Good question!
Basically, skeletest uses both a preprocessor and GHC plugin. One of the things the plugin does is search for and replace P.con with an internal conMatches function before the typechecking phase.
-- P.con User{name = P.eq "user1", email = P.contains "@"}
-- ====>
-- P.conMatches
-- "User"
-- (Just (HCons (Const "user") $ HCons (Const "email") $ HNil))
-- ( \case
-- User{name, email} -> Just (HCons (pure name) $ HCons (pure email) $ HNil)
-- _ -> Nothing
-- )
-- (HCons (P.eq "user1") $ HCons (P.contains "@") $ HNil)
replaceConMatch :: Ctx -> HsExpr GhcRn -> HsExpr GhcRn
replaceConMatch ctx e =
case getExpr e of
-- Matches:
-- P.con User{name = ...}
-- P.con (User "...")
HsExprApps (getExpr -> HsExprVar name) [arg]
| isCon name ->
convertCon arg
-- Matches:
-- P.con $ User "..."
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