Add _Enum! macros to replace _ arms when matching shared enums

book/src/shared.md

@@ -78,14 +78,19 @@ impl Suit {
     pub const Hearts: Self = Suit { repr: 2 };
     pub const Spades: Self = Suit { repr: 3 };
 }
+macro_rules! _Suit {
+    () => { Suit{ repr: 4.. } };
+    ($i:ident) => { Suit { repr: $i @ 4.. } };
+}
 ```
 
 Notice you're free to treat the enum as an integer in Rust code via the public
 `repr` field.
 
-Pattern matching with `match` still works but will require you to write wildcard
-arms to handle the situation of an enum value that is not one of the listed
-variants.
+Pattern matching with `match` still works but will require you to write a
+fallback arm to handle the situation of an enum value that is not one of the
+listed variants. A convenience `_Enum!` macro is generated to statically ensure
+that all listed variants are covered:
 
 ```rust,noplayground
 fn main() {
@@ -95,7 +100,7 @@ fn main() {
         Suit::Diamonds => ...,
         Suit::Hearts => ...,
         Suit::Spades => ...,
-        _ => ...,  // fallback arm
+        _Suit!(unlisted) => ...,  // fallback arm
     }
 }
 ```

macro/src/expand.rs

@@ -68,7 +68,7 @@ fn expand(ffi: Module, doc: Doc, attrs: OtherAttrs, apis: &[Api], types: &Types)
                 hidden.extend(expand_struct_operators(strct));
                 forbid.extend(expand_struct_forbid_drop(strct));
             }
-            Api::Enum(enm) => expanded.extend(expand_enum(enm)),
+            Api::Enum(enm) => expanded.extend(expand_enum(enm, &ffi.ident)),
             Api::CxxType(ety) => {
                 let ident = &ety.name.rust;
                 if !types.structs.contains_key(ident) && !types.enums.contains_key(ident) {
@@ -317,7 +317,7 @@ fn expand_struct_forbid_drop(strct: &Struct) -> TokenStream {
     }
 }
 
-fn expand_enum(enm: &Enum) -> TokenStream {
+fn expand_enum(enm: &Enum, ffi: &Ident) -> TokenStream {
     let ident = &enm.name.rust;
     let doc = &enm.doc;
     let attrs = &enm.attrs;
@@ -352,6 +352,49 @@ fn expand_enum(enm: &Enum) -> TokenStream {
         }
     };
 
+    let gaps = {
+        // discriminants must be sorted to build the range patterns of the gaps between them
+        let discriminants = {
+            let mut discriminants: Vec<_> = enm
+                .variants
+                .iter()
+                .map(|variant| variant.discriminant)
+                .collect();
+            discriminants.sort();
+            discriminants
+        };
+
+        // When map_windows gets stabilized:
+        // let gaps = std::iter::once(None)
+        //     .chain(discriminants.into_iter().map(Some))
+        //     .chain(std::iter::once(None))
+        //     .map_windows::<_, _, 2>(|&[start, end]| match (start, end) ...);
+        let gaps = {
+            let starts =
+                std::iter::once(None).chain(discriminants.iter().map(|d| d.checked_succ()));
+            let ends = discriminants
+                .iter()
+                .copied()
+                .map(Some)
+                .chain(std::iter::once(None));
+
+            starts.zip(ends)
+        };
+
+        let gaps = gaps.flat_map(|(start, end)| match (start, end) {
+            (None, None) => None,
+            // ..#end is not a valid range pattern when end is the first admissible value for the discriminant (typically 0 for unsigned discriminants)
+            (None, Some(end)) if end == enm.repr.limits().min => None,
+            (None, Some(end)) => Some(quote! { ..#end }),
+            (Some(start), None) => Some(quote! { #start.. }),
+            (Some(start), Some(end)) if start == end => None,
+            (Some(start), Some(end)) => Some(quote! { #start..#end }),
+        });
+
+        quote! { #(#gaps)|* }
+    };
+    let gaps_macro = format_ident!("_{}", ident);
+
     quote! {
         #doc
         #derives
@@ -364,6 +407,11 @@ fn expand_enum(enm: &Enum) -> TokenStream {
             #(#variants)*
         }
 
+        #[macro_export] macro_rules! #gaps_macro {
+            () => { $crate::#ffi::#ident{ repr: #gaps } };
+            ($i:ident) => { $crate::#ffi::#ident{ repr: $i @ #gaps } };
+        }
+
         unsafe impl ::cxx::ExternType for #ident {
             #[allow(unused_attributes)] // incorrect lint
             #[doc(hidden)]

syntax/discriminant.rs

@@ -1,4 +1,7 @@
-use crate::syntax::Atom::{self, *};
+use crate::syntax::{
+    Atom::{self, *},
+    EnumRepr,
+};
 use proc_macro2::{Literal, Span, TokenStream};
 use quote::ToTokens;
 use std::cmp::Ordering;
@@ -179,7 +182,7 @@ impl Discriminant {
         }
     }
 
-    #[cfg(feature = "experimental-enum-variants-from-header")]
+    #[allow(dead_code)] // only used by cxxbridge-macro, not cxx-build
     pub(crate) const fn checked_succ(self) -> Option<Self> {
         match self.sign {
             Sign::Negative => {
@@ -274,10 +277,10 @@ fn parse_int_suffix(suffix: &str) -> Result<Option<Atom>> {
 }
 
 #[derive(Copy, Clone)]
-struct Limits {
+pub(crate) struct Limits {
     repr: Atom,
-    min: Discriminant,
-    max: Discriminant,
+    pub min: Discriminant,
+    pub max: Discriminant,
 }
 
 impl Limits {
@@ -333,3 +336,16 @@ const LIMITS: [Limits; 8] = [
         max: Discriminant::pos(i64::MAX as u64),
     },
 ];
+
+impl EnumRepr {
+    #[allow(dead_code)] // only used by cxxbridge-macro, not cxx-build
+    pub fn limits(&self) -> Limits {
+        match self {
+            &EnumRepr::Native { atom, repr_type: _ } => {
+                Limits::of(atom).expect("Unexpected EnumRepr")
+            }
+            #[cfg(feature = "experimental-enum-variants-from-header")]
+            &EnumRepr::Foreign { rust_type: _ } => todo!(),
+        }
+    }
+}

tests/test.rs

@@ -80,23 +80,33 @@ fn test_c_return() {
     assert_eq!(2021, ffi::c_return_sum(2020, 1));
     match ffi::c_return_enum(0) {
         enm @ ffi::Enum::AVal => assert_eq!(0, enm.repr),
-        _ => assert!(false),
+        ffi::Enum::BVal => assert!(false),
+        ffi::Enum::LastVal => assert!(false),
+        cxx_test_suite::_Enum!() => assert!(false),
     }
     match ffi::c_return_enum(1) {
+        ffi::Enum::AVal => assert!(false),
         enm @ ffi::Enum::BVal => assert_eq!(2020, enm.repr),
-        _ => assert!(false),
+        ffi::Enum::LastVal => assert!(false),
+        cxx_test_suite::_Enum!() => assert!(false),
     }
     match ffi::c_return_enum(2021) {
+        ffi::Enum::AVal => assert!(false),
+        ffi::Enum::BVal => assert!(false),
         enm @ ffi::Enum::LastVal => assert_eq!(2021, enm.repr),
-        _ => assert!(false),
+        cxx_test_suite::_Enum!() => assert!(false),
     }
     match ffi::c_return_ns_enum(0) {
         enm @ ffi::AEnum::AAVal => assert_eq!(0, enm.repr),
-        _ => assert!(false),
+        ffi::AEnum::ABVal => assert!(false),
+        ffi::AEnum::ACVal => assert!(false),
+        cxx_test_suite::_AEnum!() => assert!(false),
     }
     match ffi::c_return_nested_ns_enum(0) {
         enm @ ffi::ABEnum::ABAVal => assert_eq!(0, enm.repr),
-        _ => assert!(false),
+        ffi::ABEnum::ABBVal => assert!(false),
+        ffi::ABEnum::ABCVal => assert!(false),
+        cxx_test_suite::_ABEnum!() => assert!(false),
     }
 }