ast, native, cgen: add support for $if native {} (#19500)

Author: Spydr06

vlib/v/ast/comptime_valid_idents.v

@@ -9,7 +9,8 @@ pub const (
 	valid_comptime_if_cpu_features   = ['x64', 'x32', 'little_endian', 'big_endian']
 	valid_comptime_if_other          = ['apk', 'js', 'debug', 'prod', 'test', 'glibc', 'prealloc',
 		'no_bounds_checking', 'freestanding', 'threads', 'js_node', 'js_browser', 'js_freestanding',
-		'interpreter', 'es5', 'profile', 'wasm32', 'wasm32_emscripten', 'wasm32_wasi', 'fast_math']
+		'interpreter', 'es5', 'profile', 'wasm32', 'wasm32_emscripten', 'wasm32_wasi', 'fast_math',
+		'native']
 	valid_comptime_not_user_defined  = all_valid_comptime_idents()
 	valid_comptime_compression_types = ['none', 'zlib']
 )

vlib/v/gen/c/comptime.v

@@ -1078,6 +1078,9 @@ fn (mut g Gen) comptime_if_to_ifdef(name string, is_comptime_option bool) !strin
 		'wasm32_emscripten' {
 			return '__EMSCRIPTEN__'
 		}
+		'native' {
+			return '_VNATIVE' // when using the native backend, cgen is inactive
+		}
 		// compilers:
 		'gcc' {
 			return '__V_GCC__'

vlib/v/gen/js/comptime.v

@@ -236,6 +236,9 @@ fn (mut g JsGen) comptime_if_to_ifdef(name string, is_comptime_option bool) !str
 		'js' {
 			return 'true'
 		}
+		'native' {
+			return 'false'
+		}
 		// compilers:
 		'gcc' {
 			return 'false'

vlib/v/gen/native/comptime.v

@@ -173,6 +173,9 @@ fn (mut g Gen) comptime_ident(name string, is_comptime_option bool) bool {
 		//
 		// Other
 		//
+		'native' {
+			true
+		}
 		'debug' {
 			g.pref.is_debug
 		}

vlib/v/gen/native/tests/comptime.vv

@@ -7,6 +7,10 @@ fn comptime_if() {
 		println('linux or windows or macos')
 	}
 
+	$if native {
+		println('using the native backend')
+	}
+
 	os := $if linux || windows || macos {
 		'linux or windows or macos'
 	} $else {

vlib/v/gen/native/tests/comptime.vv.out

@@ -1,4 +1,5 @@
 linux or windows or macos
+using the native backend
 linux or windows or macos
 custom defines work
 printing directly