resolve: Use item_name and opt_parent in Resolver::get_module

This is a cleanup that doesn't introduce new query calls, but this way `def_key` is decoded twice which may matter for performance or may not

Author: petrochenkov

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -27,7 +27,6 @@ use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
 use rustc_metadata::creader::LoadedMacro;
 use rustc_middle::metadata::ModChild;
 use rustc_middle::{bug, ty};
-use rustc_session::cstore::CrateStore;
 use rustc_span::hygiene::{ExpnId, LocalExpnId, MacroKind};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::Span;
@@ -118,20 +117,14 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             let def_kind = self.cstore().def_kind(def_id);
             match def_kind {
                 DefKind::Mod | DefKind::Enum | DefKind::Trait => {
-                    let def_key = self.cstore().def_key(def_id);
-                    let parent = def_key.parent.map(|index| {
-                        self.get_nearest_non_block_module(DefId { index, krate: def_id.krate })
-                    });
-                    let name = if let Some(cnum) = def_id.as_crate_root() {
-                        self.cstore().crate_name(cnum)
-                    } else {
-                        def_key.disambiguated_data.data.get_opt_name().expect("module without name")
-                    };
-
+                    let parent = self
+                        .tcx
+                        .opt_parent(def_id)
+                        .map(|parent_id| self.get_nearest_non_block_module(parent_id));
                     let expn_id = self.cstore().module_expansion_untracked(def_id, &self.tcx.sess);
                     Some(self.new_module(
                         parent,
-                        ModuleKind::Def(def_kind, def_id, name),
+                        ModuleKind::Def(def_kind, def_id, self.tcx.item_name(def_id)),
                         expn_id,
                         self.def_span(def_id),
                         // FIXME: Account for `#[no_implicit_prelude]` attributes.