Error with hax_lib::fstar::before on impl method

[maximebuyse]

struct Test();

impl Test {
    #[hax_lib::fstar::before(r"assume val v: i32")]
    fn f() {}
}

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With the rust code above, when extracting to F*, I get the following error:

error: `const` items in this context need a name
 --> src/lib.rs:5:5
  |
5 |     #[hax_lib::fstar::before(r"assume val v: i32")]
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `_` is not a valid name for this `const` item
  |
  = note: this error originates in the attribute macro `hax_lib::fstar::before` (in Nightly builds, run with -Z macro-backtrace for more info)

The generated F* corresponds to what is expected so that's not a big problem but it would be better if we didn't get the error.