Merge `TRANSMUTE()` macro from branch ref: strings/87e102ded928dcc4d9c4c79e22c68b709ea91c40.

Fortune for naka's current commit: Future blessing − 末吉

include/macros.h

@@ -206,6 +206,14 @@ static_assert_eq(bswap(128lu), 9223372036854775808lu, "bswap128 (lu) failed (1)"
 static_assert_eq(128lu, bswap(9223372036854775808lu), "bswap128 (lu) failed (2)");
 static_assert_eq(bswap(bswap(128lu)), 128, "bswap128 (lu) failed (3)");
 
+// Transmute
+
+#define TRANSMUTE(val, type) ({ union _trans { var(val) input; type output; }; \
+				_Static_assert(sizeof(union _trans) == sizeof(type), "Cannot transmute values of different sizes"); \
+				/* XXX: Do we need do check this for equality? Can we re-word it or do we even need it at all? _Static_assert(_Alignof(union _trans) == _Alignof(type), "Cannot transmute values of different alignments");*/ \
+				union _trans _trans__value = { .input = (val), }; \
+				_trans__value.output; })
+
 //  Trace message output
 
 #include "trace.h"

src/tests/transmute.c

@@ -0,0 +1,23 @@
+
+#include <string.h>
+
+#include <ints.h>
+#include <macros.h>
+#include <tests.h>
+
+DEFTEST(transmute_copy)
+{
+	static const u64 EXPECTED = 18446743474098365025lu;
+	struct { char i[4]; i32 u; } input = { .i = {'a','b','c','d'}, .u = -140 };
+	let output = TRANSMUTE(input, u64);
+	TRACE("Transmute test, expected 0x%lx, got 0x%lx", EXPECTED, output);
+	//XXX: TODO: This test will fail on Big Endian machines.
+	TEST_ASSERT(output == EXPECTED);
+	TRACE("Alignment of output: %lu, alignment of input: %lu", _Alignof(output), _Alignof(input));
+	let input2 = TRANSMUTE(output, var(input));
+	TRACE("Transmute output passed, trying to transmute back");
+	TEST_ASSERT( memcmp(&input2, &input, sizeof(input)) == 0 );
+
+	return TEST_OK;
+}
+RUNTEST_DEBUG(transmute_copy);