fix derive-inside-macro

The `syn` crate differentiates between `[u64; 4]` and `[u64; $limbs]` inside
a macro, previously only the former case was handled. This commit fixes.

Also, adding the "full" feature to syn in the ff_derive Cargo.toml is needed
in order to run cargo check on this crate.

ff_derive/Cargo.toml

@@ -22,7 +22,7 @@ num-traits = "0.2"
 num-integer = "0.1"
 proc-macro2 = "1"
 quote = "1"
-syn = "1"
+syn = { version = "1", features = ["full"] }
 
 [badges]
 maintenance = { status = "passively-maintained" }

ff_derive/src/lib.rs

@@ -252,11 +252,16 @@ fn validate_struct(ast: &syn::DeriveInput, limbs: usize) -> Option<proc_macro2::
     }
 
     // The array's length should be a literal int equal to `limbs`.
-    let lit_int = match match &arr.len {
-        syn::Expr::Lit(expr_lit) => match &expr_lit.lit {
-            syn::Lit::Int(lit_int) => Some(lit_int),
+    let expr_lit = match &arr.len {
+        syn::Expr::Lit(expr_lit) => Some(&expr_lit.lit),
+        syn::Expr::Group(expr_group) => match &*expr_group.expr {
+            syn::Expr::Lit(expr_lit) => Some(&expr_lit.lit),
             _ => None,
-        },
+        }
+        _ => None
+    };
+    let lit_int = match match expr_lit {
+        Some(syn::Lit::Int(lit_int)) => Some(lit_int),
         _ => None,
     } {
         Some(x) => x,