Remove full-syntax feature

The primary use-case of full-syntax (complex discriminants) is now
covered by syn even without full, so this feature is unnecessary.

Since bumping the MSRV for syn to 1.56 is a breaking change, we can drop
the full-syntax feature at the same time.

Cargo.toml

@@ -22,9 +22,6 @@ syn = "2"
 num = "0.3"
 num-traits = "0.2"
 
-[features]
-full-syntax = ["syn/full"]
-
 [lib]
 name = "num_derive"
 proc-macro = true

ci/test_full.sh

@@ -27,7 +27,7 @@ if ! check_version $MSRV ; then
   exit 1
 fi
 
-FEATURES=(full-syntax)
+FEATURES=()
 echo "Testing supported features: ${FEATURES[*]}"
 
 set -x

src/lib.rs

@@ -71,19 +71,11 @@ use quote::quote;
 use syn::{Data, Fields, Ident};
 
 /// Try to parse the tokens, or else return a compilation error
-/// suggesting "full-syntax" if that's not already enabled.
 macro_rules! parse {
     ($tokens:ident as $type:ty) => {
         match syn::parse::<$type>($tokens) {
             Ok(parsed) => parsed,
-            Err(mut error) => {
-                if cfg!(not(feature = "full-syntax")) {
-                    let hint = syn::Error::new(
-                        Span::call_site(),
-                        r#"this might need the "full-syntax" feature of `num-derive`"#,
-                    );
-                    error.combine(hint);
-                }
+            Err(error) => {
                 return TokenStream::from(error.to_compile_error());
             }
         }

tests/with_custom_values.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![cfg(feature = "full-syntax")]
-
 extern crate num as num_renamed;
 #[macro_use]
 extern crate num_derive;