Allow more constexpr constants.

CHANGES

@@ -1,3 +1,9 @@
+## 0.23.0
+
+      * Better support for constexpr. (#756)
+      * constexpr is now enabled by default in C++ mode. You can use const.allow_constexpr=false to revert to previous behavior. (#756)
+      * Minimum syn version no longer parses old rust code. (#754)
+
 ## 0.22.0
 
       * Support rename rule for union body members (#751).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cbindgen"
-version = "0.22.0"
+version = "0.23.0"
 authors = [
   "Emilio Cobos Álvarez <emilio@crisal.io>",
   "Jeff Muizelaar <jmuizelaar@mozilla.com>",

docs.md

@@ -913,7 +913,7 @@ allow_static_const = true
 
 # Whether a generated constant can be constexpr in C++ mode.
 #
-# default: false
+# default: true
 allow_constexpr = false
 
 # This rule specifies the order in which constants will be sorted.

src/bindgen/config.rs

@@ -706,7 +706,7 @@ impl Default for ConstantConfig {
     fn default() -> ConstantConfig {
         ConstantConfig {
             allow_static_const: true,
-            allow_constexpr: false,
+            allow_constexpr: true,
             sort_by: None,
         }
     }

src/bindgen/ir/constant.rs

@@ -183,22 +183,60 @@ impl Literal {
         }
     }
 
-    pub fn uses_only_primitive_types(&self) -> bool {
+    fn can_be_constexpr(&self) -> bool {
+        !self.has_pointer_casts()
+    }
+
+    fn visit(&self, visitor: &mut impl FnMut(&Self) -> bool) -> bool {
+        if !visitor(self) {
+            return false;
+        }
         match self {
             Literal::Expr(..) | Literal::Path { .. } => true,
-            Literal::PostfixUnaryOp { ref value, .. } => value.uses_only_primitive_types(),
+            Literal::PostfixUnaryOp { ref value, .. } => value.visit(visitor),
             Literal::BinOp {
                 ref left,
                 ref right,
                 ..
-            } => left.uses_only_primitive_types() & right.uses_only_primitive_types(),
-            Literal::FieldAccess { ref base, .. } => base.uses_only_primitive_types(),
-            Literal::Struct { .. } => false,
-            Literal::Cast { ref value, ref ty } => {
-                value.uses_only_primitive_types() && ty.is_primitive_or_ptr_primitive()
+            } => left.visit(visitor) && right.visit(visitor),
+            Literal::FieldAccess { ref base, .. } => base.visit(visitor),
+            Literal::Struct { ref fields, .. } => {
+                for (_name, field) in fields.iter() {
+                    if !field.visit(visitor) {
+                        return false;
+                    }
+                }
+                true
             }
+            Literal::Cast { ref value, .. } => value.visit(visitor),
         }
     }
+
+    fn has_pointer_casts(&self) -> bool {
+        let mut has_pointer_casts = false;
+        self.visit(&mut |lit| {
+            if let Literal::Cast { ref ty, .. } = *lit {
+                has_pointer_casts = has_pointer_casts || ty.is_ptr();
+            }
+            !has_pointer_casts
+        });
+        has_pointer_casts
+    }
+
+    pub fn uses_only_primitive_types(&self) -> bool {
+        let mut uses_only_primitive_types = true;
+        self.visit(&mut |lit| {
+            // XXX This is a bit sketchy, but alas.
+            uses_only_primitive_types = uses_only_primitive_types
+                && match *lit {
+                    Literal::Struct { .. } => false,
+                    Literal::Cast { ref ty, .. } => ty.is_primitive_or_ptr_primitive(),
+                    _ => true,
+                };
+            uses_only_primitive_types
+        });
+        uses_only_primitive_types
+    }
 }
 
 impl Literal {
@@ -720,12 +758,7 @@ impl Constant {
 
         self.documentation.write(config, out);
 
-        let allow_constexpr = if let Type::Primitive(..) = self.ty {
-            config.constant.allow_constexpr
-        } else {
-            false
-        };
-
+        let allow_constexpr = config.constant.allow_constexpr && self.value.can_be_constexpr();
         match config.language {
             Language::Cxx if config.constant.allow_static_const || allow_constexpr => {
                 if allow_constexpr {

src/bindgen/ir/ty.rs

@@ -501,6 +501,10 @@ impl Type {
         Ok(Some(converted))
     }
 
+    pub fn is_ptr(&self) -> bool {
+        matches!(*self, Type::Ptr { .. } | Type::FuncPtr { .. })
+    }
+
     pub fn is_primitive_or_ptr_primitive(&self) -> bool {
         match *self {
             Type::Primitive(..) => true,

tests/expectations/assoc_const_conflict.cpp

@@ -4,4 +4,4 @@
 #include <ostream>
 #include <new>
 
-static const uint32_t Foo_FOO = 42;
+constexpr static const uint32_t Foo_FOO = 42;

tests/expectations/assoc_constant.cpp

@@ -7,8 +7,8 @@
 struct Foo {
 
 };
-static const int32_t Foo_GA = 10;
-static const float Foo_ZO = 3.14;
+constexpr static const int32_t Foo_GA = 10;
+constexpr static const float Foo_ZO = 3.14;
 
 extern "C" {
 

tests/expectations/associated_in_body.cpp

@@ -47,26 +47,26 @@ struct StyleAlignFlags {
   static const StyleAlignFlags MIXED_SELF;
 };
 /// 'auto'
-inline const StyleAlignFlags StyleAlignFlags::AUTO = StyleAlignFlags{ /* .bits = */ (uint8_t)0 };
+constexpr inline const StyleAlignFlags StyleAlignFlags::AUTO = StyleAlignFlags{ /* .bits = */ (uint8_t)0 };
 /// 'normal'
-inline const StyleAlignFlags StyleAlignFlags::NORMAL = StyleAlignFlags{ /* .bits = */ (uint8_t)1 };
+constexpr inline const StyleAlignFlags StyleAlignFlags::NORMAL = StyleAlignFlags{ /* .bits = */ (uint8_t)1 };
 /// 'start'
-inline const StyleAlignFlags StyleAlignFlags::START = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 1) };
+constexpr inline const StyleAlignFlags StyleAlignFlags::START = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 1) };
 /// 'end'
-inline const StyleAlignFlags StyleAlignFlags::END = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 2) };
-inline const StyleAlignFlags StyleAlignFlags::ALIAS = StyleAlignFlags{ /* .bits = */ (uint8_t)(StyleAlignFlags::END).bits };
+constexpr inline const StyleAlignFlags StyleAlignFlags::END = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 2) };
+constexpr inline const StyleAlignFlags StyleAlignFlags::ALIAS = StyleAlignFlags{ /* .bits = */ (uint8_t)(StyleAlignFlags::END).bits };
 /// 'flex-start'
-inline const StyleAlignFlags StyleAlignFlags::FLEX_START = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 3) };
-inline const StyleAlignFlags StyleAlignFlags::MIXED = StyleAlignFlags{ /* .bits = */ (uint8_t)(((1 << 4) | (StyleAlignFlags::FLEX_START).bits) | (StyleAlignFlags::END).bits) };
-inline const StyleAlignFlags StyleAlignFlags::MIXED_SELF = StyleAlignFlags{ /* .bits = */ (uint8_t)(((1 << 5) | (StyleAlignFlags::FLEX_START).bits) | (StyleAlignFlags::END).bits) };
+constexpr inline const StyleAlignFlags StyleAlignFlags::FLEX_START = StyleAlignFlags{ /* .bits = */ (uint8_t)(1 << 3) };
+constexpr inline const StyleAlignFlags StyleAlignFlags::MIXED = StyleAlignFlags{ /* .bits = */ (uint8_t)(((1 << 4) | (StyleAlignFlags::FLEX_START).bits) | (StyleAlignFlags::END).bits) };
+constexpr inline const StyleAlignFlags StyleAlignFlags::MIXED_SELF = StyleAlignFlags{ /* .bits = */ (uint8_t)(((1 << 5) | (StyleAlignFlags::FLEX_START).bits) | (StyleAlignFlags::END).bits) };
 
 /// An arbitrary identifier for a native (OS compositor) surface
 struct StyleNativeSurfaceId {
   uint64_t _0;
   static const StyleNativeSurfaceId DEBUG_OVERLAY;
 };
 /// A special id for the native surface that is used for debug / profiler overlays.
-inline const StyleNativeSurfaceId StyleNativeSurfaceId::DEBUG_OVERLAY = StyleNativeSurfaceId{ /* ._0 = */ UINT64_MAX };
+constexpr inline const StyleNativeSurfaceId StyleNativeSurfaceId::DEBUG_OVERLAY = StyleNativeSurfaceId{ /* ._0 = */ UINT64_MAX };
 
 struct StyleNativeTileId {
   StyleNativeSurfaceId surface_id;
@@ -75,7 +75,7 @@ struct StyleNativeTileId {
   static const StyleNativeTileId DEBUG_OVERLAY;
 };
 /// A special id for the native surface that is used for debug / profiler overlays.
-inline const StyleNativeTileId StyleNativeTileId::DEBUG_OVERLAY = StyleNativeTileId{ /* .surface_id = */ StyleNativeSurfaceId::DEBUG_OVERLAY, /* .x = */ 0, /* .y = */ 0 };
+constexpr inline const StyleNativeTileId StyleNativeTileId::DEBUG_OVERLAY = StyleNativeTileId{ /* .surface_id = */ StyleNativeSurfaceId::DEBUG_OVERLAY, /* .x = */ 0, /* .y = */ 0 };
 
 extern "C" {
 

tests/expectations/bitflags.cpp

@@ -39,18 +39,18 @@ struct AlignFlags {
   }
 };
 /// 'auto'
-static const AlignFlags AlignFlags_AUTO = AlignFlags{ /* .bits = */ (uint8_t)0 };
+constexpr static const AlignFlags AlignFlags_AUTO = AlignFlags{ /* .bits = */ (uint8_t)0 };
 /// 'normal'
-static const AlignFlags AlignFlags_NORMAL = AlignFlags{ /* .bits = */ (uint8_t)1 };
+constexpr static const AlignFlags AlignFlags_NORMAL = AlignFlags{ /* .bits = */ (uint8_t)1 };
 /// 'start'
-static const AlignFlags AlignFlags_START = AlignFlags{ /* .bits = */ (uint8_t)(1 << 1) };
+constexpr static const AlignFlags AlignFlags_START = AlignFlags{ /* .bits = */ (uint8_t)(1 << 1) };
 /// 'end'
-static const AlignFlags AlignFlags_END = AlignFlags{ /* .bits = */ (uint8_t)(1 << 2) };
-static const AlignFlags AlignFlags_ALIAS = AlignFlags{ /* .bits = */ (uint8_t)(AlignFlags_END).bits };
+constexpr static const AlignFlags AlignFlags_END = AlignFlags{ /* .bits = */ (uint8_t)(1 << 2) };
+constexpr static const AlignFlags AlignFlags_ALIAS = AlignFlags{ /* .bits = */ (uint8_t)(AlignFlags_END).bits };
 /// 'flex-start'
-static const AlignFlags AlignFlags_FLEX_START = AlignFlags{ /* .bits = */ (uint8_t)(1 << 3) };
-static const AlignFlags AlignFlags_MIXED = AlignFlags{ /* .bits = */ (uint8_t)(((1 << 4) | (AlignFlags_FLEX_START).bits) | (AlignFlags_END).bits) };
-static const AlignFlags AlignFlags_MIXED_SELF = AlignFlags{ /* .bits = */ (uint8_t)(((1 << 5) | (AlignFlags_FLEX_START).bits) | (AlignFlags_END).bits) };
+constexpr static const AlignFlags AlignFlags_FLEX_START = AlignFlags{ /* .bits = */ (uint8_t)(1 << 3) };
+constexpr static const AlignFlags AlignFlags_MIXED = AlignFlags{ /* .bits = */ (uint8_t)(((1 << 4) | (AlignFlags_FLEX_START).bits) | (AlignFlags_END).bits) };
+constexpr static const AlignFlags AlignFlags_MIXED_SELF = AlignFlags{ /* .bits = */ (uint8_t)(((1 << 5) | (AlignFlags_FLEX_START).bits) | (AlignFlags_END).bits) };
 
 struct DebugFlags {
   uint32_t bits;
@@ -84,7 +84,7 @@ struct DebugFlags {
   }
 };
 /// Flag with the topmost bit set of the u32
-static const DebugFlags DebugFlags_BIGGEST_ALLOWED = DebugFlags{ /* .bits = */ (uint32_t)(1 << 31) };
+constexpr static const DebugFlags DebugFlags_BIGGEST_ALLOWED = DebugFlags{ /* .bits = */ (uint32_t)(1 << 31) };
 
 extern "C" {
 

tests/expectations/cfg_2.cpp

@@ -11,11 +11,11 @@ DEF NOT_DEFINED = 0
 #include <new>
 
 #if defined(NOT_DEFINED)
-static const int32_t DEFAULT_X = 8;
+constexpr static const int32_t DEFAULT_X = 8;
 #endif
 
 #if defined(DEFINED)
-static const int32_t DEFAULT_X = 42;
+constexpr static const int32_t DEFAULT_X = 42;
 #endif
 
 #if (defined(NOT_DEFINED) || defined(DEFINED))

tests/expectations/const_conflict.cpp

@@ -4,4 +4,4 @@
 #include <ostream>
 #include <new>
 
-static const uint32_t Foo_FOO = 42;
+constexpr static const uint32_t Foo_FOO = 42;

tests/expectations/const_transparent.cpp

@@ -6,4 +6,4 @@
 
 using Transparent = uint8_t;
 
-static const Transparent FOO = 0;
+constexpr static const Transparent FOO = 0;

tests/expectations/constant.cpp

@@ -4,44 +4,44 @@
 #include <ostream>
 #include <new>
 
-static const int32_t FOO = 10;
+constexpr static const int32_t FOO = 10;
 
-static const uint32_t DELIMITER = ':';
+constexpr static const uint32_t DELIMITER = ':';
 
-static const uint32_t LEFTCURLY = '{';
+constexpr static const uint32_t LEFTCURLY = '{';
 
-static const uint32_t QUOTE = '\'';
+constexpr static const uint32_t QUOTE = '\'';
 
-static const uint32_t TAB = '\t';
+constexpr static const uint32_t TAB = '\t';
 
-static const uint32_t NEWLINE = '\n';
+constexpr static const uint32_t NEWLINE = '\n';
 
-static const uint32_t HEART = U'\U00002764';
+constexpr static const uint32_t HEART = U'\U00002764';
 
-static const uint32_t EQUID = U'\U00010083';
+constexpr static const uint32_t EQUID = U'\U00010083';
 
-static const float ZOM = 3.14;
+constexpr static const float ZOM = 3.14;
 
 /// A single-line doc comment.
-static const int8_t POS_ONE = 1;
+constexpr static const int8_t POS_ONE = 1;
 
 /// A
 /// multi-line
 /// doc
 /// comment.
-static const int8_t NEG_ONE = -1;
+constexpr static const int8_t NEG_ONE = -1;
 
-static const int64_t SHIFT = 3;
+constexpr static const int64_t SHIFT = 3;
 
-static const int64_t XBOOL = 1;
+constexpr static const int64_t XBOOL = 1;
 
-static const int64_t XFALSE = ((0 << SHIFT) | XBOOL);
+constexpr static const int64_t XFALSE = ((0 << SHIFT) | XBOOL);
 
-static const int64_t XTRUE = (1 << (SHIFT | XBOOL));
+constexpr static const int64_t XTRUE = (1 << (SHIFT | XBOOL));
 
-static const uint8_t CAST = (uint8_t)'A';
+constexpr static const uint8_t CAST = (uint8_t)'A';
 
-static const uint32_t DOUBLE_CAST = (uint32_t)(float)1;
+constexpr static const uint32_t DOUBLE_CAST = (uint32_t)(float)1;
 
 struct Foo {
   int32_t x[FOO];

tests/expectations/constant_big.cpp

@@ -4,10 +4,10 @@
 #include <ostream>
 #include <new>
 
-static const uint64_t UNSIGNED_NEEDS_ULL_SUFFIX = 9223372036854775808ULL;
+constexpr static const uint64_t UNSIGNED_NEEDS_ULL_SUFFIX = 9223372036854775808ULL;
 
-static const uint64_t UNSIGNED_DOESNT_NEED_ULL_SUFFIX = 8070450532247928832;
+constexpr static const uint64_t UNSIGNED_DOESNT_NEED_ULL_SUFFIX = 8070450532247928832;
 
-static const int64_t SIGNED_NEEDS_ULL_SUFFIX = -9223372036854775808ULL;
+constexpr static const int64_t SIGNED_NEEDS_ULL_SUFFIX = -9223372036854775808ULL;
 
-static const int64_t SIGNED_DOESNT_NEED_ULL_SUFFIX = -9223372036854775807;
+constexpr static const int64_t SIGNED_DOESNT_NEED_ULL_SUFFIX = -9223372036854775807;

tests/expectations/constant_constexpr.cpp

@@ -4,18 +4,18 @@
 #include <ostream>
 #include <new>
 
-constexpr static const int64_t CONSTANT_I64 = 216;
+static const int64_t CONSTANT_I64 = 216;
 
-constexpr static const float CONSTANT_FLOAT32 = 312.292;
+static const float CONSTANT_FLOAT32 = 312.292;
 
-constexpr static const uint32_t DELIMITER = ':';
+static const uint32_t DELIMITER = ':';
 
-constexpr static const uint32_t LEFTCURLY = '{';
+static const uint32_t LEFTCURLY = '{';
 
 struct Foo {
   int32_t x;
   static const int64_t CONSTANT_I64_BODY;
 };
-constexpr inline const int64_t Foo::CONSTANT_I64_BODY = 216;
+inline const int64_t Foo::CONSTANT_I64_BODY = 216;
 
 static const Foo SomeFoo = Foo{ /* .x = */ 99 };

tests/expectations/constant_sort_name.cpp

@@ -4,9 +4,9 @@
 #include <ostream>
 #include <new>
 
-static const uint8_t A = 0;
+constexpr static const uint8_t A = 0;
 
-static const uint8_t B = 0;
+constexpr static const uint8_t B = 0;
 
 extern "C" {
 

tests/expectations/constant_sort_none.cpp

@@ -4,9 +4,9 @@
 #include <ostream>
 #include <new>
 
-static const uint8_t B = 0;
+constexpr static const uint8_t B = 0;
 
-static const uint8_t A = 0;
+constexpr static const uint8_t A = 0;
 
 extern "C" {
 

tests/expectations/constant_user_defined_type.cpp

@@ -14,8 +14,8 @@ struct S {
 
 using A = uint8_t;
 
-static const S C1 = S{ /* .field = */ 0 };
+constexpr static const S C1 = S{ /* .field = */ 0 };
 
-static const E C2 = V;
+constexpr static const E C2 = V;
 
-static const A C3 = 0;
+constexpr static const A C3 = 0;