the trait `ToTokens` is not implemented for `VariantDefaultAttribute`

[idersan]

from libbpf-rs dependence ^v0.5, so download v0.5.1 to use.

error message:

error[E0277]: the trait bound `VariantDefaultAttribute: ToTokens` is not satisfied
  --> external/rust/crates/num_enum/num_enum_derive/src/lib.rs:81:18
   |
81 | impl Spanned for VariantDefaultAttribute {
   |                  ^^^^^^^^^^^^^^^^^^^^^^^ the trait `ToTokens` is not implemented for `VariantDefaultAttribute`
   |
   = help: the following other types implement trait `ToTokens`:
             &'a T
             &'a mut T
             Abi
             Abstract
             AndAnd
             AndEq
             AngleBracketedGenericArguments
             Arm
           and 301 others
   = note: required for `VariantDefaultAttribute` to implement `quote::spanned::Spanned`
   = note: required for `VariantDefaultAttribute` to implement `syn::spanned::private::Sealed`
note: required by a bound in `syn::spanned::Spanned`
  --> ./external/rust/crates/syn/src/spanned.rs:96:1

error[E0277]: the trait bound `VariantAlternativesAttribute: ToTokens` is not satisfied
   --> external/rust/crates/num_enum/num_enum_derive/src/lib.rs:106:18
    |
106 | impl Spanned for VariantAlternativesAttribute {
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `ToTokens` is not implemented for `VariantAlternativesAttribute`
    |
    = help: the following other types implement trait `ToTokens`:
              &'a T
              &'a mut T
              Abi
              Abstract
              AndAnd
              AndEq
              AngleBracketedGenericArguments
              Arm
            and 301 others
    = note: required for `VariantAlternativesAttribute` to implement `quote::spanned::Spanned`
    = note: required for `VariantAlternativesAttribute` to implement `syn::spanned::private::Sealed`
note: required by a bound in `syn::spanned::Spanned`
   --> ./external/rust/crates/syn/src/spanned.rs:96:1

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.
13:41:23 ninja failed with: exit status 1

#### failed to build some targets (3 seconds) ####

[idersan]

problem solved: change syn to 1.0.x.