evaluate inherit (from) exprs only once per directive

desugaring inherit-from to syntactic duplication of the source expr also
duplicates side effects of the source expr (such as trace calls) and
expensive computations (such as derivationStrict).

(cherry picked from commit cefd0302b55b3360dbca59cfcb4bf6a750d6cdcf)
Change-Id: Iff519f991adef2e51683ba2c552d37a3df7a179e

src/libexpr/parser.y

@@ -319,14 +319,17 @@ binds
     }
   | binds INHERIT '(' expr ')' attrs ';'
     { $$ = $1;
-      /* !!! Should ensure sharing of the expression in $4. */
+      if (!$$->inheritFromExprs)
+          $$->inheritFromExprs = std::make_unique<std::vector<Expr *>>();
+      $$->inheritFromExprs->push_back($4);
+      auto from = new nix::ExprInheritFrom(state->at(@4), $$->inheritFromExprs->size() - 1);
       for (auto & i : *$6) {
           if ($$->attrs.find(i.symbol) != $$->attrs.end())
               state->dupAttr(i.symbol, state->at(@6), $$->attrs[i.symbol].pos);
           $$->attrs.emplace(
               i.symbol,
               ExprAttrs::AttrDef(
-                  new ExprSelect(CUR_POS, $4, i.symbol),
+                  new ExprSelect(CUR_POS, from, i.symbol),
                   state->at(@6),
                   ExprAttrs::AttrDef::Kind::InheritedFrom));
       }