arbitrary expressions in key-value attributes are unstable

[aliemjay]

The key-value attributes, which are used by the new syntax (#99), accept only literals on stable compiler. Trying to use any other expression yields the following error message:

error[E0658]: arbitrary expressions in key-value attributes are unstable
note: see issue #78835 rust-lang/rust#78835 for more information

This makes it impossible to set trivial defaults such as None or &[].

Here is the test code:

// rustc 1.51
// rstest 0.7.0
use rstest::{fixture, rstest};

// Old syntax is OK
#[fixture(val=1+1)]
fn echo(val: i32) -> i32 {
    val
}

// New syntax fails
#[fixture]
fn echo2(#[default=1+1] val: i32) -> i32 {
    val
}

[la10736]

Ok, I missed it. I'll switch to #[deafult( 1 + 1 )] syntax till the syntax become stable.