diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 45ab423c18..5ae5a1e3cf 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -82,8 +82,6 @@ void copyContext(const Value & v, PathSet & context);
    paths. */
 typedef std::map<Path, Path> SrcToStore;
 
-struct EvalState;
-
 
 std::ostream & operator << (std::ostream & str, const Value & v);
 
@@ -216,9 +214,9 @@ private:
 
     inline Value * lookupVar(Env * env, const ExprVar & var, bool noEval);
 
-    friend class ExprVar;
-    friend class ExprAttrs;
-    friend class ExprLet;
+    friend struct ExprVar;
+    friend struct ExprAttrs;
+    friend struct ExprLet;
 
     Expr * parse(const char * text, const Path & path,
         const Path & basePath, StaticEnv & staticEnv);
@@ -278,9 +276,9 @@ private:
     typedef std::map<Pos, unsigned int> AttrSelects;
     AttrSelects attrSelects;
 
-    friend class ExprOpUpdate;
-    friend class ExprOpConcatLists;
-    friend class ExprSelect;
+    friend struct ExprOpUpdate;
+    friend struct ExprOpConcatLists;
+    friend struct ExprSelect;
     friend void prim_getAttr(EvalState & state, Value * * args, Value & v);
 };
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index bc6993477c..7586fffb83 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -48,7 +48,7 @@ std::ostream & operator << (std::ostream & str, const Pos & pos);
 
 struct Env;
 struct Value;
-struct EvalState;
+class EvalState;
 struct StaticEnv;
 struct Expr;
 
@@ -71,6 +71,7 @@ string showAttrPath(const AttrPath & attrPath);
 
 struct Expr
 {
+    virtual ~Expr() { };
     virtual void show(std::ostream & str);
     virtual void bindVars(const StaticEnv & env);
     virtual void eval(EvalState & state, Env & env, Value & v);
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index d89d92c7a2..8a084fea06 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -252,7 +252,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
   nix::Formals * formals;
   nix::Formal * formal;
   nix::NixInt n;
-  char * id; // !!! -> Symbol
+  const char * id; // !!! -> Symbol
   char * path;
   char * uri;
   std::vector<nix::AttrName> * attrNames;
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 9825827934..2feb2f9492 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -22,13 +22,13 @@ typedef enum {
 } ValueType;
 
 
-struct Bindings;
+class Bindings;
 struct Env;
 struct Expr;
 struct ExprLambda;
 struct PrimOp;
 struct PrimOp;
-struct Symbol;
+class Symbol;
 
 
 typedef long NixInt;
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 4329d9a22b..1b379752fa 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -75,12 +75,9 @@ using std::map;
 static string pathNullDevice = "/dev/null";
 
 
-static const uid_t rootUserId = 0;
-
-
 /* Forward definition. */
 class Worker;
-class HookInstance;
+struct HookInstance;
 
 
 /* A pointer to a goal. */
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index d212259f3b..79bd7d56b3 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -22,8 +22,6 @@ static string gcLockName = "gc.lock";
 static string tempRootsDir = "temproots";
 static string gcRootsDir = "gcroots";
 
-static const int defaultGcLevel = 1000;
-
 
 /* Acquire the global GC lock.  This is used to prevent new Nix
    processes from starting after the temporary root files have been