cstore::CStore,
     decoder::get_trait_method_def_ids(cdata, def.node)
 }
 
+pub fn get_item_variances(cstore: @mut cstore::CStore,
+                          def: ast::DefId) -> ty::ItemVariances {
+    let cdata = cstore::get_crate_data(cstore, def.crate);
+    decoder::get_item_variances(cdata, def.node)
+}
+
 pub fn get_provided_trait_methods(tcx: ty::ctxt,
                                   def: ast::DefId)
                                -> ~[@ty::Method] {

src/librustc/metadata/decoder.rs

@@ -1088,6 +1088,14 @@ pub fn get_trait_method_def_ids(cdata: Cmd,
     result
 }
 
+pub fn get_item_variances(cdata: Cmd, id: ast::NodeId) -> ty::ItemVariances {
+    let data = cdata.data;
+    let item_doc = lookup_item(id, data);
+    let variance_doc = reader::get_doc(item_doc, tag_item_variances);
+    let mut decoder = reader::Decoder(variance_doc);
+    Decodable::decode(&mut decoder)
+}
+
 pub fn get_provided_trait_methods(intr: @ident_interner, cdata: Cmd,
                                   id: ast::NodeId, tcx: ty::ctxt) ->
         ~[@ty::Method] {

src/librustc/metadata/encoder.rs

@@ -211,6 +211,15 @@ fn encode_region_param_defs(ebml_w: &mut writer::Encoder,
     }
 }
 
+fn encode_item_variances(ebml_w: &mut writer::Encoder,
+                         ecx: &EncodeContext,
+                         id: ast::NodeId) {
+    let v = ty::item_variances(ecx.tcx, ast_util::local_def(id));
+    ebml_w.start_tag(tag_item_variances);
+    v.encode(ebml_w);
+    ebml_w.end_tag();
+}
+
 fn encode_bounds_and_type(ebml_w: &mut writer::Encoder,
                           ecx: &EncodeContext,
                           tpt: &ty::ty_param_bounds_and_ty) {
@@ -992,6 +1001,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         ebml_w.start_tag(tag_items_data_item);
         encode_def_id(ebml_w, def_id);
         encode_family(ebml_w, 't');
+        encode_item_variances(ebml_w, ecx, item.id);
         encode_bounds_and_type(ebml_w, ecx, &lookup_item_type(tcx, def_id));
         encode_name(ecx, ebml_w, item.ident);
         encode_attributes(ebml_w, item.attrs);
@@ -1032,6 +1042,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         encode_family(ebml_w, 'S');
         encode_bounds_and_type(ebml_w, ecx, &lookup_item_type(tcx, def_id));
 
+        encode_item_variances(ebml_w, ecx, item.id);
         encode_name(ecx, ebml_w, item.ident);
         encode_attributes(ebml_w, item.attrs);
         encode_path(ecx, ebml_w, path, ast_map::path_name(item.ident));
@@ -1138,6 +1149,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         ebml_w.start_tag(tag_items_data_item);
         encode_def_id(ebml_w, def_id);
         encode_family(ebml_w, 'I');
+        encode_item_variances(ebml_w, ecx, item.id);
         let trait_def = ty::lookup_trait_def(tcx, def_id);
         encode_ty_type_param_defs(ebml_w, ecx,
                                   trait_def.generics.type_param_defs,