Add feature flag to switch functionality of count

This feature flag is off by default meaning excluding disabled variants
from the COUNT is disabled by default.

Sometime in the future, this flag could be switched on by default and
thus introduce the breaking change.

strum/Cargo.toml

@@ -27,6 +27,7 @@ travis-ci = { repository = "Peternator7/strum" }
 [features]
 default = ["std"]
 derive = ["strum_macros"]
+enum-count-excludes-disabled = ["strum_macros/enum-count-excludes-disabled"]
 std = []
 
 [package.metadata.docs.rs]

strum_macros/Cargo.toml

@@ -18,6 +18,10 @@ readme = "../README.md"
 proc-macro = true
 name = "strum_macros"
 
+[features]
+default = []
+enum-count-excludes-disabled = []
+
 [dependencies]
 heck = "0.4.1"
 proc-macro2 = "1.0"

strum_macros/src/macros/enum_count.rs

@@ -2,11 +2,15 @@ use proc_macro2::TokenStream;
 use quote::quote;
 use syn::{Data, DeriveInput};
 
+#[cfg(feature = "enum-count-excludes-disabled")]
 use crate::helpers::variant_props::HasStrumVariantProperties;
 use crate::helpers::{non_enum_error, HasTypeProperties};
 
 pub(crate) fn enum_count_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
     let n = match &ast.data {
+        #[cfg(not(feature = "enum-count-excludes-disabled"))]
+        Data::Enum(v) => v.variants.len(),
+        #[cfg(feature = "enum-count-excludes-disabled")]
         Data::Enum(v) => v.variants.iter().try_fold(0usize, |acc, v| {
             if v.get_variant_properties()?.disabled.is_none() {
                 Ok::<usize, syn::Error>(acc + 1usize)

strum_tests/Cargo.toml

@@ -10,7 +10,7 @@ test_phf = ["strum/phf"]
 
 [dependencies]
 strum = { path = "../strum", features = ["derive"] }
-strum_macros = { path = "../strum_macros", features = [] }
+strum_macros = { path = "../strum_macros", features = ["enum-count-excludes-disabled"] }
 clap = "=2.33.0"
 enum_variant_type = "=0.2.0"
 structopt = "0.2.18"