Allow compilation of C extensions with `-Wconversion`

C extension maintainers can now compile with this warning option and
the Ruby header files will generate no warnings.

[Feature #20507]

include/ruby/internal/arithmetic/long.h

@@ -114,11 +114,11 @@ RB_INT2FIX(long i)
 
     /* :NOTE: VALUE can be wider than long.  As j being unsigned, 2j+1 is fully
      * defined. Also it can be compiled into a single LEA instruction. */
-    const unsigned long j = i;
+    const unsigned long j = RBIMPL_CAST((unsigned long)i);
     const unsigned long k = (j << 1) + RUBY_FIXNUM_FLAG;
-    const long          l = k;
+    const long          l = RBIMPL_CAST((long)k);
     const SIGNED_VALUE  m = l; /* Sign extend */
-    const VALUE         n = m;
+    const VALUE         n = RBIMPL_CAST((VALUE)m);
 
     RBIMPL_ASSERT_OR_ASSUME(RB_FIXNUM_P(n));
     return n;
@@ -166,7 +166,7 @@ rbimpl_fix2long_by_idiv(VALUE x)
     /* :NOTE: VALUE  can be wider  than long.  (x-1)/2 never  overflows because
      * RB_FIXNUM_P(x)  holds.   Also it  has  no  portability issue  like  y>>1
      * below. */
-    const SIGNED_VALUE y = x - RUBY_FIXNUM_FLAG;
+    const SIGNED_VALUE y = RBIMPL_CAST((SIGNED_VALUE)(x - RUBY_FIXNUM_FLAG));
     const SIGNED_VALUE z = y / 2;
     const long         w = RBIMPL_CAST((long)z);
 
@@ -193,7 +193,7 @@ rbimpl_fix2long_by_shift(VALUE x)
 
     /* :NOTE: VALUE can be wider than long.  If right shift is arithmetic, this
      * is noticeably faster than above. */
-    const SIGNED_VALUE y = x;
+    const SIGNED_VALUE y = RBIMPL_CAST((SIGNED_VALUE)x);
     const SIGNED_VALUE z = y >> 1;
     const long         w = RBIMPL_CAST((long)z);
 
@@ -252,7 +252,7 @@ static inline unsigned long
 rb_fix2ulong(VALUE x)
 {
     RBIMPL_ASSERT_OR_ASSUME(RB_FIXNUM_P(x));
-    return rb_fix2long(x);
+    return RBIMPL_CAST((unsigned long)rb_fix2long(x));
 }
 
 /**
@@ -323,7 +323,7 @@ static inline VALUE
 rb_ulong2num_inline(unsigned long v)
 {
     if (RB_POSFIXABLE(v))
-        return RB_LONG2FIX(v);
+        return RB_LONG2FIX(RBIMPL_CAST((long)v));
     else
         return rb_uint2big(v);
 }

include/ruby/internal/arithmetic/long_long.h

@@ -127,7 +127,7 @@ static inline unsigned LONG_LONG
 rb_num2ull_inline(VALUE x)
 {
     if (RB_FIXNUM_P(x))
-        return RB_FIX2LONG(x);
+        return RBIMPL_CAST((unsigned LONG_LONG)RB_FIX2LONG(x));
     else
         return rb_num2ull(x);
 }

include/ruby/internal/arithmetic/st_data_t.h

@@ -58,7 +58,7 @@ RBIMPL_ATTR_ARTIFICIAL()
 static inline VALUE
 RB_ST2FIX(st_data_t i)
 {
-    SIGNED_VALUE x = i;
+    SIGNED_VALUE x = RBIMPL_CAST((SIGNED_VALUE)i);
 
     if (x >= 0) {
         x &= RUBY_FIXNUM_MAX;
@@ -69,7 +69,7 @@ RB_ST2FIX(st_data_t i)
 
     RBIMPL_ASSERT_OR_ASSUME(RB_FIXABLE(x));
     unsigned long y = RBIMPL_CAST((unsigned long)x);
-    return RB_LONG2FIX(y);
+    return RB_LONG2FIX(RBIMPL_CAST((long)y));
 }
 
 #endif /* RBIMPL_ARITHMETIC_ST_DATA_T_H */

include/ruby/internal/encoding/encoding.h

@@ -80,7 +80,7 @@ enum ruby_encoding_consts {
 static inline void
 RB_ENCODING_SET_INLINED(VALUE obj, int encindex)
 {
-    VALUE f = /* upcast */ encindex;
+    VALUE f = /* upcast */ RBIMPL_CAST((VALUE)encindex);
 
     f <<= RUBY_ENCODING_SHIFT;
     RB_FL_UNSET_RAW(obj, RUBY_ENCODING_MASK);

include/ruby/internal/memory.h

@@ -643,7 +643,7 @@ rbimpl_size_mul_or_raise(size_t x, size_t y)
 static inline void *
 rb_alloc_tmp_buffer2(volatile VALUE *store, long count, size_t elsize)
 {
-    const size_t total_size = rbimpl_size_mul_or_raise(count, elsize);
+    const size_t total_size = rbimpl_size_mul_or_raise(RBIMPL_CAST((size_t)count), elsize);
     const size_t cnt = (total_size + sizeof(VALUE) - 1) / sizeof(VALUE);
     return rb_alloc_tmp_buffer_with_count(store, total_size, cnt);
 }

include/ruby/internal/special_consts.h

@@ -156,7 +156,7 @@ RB_TEST(VALUE obj)
      *
      *  RTEST(v) can be 0 if and only if (v == Qfalse || v == Qnil).
      */
-    return obj & ~RUBY_Qnil;
+    return obj & RBIMPL_CAST((VALUE)~RUBY_Qnil);
 }
 
 RBIMPL_ATTR_CONST()
@@ -226,7 +226,7 @@ RB_NIL_OR_UNDEF_P(VALUE obj)
      *
      *  NIL_OR_UNDEF_P(v) can be true only when v is Qundef or Qnil.
      */
-    const VALUE mask = ~(RUBY_Qundef ^ RUBY_Qnil);
+    const VALUE mask = RBIMPL_CAST((VALUE)~(RUBY_Qundef ^ RUBY_Qnil));
     const VALUE common_bits = RUBY_Qundef & RUBY_Qnil;
     return (obj & mask) == common_bits;
 }