Remove async feature of wit-bindgen-rust-macros (#1595)

This removes the `async` feature which is purely used for an error
message and that's it. While the error message is useful the way that
the feature is activated results in an unfortunate interaction with
Cargo dependencies and how lock files are generated. Notably the
conditional `a?/b` syntax means that even when a Cargo feature is
disabled the full dependency tree shows up in lock files (for now). The
dependency tree of `wit-bindgen-rust-macros` is quite large, which is
why it's an optional feature, so this is undesirable.

Author: alexcrichton

crates/guest-rust/Cargo.toml

@@ -38,7 +38,7 @@ default = ["macros", "realloc", "async", "std", "bitflags", "macro-string"]
 macros = ["dep:wit-bindgen-rust-macro"]
 realloc = []
 std = []
-async = ["wit-bindgen-rust-macro?/async"]
+async = []
 bitflags = ["dep:bitflags"]
 async-spawn = ['async', 'dep:futures', 'std']
 futures-stream = ['async', 'dep:futures']

crates/guest-rust/macro/Cargo.toml

@@ -27,5 +27,4 @@ macro-string = { workspace = true, optional = true }
 
 [features]
 default = ["macro-string"]
-async = []
 macro-string = ["dep:macro-string"]

crates/guest-rust/macro/src/lib.rs

@@ -163,12 +163,6 @@ impl Parse for Config {
                             return Err(Error::new(span, "cannot specify second async config"));
                         }
                         async_configured = true;
-                        if val.any_enabled() && !cfg!(feature = "async") {
-                            return Err(Error::new(
-                                span,
-                                "must enable `async` feature to enable async imports and/or exports",
-                            ));
-                        }
                         opts.async_ = val;
                     }
                     Opt::EnableMethodChaining(enable) => {