parser: generate all flagged enum functions, no matter what the backend is; tag them with @[flag_enum_fn], and let the backends filter them out, if they do not support them (#22543)

vlib/v/gen/native/amd64.v

@@ -3082,6 +3082,10 @@ fn (mut c Amd64) infloop() {
 }
 
 fn (mut c Amd64) fn_decl(node ast.FnDecl) {
+	if node.attrs.contains('flag_enum_fn') {
+		// TODO: remove, when the native backend can process all flagged enum generated functions
+		return
+	}
 	c.push(Amd64Register.rbp)
 	c.mov_reg(Amd64Register.rbp, Amd64Register.rsp)
 	local_alloc_pos := c.g.pos()

vlib/v/gen/native/arm64.v

@@ -109,6 +109,10 @@ fn (mut c Arm64) sub_sp(v i32) {
 }
 
 pub fn (mut c Arm64) fn_decl(node ast.FnDecl) {
+	if node.attrs.contains('flag_enum_fn') {
+		// TODO: remove, when the native backend can process all flagged enum generated functions
+		return
+	}
 	c.g.gen_arm64_helloworld()
 	/*
 	0x100003f6c      ff8300d1       sub sp, sp, 0x20           ; [00] -r-x section size 52 named 0.__TEXT.__text

vlib/v/gen/native/comptime.v

@@ -53,6 +53,10 @@ fn (mut g Gen) comptime_is_truthy(cond ast.Expr) bool {
 				.ne {
 					return g.comptime_is_truthy(cond.left) != g.comptime_is_truthy(cond.right)
 				}
+				.key_is {
+					// TODO: implement properly, to support @[flag_enum_fn] functions
+					return true
+				}
 				else {
 					g.n_error('Compile time infix expr `${cond}` is not handled by the native backend.')
 				}

vlib/v/gen/native/gen.v

@@ -1035,6 +1035,11 @@ fn (mut g Gen) delay_fn_call(name string) {
 }
 
 fn (mut g Gen) fn_decl(node ast.FnDecl) {
+	if node.attrs.contains('flag_enum_fn') {
+		// TODO: remove, when the native backend can process all flagged enum generated functions
+		return
+	}
+
 	name := if node.is_method {
 		'${g.table.get_type_name(node.receiver.typ)}.${node.name}'
 	} else {

vlib/v/gen/wasm/gen.v

@@ -179,6 +179,11 @@ pub fn (mut g Gen) fn_decl(node ast.FnDecl) {
 		return
 	}
 
+	if node.attrs.contains('flag_enum_fn') {
+		// TODO: remove, when support for fn results is done
+		return
+	}
+
 	name := if node.is_method {
 		'${g.table.get_type_name(node.receiver.typ)}.${node.name}'
 	} else {

vlib/v/parser/parser.v

@@ -4241,7 +4241,7 @@ fn (mut p Parser) enum_decl() ast.EnumDecl {
 	p.check(.rcbr)
 	is_flag := p.attrs.contains('flag')
 	is_multi_allowed := p.attrs.contains('_allow_multiple_values')
-	pubfn := if p.mod == 'main' { 'fn' } else { 'pub fn' }
+	pubfn := if p.mod == 'main' { '@[flag_enum_fn] fn' } else { '@[flag_enum_fn] pub fn' }
 	if is_flag {
 		if fields.len > 64 {
 			p.error('when an enum is used as bit field, it must have a max of 64 fields')
@@ -4317,12 +4317,7 @@ fn (mut p Parser) enum_decl() ast.EnumDecl {
 		}
 	}
 	if enum_name[0].is_capital() && fields.len > 0 {
-		// TODO: this check is to prevent avoidable later stage checker errors for generated code,
+		p.codegen(code_for_from_fn)
-		// since currently there is no way to show the proper source context :-|.
-		if p.pref.backend == .c {
-			// TODO: improve the other backends, to the point where they can handle generics or comptime checks too
-			p.codegen(code_for_from_fn)
-		}
 	}
 
 	idx := p.table.register_sym(ast.TypeSymbol{