use the smallest possible size int for descriminant reading/writing

bitbuffer_derive/src/lib.rs

@@ -144,7 +144,7 @@ use quote::{quote, quote_spanned};
 use syn::spanned::Spanned;
 use syn::{
     parse_macro_input, parse_quote, parse_str, Attribute, Data, DataStruct, DeriveInput, Expr,
-    Fields, GenericParam, Ident, Lit, LitStr, Path,
+    Fields, GenericParam, Ident, Lit, LitInt, LitStr, Path,
 };
 use syn_util::get_attribute_value;
 
@@ -414,14 +414,16 @@ fn parse(data: Data, struct_name: &Ident, attrs: &[Attribute], unchecked: bool)
 
                 let discriminant_token: TokenStream = match Discriminant::from(variant) {
                     Discriminant::Int(discriminant) => {
+                        let lit = LitInt::new(&format!("{}", discriminant), span);
                         last_discriminant = discriminant as isize;
-                        quote_spanned! { span => #discriminant }
+                        quote_spanned! { span => #lit }
                     }
                     Discriminant::Wildcard => quote_spanned! { span => _ },
                     Discriminant::Default => {
                         let new_discriminant = (last_discriminant + 1) as usize;
+                        let lit = LitInt::new(&format!("{}", new_discriminant), span);
                         last_discriminant += 1;
-                        quote_spanned! { span => #new_discriminant }
+                        quote_spanned! { span => #lit }
                     }
                 };
                 quote_spanned! {span=>
@@ -431,13 +433,15 @@ fn parse(data: Data, struct_name: &Ident, attrs: &[Attribute], unchecked: bool)
 
             let span = data.enum_token.span();
 
+            let repr = repr_for_bits(discriminant_bits);
+
             let enum_name = Lit::Str(LitStr::new(&struct_name.to_string(), struct_name.span()));
             quote_spanned! {span=>
-                let discriminant:usize = stream.read_int(#discriminant_bits as usize)?;
+                let discriminant:#repr = stream.read_int(#discriminant_bits as usize)?;
                 Ok(match discriminant {
                     #(#match_arms)*
                     _ => {
-                        return Err(::bitbuffer::BitError::UnmatchedDiscriminant{discriminant, enum_name: #enum_name.to_string()})
+                        return Err(::bitbuffer::BitError::UnmatchedDiscriminant{discriminant: discriminant as usize, enum_name: #enum_name.to_string()})
                     }
                 })
             }
@@ -555,3 +559,15 @@ fn get_field_size(attrs: &[Attribute], span: Span) -> Option<TokenStream> {
             })
         })
 }
+
+fn repr_for_bits(discriminant_bits: u64) -> TokenStream {
+    if discriminant_bits <= 8 {
+        quote!(u8)
+    } else if discriminant_bits <= 16 {
+        quote!(u16)
+    } else if discriminant_bits <= 32 {
+        quote!(u32)
+    } else {
+        quote!(usize)
+    }
+}

bitbuffer_derive/src/write.rs

@@ -1,11 +1,11 @@
 use crate::discriminant::Discriminant;
-use crate::size;
+use crate::{repr_for_bits, size};
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, quote_spanned};
 use syn::spanned::Spanned;
 use syn::{
     parse_macro_input, parse_quote, parse_str, Attribute, Data, DataStruct, DeriveInput, Expr,
-    Fields, GenericParam, Ident, Index, Lit, Member, Path, Type,
+    Fields, GenericParam, Ident, Index, Lit, LitInt, Member, Path, Type,
 };
 use syn_util::get_attribute_value;
 
@@ -182,17 +182,20 @@ fn write(data: Data, struct_name: &Ident, attrs: &[Attribute]) -> TokenStream {
 
                 let discriminant_token: TokenStream = match Discriminant::from(variant) {
                     Discriminant::Int(discriminant) => {
+                        let lit = LitInt::new(&format!("{}", discriminant), span);
                         last_discriminant = discriminant as isize;
-                        quote_spanned! { span => #discriminant }
+                        quote_spanned! { span => #lit }
                     }
                     Discriminant::Wildcard => {
                         let free_discriminant = max_discriminant + 1;
-                        quote_spanned! { span => #free_discriminant }
+                        let lit = LitInt::new(&format!("{}", free_discriminant), span);
+                        quote_spanned! { span => #lit }
                     }
                     Discriminant::Default => {
                         let new_discriminant = (last_discriminant + 1) as usize;
+                        let lit = LitInt::new(&format!("{}", new_discriminant), span);
                         last_discriminant += 1;
-                        quote_spanned! { span => #new_discriminant }
+                        quote_spanned! { span => #lit }
                     }
                 };
 
@@ -240,12 +243,13 @@ fn write(data: Data, struct_name: &Ident, attrs: &[Attribute]) -> TokenStream {
             });
 
             let span = data.enum_token.span();
+            let repr = repr_for_bits(discriminant_bits);
 
             quote_spanned! {span=>
-                let discriminant = match &self {
+                let discriminant:#repr = match &self {
                     #(#discriminant_value),*
                 };
-                __target__stream.write_int(discriminant as usize, #discriminant_bits as usize)?;
+                __target__stream.write_int(discriminant, #discriminant_bits as usize)?;
                 match &self {
                     #(#write_inner)*
                 }

bitbuffer_derive/tests/expand.rs

@@ -3,11 +3,13 @@
 #![allow(unused_imports)]
 
 use bitbuffer::{BitReadStream, Endianness};
-use bitbuffer_derive::{BitWrite, BitWriteSized};
+use bitbuffer_derive::{BitRead, BitWrite, BitWriteSized};
 
 #[derive(BitWrite)]
-struct TestSizeExpression {
-    size: u8,
-    #[size = "size + 2"]
-    str: String,
+#[discriminant_bits = 4]
+enum TestEnumRest {
+    Foo,
+    Bar,
+    #[discriminant = "_"]
+    Asd,
 }