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

fn suggest_restriction<'tcx>(
    tcx: TyCtxt<'tcx>, 
    generics: &Generics<'tcx>, 
    msg: &str, 
    err: &mut DiagnosticBuilder<'_>, 
    fn_sig: Option<&FnSig<'_>>, 
    projection: Option<&ProjectionTy<'_>>, 
    trait_ref: PolyTraitRef<'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.