libexpr: don't dynamic_cast Exprs after parsing is done

we'll want to wrap some exprs for debug purposes, and dynamic casts
cannot look through such wrappers. dedicated casting functions can.

Change-Id: I1fba0ec52d281a1b8de85a62e4948bfae536bcfc

lix/libexpr/flake/flake.cc

@@ -249,7 +249,7 @@ static Flake getFlake(
     Expr & flakeExpr = state.ctx.parseExprFromFile(resolvedFlakeFile);
 
     // Enforce that 'flake.nix' is a direct attrset, not a computation.
-    if (!(dynamic_cast<ExprAttrs *>(&flakeExpr))) {
+    if (!flakeExpr.try_cast<ExprAttrs>()) {
         state.ctx.errors.make<EvalError>("file '%s' must be an attribute set", resolvedFlakeFile).debugThrow();
     }
 

lix/libexpr/nixexpr.cc

@@ -122,8 +122,8 @@ void ExprAttrs::addBindingsToJSON(JSON & out, const SymbolTable & symbols) const
             out["inherit"][symbols[i->first]] = i->second.e->toJSON(symbols);
             break;
         case AttrDef::Kind::InheritedFrom: {
-            auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
-            auto & from = dynamic_cast<ExprInheritFrom &>(*select.e);
+            auto & select = i->second.e->cast<ExprSelect>();
+            auto & from = select.e->cast<ExprInheritFrom>();
             inheritsFrom[from.displ].push_back(i->first);
             break;
         }

lix/libexpr/nixexpr.hh

@@ -65,6 +65,18 @@ public:
     virtual Value * maybeThunk(EvalState & state, Env & env);
     virtual void setName(Symbol name);
     PosIdx getPos() const { return pos; }
+
+    template<typename E>
+    E & cast()
+    {
+        return dynamic_cast<E &>(*this);
+    }
+
+    template<typename E>
+    E * try_cast()
+    {
+        return dynamic_cast<E *>(this);
+    }
 };
 
 struct ExprLiteral : Expr

lix/libexpr/value.cc

@@ -41,10 +41,10 @@ bool Value::isTrivial() const
         internalType != tApp
         && internalType != tPrimOpApp
         && (internalType != tThunk
-            || (dynamic_cast<ExprSet *>(thunk.expr)
+            || (thunk.expr->try_cast<ExprSet>()
                 && static_cast<ExprSet *>(thunk.expr)->dynamicAttrs.empty())
-            || dynamic_cast<ExprLambda *>(thunk.expr)
-            || dynamic_cast<ExprList *>(thunk.expr));
+            || thunk.expr->try_cast<ExprLambda>()
+            || thunk.expr->try_cast<ExprList>());
 }
 
 PrimOp * Value::primOpAppPrimOp() const

lix/nix/flake.cc

@@ -456,10 +456,10 @@ struct CmdFlakeCheck : FlakeCommand
                 if (!v.isLambda()) {
                     throw Error("overlay is not a function, but %s instead", showType(v));
                 }
-                auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body.get());
+                auto body = v.lambda.fun->body->try_cast<ExprLambda>();
                 if (!body)
                     throw Error("overlay is not a function with two arguments, but only takes one");
-                if (dynamic_cast<ExprLambda *>(body->body.get()))
+                if (body->body->try_cast<ExprLambda>())
                     throw Error("overlay is not a function with two arguments, but takes more than two");
                 // FIXME: if we have a 'nixpkgs' input, use it to
                 // evaluate the overlay.