Add option to output constexpr generated constant primitive values

This is controlled by the [const] allow_constexpr option, similar to the
allow_static_const option.

It's only applied to primitives currently.

Author: AlaskanEmily

src/bindgen/config.rs

@@ -555,12 +555,15 @@ impl EnumConfig {
 pub struct ConstantConfig {
     /// Whether a generated constant can be a static const in C++ mode.
     pub allow_static_const: bool,
+    /// Whether a generated constant should be constexpr in C++ mode.
+    pub allow_constexpr: bool,
 }
 
 impl Default for ConstantConfig {
     fn default() -> ConstantConfig {
         ConstantConfig {
             allow_static_const: true,
+            allow_constexpr: false,
         }
     }
 }

src/bindgen/ir/constant.rs

@@ -491,12 +491,22 @@ impl Constant {
 
         self.documentation.write(config, out);
 
-        if config.constant.allow_static_const && config.language == Language::Cxx {
-            out.write(if in_body { "inline " } else { "static " });
-            if let Type::ConstPtr(..) = self.ty {
-                // Nothing.
+        let allow_constexpr = if let Type::Primitive(..) = self.ty {
+            config.constant.allow_constexpr
+        } else {
+            false
+        };
+
+        if (config.constant.allow_static_const || allow_constexpr) && config.language == Language::Cxx {
+            if allow_constexpr {
+                out.write("constexpr ")
             } else {
-                out.write("const ");
+               out.write(if in_body { "inline " } else { "static " });
+                if let Type::ConstPtr(..) = self.ty {
+                    // Nothing.
+                } else {
+                    out.write("const ");
+                }
             }
             self.ty.write(config, out);
             write!(out, " {} = ", name);

tests/expectations/constant_constexpr.c

@@ -0,0 +1,18 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define CONSTANT_FLOAT32 312.292
+
+#define CONSTANT_I64 216
+
+#define DELIMITER ':'
+
+#define LEFTCURLY '{'
+
+typedef struct {
+  int32_t x;
+} Foo;
+
+#define SomeFoo (Foo){ .x = 99 }

tests/expectations/constant_constexpr.compat.c

@@ -0,0 +1,18 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define CONSTANT_FLOAT32 312.292
+
+#define CONSTANT_I64 216
+
+#define DELIMITER ':'
+
+#define LEFTCURLY '{'
+
+typedef struct {
+  int32_t x;
+} Foo;
+
+#define SomeFoo (Foo){ .x = 99 }

tests/expectations/constant_constexpr.cpp

@@ -0,0 +1,18 @@
+#include <cstdarg>
+#include <cstdint>
+#include <cstdlib>
+#include <new>
+
+constexpr float CONSTANT_FLOAT32 = 312.292;
+
+constexpr int64_t CONSTANT_I64 = 216;
+
+constexpr uint32_t DELIMITER = ':';
+
+constexpr uint32_t LEFTCURLY = '{';
+
+struct Foo {
+  int32_t x;
+};
+
+static const Foo SomeFoo = Foo{ /* .x = */ 99 };

tests/rust/constant_constexpr.rs

@@ -0,0 +1,12 @@
+pub const SomeName: &'static str = "hello world";
+pub const CONSTANT_I64: i64 = 216;
+pub const CONSTANT_FLOAT32: f32 = 312.292;
+pub const DELIMITER: char = ':';
+pub const LEFTCURLY: char = '{';
+#[repr(C)]
+struct Foo {
+    x: i32,
+}
+
+pub const SomeFoo: Foo = Foo{ x: 99, };
+

tests/rust/constant_constexpr.toml

@@ -0,0 +1,4 @@
+[const]
+allow_constexpr = true
+allow_static_const = true
+