Undef the `DEFINE_C_API_STRUCT` macro after using it in the MLIR C API header (NFC)

[mehdi_amini]

Summary

Leaking macros isn't a good practice when defining headers. This
requires to duplicate the macro definition in every header though, but
that seems like a better tradeoff right now.

[rriddle]

mlir/include/mlir-c/AffineExpr.h

33

Newline before here?

mlir/include/mlir-c/AffineMap.h

32

Newline before here?

mlir/include/mlir-c/AffineExpr.h

 
 #include "mlir-c/AffineMap.h"
 #include "mlir-c/IR.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+/*============================================================================*/
+/** Opaque type declarations.
+ *
+ * Types are exposed to C bindings as structs containing opaque pointers. They
+ * are not supposed to be inspected from C. This allows the underlying
+ * representation to change without affecting the API users. The use of structs
+ * instead of typedefs enables some type safety as structs are not implicitly
+ * convertible to each other.
+ *
+ * Instances of these types may or may not own the underlying object. The
+ * ownership semantics is defined by how an instance of the type was obtained.
+ */
+/*============================================================================*/
+#define DEFINE_C_API_STRUCT(name, storage)                                     \
+  struct name {                                                                \
+    storage *ptr;                                                              \
+  };                                                                           \
+  typedef struct name name
+
 DEFINE_C_API_STRUCT(MlirAffineExpr, const void);
 
+#undef DEFINE_C_API_STRUCT
+
 /** Gets the context that owns the affine expression. */
 MlirContext mlirAffineExprGetContext(MlirAffineExpr affineExpr);
 
 /** Prints an affine expression by sending chunks of the string representation
  * and forwarding `userData to `callback`. Note that the callback may be called
  * several times with consecutive chunks of the string. */
 void mlirAffineExprPrint(MlirAffineExpr affineExpr, MlirStringCallback callback,
                          void *userData);

mlir/include/mlir-c/AffineMap.h

 #define MLIR_C_AFFINEMAP_H
 
 #include "mlir-c/IR.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+/*============================================================================*/
+/** Opaque type declarations.
+ *
+ * Types are exposed to C bindings as structs containing opaque pointers. They
+ * are not supposed to be inspected from C. This allows the underlying
+ * representation to change without affecting the API users. The use of structs
+ * instead of typedefs enables some type safety as structs are not implicitly
+ * convertible to each other.
+ *
+ * Instances of these types may or may not own the underlying object. The
+ * ownership semantics is defined by how an instance of the type was obtained.
+ */
+/*============================================================================*/
+#define DEFINE_C_API_STRUCT(name, storage)                                     \
+  struct name {                                                                \
+    storage *ptr;                                                              \
+  };                                                                           \
+  typedef struct name name
+
 DEFINE_C_API_STRUCT(MlirAffineMap, const void);
 
+#undef DEFINE_C_API_STRUCT
+
 /** Gets the context that the given affine map was created with*/
 MlirContext mlirAffineMapGetContext(MlirAffineMap affineMap);
 
 /** Checks whether an affine map is null. */
 inline int mlirAffineMapIsNull(MlirAffineMap affineMap) {
   return !affineMap.ptr;
 }
 

mlir/include/mlir-c/IR.h

 DEFINE_C_API_STRUCT(MlirBlock, void);
 DEFINE_C_API_STRUCT(MlirRegion, void);
 
 DEFINE_C_API_STRUCT(MlirValue, const void);
 DEFINE_C_API_STRUCT(MlirAttribute, const void);
 DEFINE_C_API_STRUCT(MlirType, const void);
 DEFINE_C_API_STRUCT(MlirLocation, const void);
 DEFINE_C_API_STRUCT(MlirModule, const void);
+#undef DEFINE_C_API_STRUCT
 
 /** Named MLIR attribute.
  *
  * A named attribute is essentially a (name, attribute) pair where the name is
  * a string.
  */
 struct MlirNamedAttribute {
   const char *name;