fmt: fix - v fmt transforms compile time options in some cases (#16351)

vlib/v/checker/constants/constants.v

@@ -0,0 +1,10 @@
+module constants
+
+// TODO: move all constants from `checker` to here, and replace the hardcoded one with the computed one
+pub const valid_comptime_not_user_defined = ['windows', 'ios', 'macos', 'mach', 'darwin', 'hpux',
+	'gnu', 'qnx', 'linux', 'freebsd', 'openbsd', 'netbsd', 'bsd', 'dragonfly', 'android', 'termux',
+	'solaris', 'haiku', 'serenity', 'vinix', 'gcc', 'tinyc', 'clang', 'mingw', 'msvc', 'cplusplus',
+	'amd64', 'i386', 'aarch64', 'arm64', 'arm32', 'rv64', 'rv32', 'x64', 'x32', 'little_endian',
+	'big_endian', 'apk', 'js', 'debug', 'prod', 'test', 'glibc', 'prealloc', 'no_bounds_checking',
+	'freestanding', 'threads', 'js_node', 'js_browser', 'js_freestanding', 'interpreter', 'es5',
+	'profile', 'wasm32_emscripten']

vlib/v/fmt/fmt.v

@@ -7,6 +7,7 @@ import strings
 import v.ast
 import v.util
 import v.pref
+import v.checker.constants
 
 const (
 	bs      = '\\'
@@ -1936,6 +1937,10 @@ pub fn (mut f Fmt) enum_val(node ast.EnumVal) {
 
 pub fn (mut f Fmt) ident(node ast.Ident) {
 	if node.info is ast.IdentVar {
+		if node.comptime && node.name in constants.valid_comptime_not_user_defined {
+			f.write(node.name)
+			return
+		}
 		if node.info.is_mut {
 			f.write(node.info.share.str() + ' ')
 		}

vlib/v/fmt/tests/conditional_compilation_keep_in_module.vv

@@ -0,0 +1,15 @@
+module log
+
+const (
+	debug = 'debug'
+	prod  = 'prod'
+)
+
+fn log() {
+	$if debug {
+		println('debug')
+	}
+	$if !prod {
+		println('prod')
+	}
+}