Pick discriminants automatically when unspecified

Closes #21

README.md

@@ -20,6 +20,7 @@ enumflags2 = "^0.6"
 ## Features
 
 - [x] Uses enums to represent individual flags—a set of flags is a separate type from a single flag.
+- [x] Automatically chooses a free bit when you don't specify.
 - [x] Detects incorrect BitFlags at compile time.
 - [x] Has a similar API compared to the popular [bitflags](https://crates.io/crates/bitflags) crate.
 - [x] Does not expose the generated types explicity. The user interacts exclusively with `struct BitFlags<Enum>;`.
@@ -37,7 +38,7 @@ use enumflags2::{bitflags, make_bitflags, BitFlags};
 enum Test {
     A = 0b0001,
     B = 0b0010,
-    C = 0b0100,
+    C, // unspecified variants pick unused bits automatically
     D = 0b1000,
 }
 

enumflags_derive/src/lib.rs

@@ -9,21 +9,28 @@ use syn::{
     parse::{Parse, ParseStream},
     parse_macro_input,
     spanned::Spanned,
-    Ident, Item, ItemEnum, Token,
+    Expr, Ident, Item, ItemEnum, Token, Variant,
 };
 
-#[derive(Debug)]
-struct Flag {
+struct Flag<'a> {
     name: Ident,
     span: Span,
-    value: FlagValue,
+    value: FlagValue<'a>,
 }
 
-#[derive(Debug)]
-enum FlagValue {
+enum FlagValue<'a> {
     Literal(u128),
     Deferred,
-    Inferred,
+    Inferred(&'a mut Variant),
+}
+
+impl FlagValue<'_> {
+    fn is_inferred(&self) -> bool {
+        match self {
+            FlagValue::Inferred(_) => true,
+            _ => false,
+        }
+    }
 }
 
 struct Parameters {
@@ -54,9 +61,9 @@ pub fn bitflags_internal(
     input: proc_macro::TokenStream,
 ) -> proc_macro::TokenStream {
     let Parameters { default } = parse_macro_input!(attr as Parameters);
-    let ast = parse_macro_input!(input as Item);
+    let mut ast = parse_macro_input!(input as Item);
     let output = match ast {
-        Item::Enum(ref item_enum) => gen_enumflags(item_enum, default),
+        Item::Enum(ref mut item_enum) => gen_enumflags(item_enum, default),
         _ => Err(syn::Error::new_spanned(
             &ast,
             "#[bitflags] requires an enum",
@@ -76,7 +83,6 @@ pub fn bitflags_internal(
 
 /// Try to evaluate the expression given.
 fn fold_expr(expr: &syn::Expr) -> Option<u128> {
-    use syn::Expr;
     match expr {
         Expr::Lit(ref expr_lit) => match expr_lit.lit {
             syn::Lit::Int(ref lit_int) => lit_int.base10_parse().ok(),
@@ -98,8 +104,8 @@ fn fold_expr(expr: &syn::Expr) -> Option<u128> {
 }
 
 fn collect_flags<'a>(
-    variants: impl Iterator<Item = &'a syn::Variant>,
-) -> Result<Vec<Flag>, syn::Error> {
+    variants: impl Iterator<Item = &'a mut Variant>,
+) -> Result<Vec<Flag<'a>>, syn::Error> {
     variants
         .map(|variant| {
             // MSRV: Would this be cleaner with `matches!`?
@@ -113,25 +119,51 @@ fn collect_flags<'a>(
                 }
             }
 
+            let name = variant.ident.clone();
+            let span = variant.span();
             let value = if let Some(ref expr) = variant.discriminant {
                 if let Some(n) = fold_expr(&expr.1) {
                     FlagValue::Literal(n)
                 } else {
                     FlagValue::Deferred
                 }
             } else {
-                FlagValue::Inferred
+                FlagValue::Inferred(variant)
             };
 
-            Ok(Flag {
-                name: variant.ident.clone(),
-                span: variant.span(),
-                value,
-            })
+            Ok(Flag { name, span, value })
         })
         .collect()
 }
 
+fn inferred_value(type_name: &Ident, previous_variants: &[Ident], repr: &Ident) -> Expr {
+    let tokens = if previous_variants.is_empty() {
+        quote!(1)
+    } else {
+        quote!(::enumflags2::_internal::next_bit(
+                #(#type_name::#previous_variants as u128)|*
+        ) as #repr)
+    };
+
+    syn::parse2(tokens).expect("couldn't parse inferred value")
+}
+
+fn infer_values<'a>(flags: &mut [Flag], type_name: &Ident, repr: &Ident) {
+    let mut previous_variants: Vec<Ident> = flags.iter()
+        .filter(|flag| !flag.value.is_inferred())
+        .map(|flag| flag.name.clone()).collect();
+
+    for flag in flags {
+        match flag.value {
+            FlagValue::Inferred(ref mut variant) => {
+                variant.discriminant = Some((<Token![=]>::default(), inferred_value(type_name, &previous_variants, repr)));
+                previous_variants.push(flag.name.clone());
+            }
+            _ => {}
+        }
+    }
+}
+
 /// Given a list of attributes, find the `repr`, if any, and return the integer
 /// type specified.
 fn extract_repr(attrs: &[syn::Attribute]) -> Result<Option<Ident>, syn::Error> {
@@ -210,10 +242,7 @@ fn check_flag(type_name: &Ident, flag: &Flag, bits: u8) -> Result<Option<TokenSt
                 Ok(None)
             }
         }
-        Inferred => Err(syn::Error::new(
-            flag.span,
-            "Please add an explicit discriminant",
-        )),
+        Inferred(_) => Ok(None),
         Deferred => {
             let variant_name = &flag.name;
             // MSRV: Use an unnamed constant (`const _: ...`).
@@ -235,33 +264,34 @@ fn check_flag(type_name: &Ident, flag: &Flag, bits: u8) -> Result<Option<TokenSt
     }
 }
 
-fn gen_enumflags(ast: &ItemEnum, default: Vec<Ident>) -> Result<TokenStream, syn::Error> {
+fn gen_enumflags(ast: &mut ItemEnum, default: Vec<Ident>) -> Result<TokenStream, syn::Error> {
     let ident = &ast.ident;
 
     let span = Span::call_site();
-    // for quote! interpolation
-    let variant_names = ast.variants.iter().map(|v| &v.ident).collect::<Vec<_>>();
-    let repeated_name = vec![&ident; ast.variants.len()];
 
-    let ty = extract_repr(&ast.attrs)?
+    let repr = extract_repr(&ast.attrs)?
         .ok_or_else(|| syn::Error::new_spanned(&ident,
                         "repr attribute missing. Add #[repr(u64)] or a similar attribute to specify the size of the bitfield."))?;
-    let bits = type_bits(&ty)?;
+    let bits = type_bits(&repr)?;
 
-    let variants = collect_flags(ast.variants.iter())?;
+    let mut variants = collect_flags(ast.variants.iter_mut())?;
     let deferred = variants
         .iter()
         .flat_map(|variant| check_flag(ident, variant, bits).transpose())
         .collect::<Result<Vec<_>, _>>()?;
 
+    infer_values(&mut variants, ident, &repr);
+
     if (bits as usize) < variants.len() {
         return Err(syn::Error::new_spanned(
-            &ty,
+            &repr,
             format!("Not enough bits for {} flags", variants.len()),
         ));
     }
 
     let std_path = quote_spanned!(span => ::enumflags2::_internal::core);
+    let variant_names = ast.variants.iter().map(|v| &v.ident).collect::<Vec<_>>();
+    let repeated_name = vec![&ident; ast.variants.len()];
 
     Ok(quote_spanned! {
         span =>
@@ -303,15 +333,15 @@ fn gen_enumflags(ast: &ItemEnum, default: Vec<Ident>) -> Result<TokenStream, syn
             }
 
             impl ::enumflags2::_internal::RawBitFlags for #ident {
-                type Numeric = #ty;
+                type Numeric = #repr;
 
                 const EMPTY: Self::Numeric = 0;
 
                 const DEFAULT: Self::Numeric =
-                    0 #(| (#repeated_name::#default as #ty))*;
+                    0 #(| (#repeated_name::#default as #repr))*;
 
                 const ALL_BITS: Self::Numeric =
-                    0 #(| (#repeated_name::#variant_names as #ty))*;
+                    0 #(| (#repeated_name::#variant_names as #repr))*;
 
                 const FLAG_LIST: &'static [Self] =
                     &[#(#repeated_name::#variant_names),*];
@@ -320,7 +350,7 @@ fn gen_enumflags(ast: &ItemEnum, default: Vec<Ident>) -> Result<TokenStream, syn
                     concat!("BitFlags<", stringify!(#ident), ">");
 
                 fn bits(self) -> Self::Numeric {
-                    self as #ty
+                    self as #repr
                 }
             }
 

src/lib.rs

@@ -13,7 +13,7 @@
 //! enum Test {
 //!     A = 0b0001,
 //!     B = 0b0010,
-//!     C = 0b0100,
+//!     C, // unspecified variants pick unused bits automatically
 //!     D = 0b1000,
 //! }
 //!
@@ -259,6 +259,11 @@ pub mod _internal {
     impl AssertionHelper for [(); 0] {
         type Status = AssertionFailed;
     }
+
+    pub const fn next_bit(x: u128) -> u128 {
+        // trailing_ones is beyond our MSRV
+        1 << (!x).trailing_zeros()
+    }
 }
 
 // Internal debug formatting implementations

test_suite/common.rs

@@ -19,7 +19,7 @@ enum Test1 {
     E = 1 << 34,
 }
 
-#[enumflags2::bitflags(default = B | C)]
+#[bitflags(default = B | C)]
 #[derive(Copy, Clone, Debug)]
 #[repr(u8)]
 enum Default6 {
@@ -129,3 +129,34 @@ fn module() {
         }
     }
 }
+
+#[test]
+fn inferred_values() {
+    #[bitflags]
+    #[derive(Copy, Clone, Debug)]
+    #[repr(u8)]
+    enum Inferred {
+        Infer2,
+        SpecifiedA = 1,
+        Infer8,
+        SpecifiedB = 4,
+    }
+
+    assert_eq!(Inferred::Infer2 as u8, 2);
+    assert_eq!(Inferred::Infer8 as u8, 8);
+
+    #[bitflags]
+    #[derive(Copy, Clone, Debug)]
+    #[repr(u8)]
+    enum OnlyInferred {
+        Infer1,
+        Infer2,
+        Infer4,
+        Infer8,
+    }
+
+    assert_eq!(OnlyInferred::Infer1 as u8, 1);
+    assert_eq!(OnlyInferred::Infer2 as u8, 2);
+    assert_eq!(OnlyInferred::Infer4 as u8, 4);
+    assert_eq!(OnlyInferred::Infer8 as u8, 8);
+}