pattern matching - Cases with Alternatives failed

Consider this list of rules

rules = {a -> 1, b -> 2, c -> 3}

I'd like to fish some of the rules out of such a list using Cases. All is well if I hard code the Alternatives to fish out:

Cases[rules, HoldPattern[a|b -> _]]

{a -> 1, b -> 2}

But, if I try to dynamically generate the Alternatives from data, it fails:

targets = {a, b}
Cases[rules, HoldPattern[Alternatives @@ targets -> _]]

{}

I've tried the following shots-in-the-dark

Cases[rules, HoldPattern[Evaluate[Alternatives @@ targets] -> _]]

{}

Cases[rules, HoldPattern[ReleaseHold[Alternatives @@ targets] -> _]]

{}

MMA 10.0.2.0 on Mac Mavericks.

Answer

HoldPattern has attribute HoldAll

Attributes[HoldPattern]
(* {HoldAll, Protected} *)

So one can use the following injection

Cases[rules, HoldPattern[Alternatives@## -> _] & @@ targets]
(* {a -> 1, b -> 2} *)

P.S. Conceptually, HoldPattern is here the wrong tool for the job, even if this works. See Leonid's answer.

Comments

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