Function rustc_trait_selection::traits::error_reporting::suggestions::suggest_restriction

fn suggest_restriction<'tcx>(
    tcx: TyCtxt<'tcx>,
    item_id: LocalDefId,
    hir_generics: &Generics<'tcx>,
    msg: &str,
    err: &mut Diagnostic,
    fn_sig: Option<&FnSig<'_>>,
    projection: Option<&AliasTy<'_>>,
    trait_pred: PolyTraitPredicate<'tcx>,
    super_traits: Option<(&Ident, &GenericBounds<'_>)>
)

Type parameter needs more bounds. The trivial case is T where T: Bound, but it can also be an impl Trait param that needs to be decomposed to a type param for cleaner code.