How can I evaluate only the (bound) variables in an expression?

Question

Given an arbitrary expression, how can I have Mathematica only evaluate the (bound) symbols, i.e. replace them by their respective values, but not do any further simplification/evaluation of the expression? Trace shows me that indeed the evaluation of the symbols is done first, but how to stop the standard evaluation procedure at this point?

Example:

a = 7
expr = d + a + 3

What I would like to obtain is d+7+3

I tried

HoldForm[expr] /. x_Symbol -> Evaluate[x]

but this gives the plain HoldForm of the expression. Symbol, by the way, is apparently not the Head of a bound symbol. In this case it is Integer. Apart from that, the replacement rule with the selected evaluation does not work at all. But I can't find anything better.

Answer 1

Update: For the general case where you don't know which symbols have values you can use the methods from this answer by WReach and this by Leonid Shiffrin in combination with ValueQ:

a = 7; b = 5;
Defer[d + a + b + 3] /. s_Symbol :> RuleCondition[s, ValueQ[s]]
Defer[d + a + b + 3] /. s_Symbol /; ValueQ@s :> RuleCondition[s] 
Defer[d + a + b + 3] /. s_Symbol :> With[{e = s}, e /; ValueQ[s]] 
Defer[d + a + b + 3] /. s_Symbol /; ValueQ@s :> With[{e = s}, e /; True] 

and the same expressions with HoldForm in place of Defer all give

d + 7 + 5 + 3

Original answer:

expr = With[{a = a}, Defer[d + a + 3]] 

d + 7 + 3

Also

Defer[d + # + 3] & @ a 
With[{a = a}, HoldForm[d + a + 3]]
HoldForm[d + # + 3] & @ a

and

Inactivate[d+a+3]