Add MBED_FALLTHROUGH attribute

C++17 standardised `[[fallthrough]]` for `switch` statements to suppress
compiler warnings. Provide access to it, or compiler-specific
alternatives.

Author: kjbracey

TESTS/mbedmicro-mbed/attributes/attributes.c

@@ -145,6 +145,24 @@ int testNoReturn()
 }
 
 
+int testFallthrough1(int i)
+{
+    switch (i) {
+        case 1:
+        case 2:
+            i *= 2;
+            MBED_FALLTHROUGH;
+        default:
+            return i;
+    }
+}
+
+int testFallthrough()
+{
+    return testFallthrough1(0);
+}
+
+
 int testUnreachable1(int i)
 {
     switch (i) {

TESTS/mbedmicro-mbed/attributes/main.cpp

@@ -33,6 +33,7 @@ extern "C" {
     int testPure();
     int testForceInline();
     int testNoReturn();
+    int testFallthrough();
     int testUnreachable();
     int testDeprecated();
 }
@@ -59,6 +60,7 @@ Case cases[] = {
     Case("Testing PURE attribute",          test_wrapper<testPure>),
     Case("Testing FORCEINLINE attribute",   test_wrapper<testForceInline>),
     Case("Testing NORETURN attribute",      test_wrapper<testNoReturn>),
+    Case("Testing FALLTHROUGH attribute",   test_wrapper<testFallthrough>),
     Case("Testing UNREACHABLE attribute",   test_wrapper<testUnreachable>),
     Case("Testing DEPRECATED attribute",    test_wrapper<testDeprecated>),
 };

platform/mbed_toolchain.h

@@ -340,6 +340,44 @@
 #endif
 #endif
 
+/** MBED_FALLTHROUGH
+ *  Marks a point in a switch statement where fallthrough can occur.
+ *  Should be placed as the last statement before a label.
+ *
+ *  @code
+ *  #include "mbed_toolchain.h"
+ *
+ *  int foo(int arg) {
+ *      switch (arg) {
+ *          case 1:
+ *          case 2:
+ *          case 3:
+ *              arg *= 2;
+ *              MBED_FALLTHROUGH;
+ *          default:
+ *              return arg;
+ *      }
+ *  }
+ *  @endcode
+ */
+#ifndef MBED_FALLTHROUGH
+#if __cplusplus >= 201703
+#define MBED_FALLTHROUGH [[fallthrough]]
+#elif defined(__clang__)
+#if __cplusplus >= 201103
+#define MBED_FALLTHROUGH [[clang::fallthrough]]
+#elif __has_attribute(fallthrough)
+#define MBED_FALLTHROUGH __attribute__((fallthrough))
+#else
+#define MBED_FALLTHROUGH
+#endif
+#elif defined (__GNUC__) && !defined(__CC_ARM)
+#define MBED_FALLTHROUGH __attribute__((fallthrough))
+#else
+#define MBED_FALLTHROUGH
+#endif
+#endif
+
 /** MBED_DEPRECATED("message string")
  *  Mark a function declaration as deprecated, if it used then a warning will be
  *  issued by the compiler possibly including the provided message. Note that not