Report introduced names that are associated with a thing but never referenced:

• variables in patterns
• let declared values and functions
• module declared values, functions, type aliases and choice types

An unused stray binding might be a sign that someone wanted to use it for something but didn't do so, yet. If that's not the case, explicitly say so (usually by replacing the variable with _ or removing the declaration)

not reported

a =
    \used ->
        used


b used =
    used


c (Variant ( _, { used } )) =
    used


d argument =
    case argument of
        used ->
            used


e =
    let
        { used } =
            { used = 0 }
    in
    used


f =
    let
        used =
            1
    in
    used

reported

a =
    \unused ->
        ""


b unused =
    ""


c (Variant ( _, { unused } )) =
    ""


d argument =
    case argument of
        unused ->
            ""


e =
    let
        { unused } =
            x
    in
    ""


f =
    let
        unused =
            1
    in
    ""


type ChoiceTypeUnused
    = VariantUnused


type alias TypeALiasUnused =
    String