compiler: add support for `[if myflag]` void fn/methods

vlib/v/ast/ast.v

@@ -211,6 +211,7 @@ pub:
 	is_js         bool
 	no_body       bool // just a definition `fn C.malloc()`
 	is_builtin    bool // this function is defined in builtin/strconv
+	ctdefine      string // has [if myflag] tag
 	pos           token.Position
 }
 
@@ -235,6 +236,7 @@ mut:
 	left_type          table.Type // type of `user`
 	receiver_type      table.Type // User
 	return_type        table.Type
+	should_be_skipped  bool
 }
 
 pub struct CallArg {

vlib/v/checker/checker.v

@@ -120,6 +120,11 @@ fn (c mut Checker) check_file_in_main(file ast.File) bool {
 						c.warn('function `$it.name` $no_pub_in_main_warning', it.pos)
 					}
 				}
+				if it.ctdefine.len > 0 {
+					if it.return_type != table.void_type {
+						c.error('only functions that do NOT return values can have `[if ${it.ctdefine}]` tags', it.pos)
+					}
+				}
 			}
 			ast.StructDecl {
 				if it.is_pub {
@@ -497,6 +502,9 @@ pub fn (c mut Checker) call_method(call_expr mut ast.CallExpr) table.Type {
 			//println('warn $method_name lef.mod=$left_type_sym.mod c.mod=$c.mod')
 			c.error('method `${left_type_sym.name}.$method_name` is private', call_expr.pos)
 		}
+		if method.return_type == table.void_type && method.ctdefine.len > 0 && method.ctdefine !in c.pref.compile_defines {
+			call_expr.should_be_skipped = true
+		}
 		nr_args := if method.args.len == 0 {  0 } else {method.args.len - 1}
 		min_required_args := method.args.len - if method.is_variadic && method.args.len > 1 { 2 } else { 1 }
 		if call_expr.args.len < min_required_args {
@@ -612,6 +620,11 @@ pub fn (c mut Checker) call_fn(call_expr mut ast.CallExpr) table.Type {
 		return table.void_type
 	}
 	call_expr.return_type = f.return_type
+
+	if f.return_type == table.void_type && f.ctdefine.len > 0 && f.ctdefine !in c.pref.compile_defines {
+		call_expr.should_be_skipped = true
+	}
+
 	if f.is_c || call_expr.is_c || f.is_js || call_expr.is_js {
 		for arg in call_expr.args {
 			c.expr(arg.expr)

vlib/v/gen/fn.v

@@ -224,6 +224,9 @@ fn (mut g Gen) fn_args(args []table.Arg, is_variadic bool) {
 }
 
 fn (mut g Gen) call_expr(node ast.CallExpr) {
+	if node.should_be_skipped {
+		return
+	}
 	gen_or := !g.is_assign_rhs && node.or_block.stmts.len > 0
 	tmp_opt := if gen_or { g.new_tmp_var() } else { '' }
 	if gen_or {

vlib/v/parser/fn.v

@@ -170,6 +170,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		end_pos = p.tok.position()
 		return_type = p.parse_type()
 	}
+	ctdefine := p.attr_ctdefine
 	// Register
 	if is_method {
 		mut type_sym := p.table.get_type_symbol(rec_type)
@@ -181,6 +182,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			is_variadic: is_variadic
 			is_generic: is_generic
 			is_pub: is_pub
+			ctdefine: ctdefine
 		})
 	} else {
 		if is_c {
@@ -202,6 +204,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			is_js: is_js
 			is_generic: is_generic
 			is_pub: is_pub
+			ctdefine: ctdefine
 		})
 	}
 	// Body
@@ -212,6 +215,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	}
 	p.close_scope()
 	p.attr = ''
+	p.attr_ctdefine = ''
 	return ast.FnDecl{
 		name: name
 		stmts: stmts
@@ -231,6 +235,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		no_body: no_body
 		pos: start_pos.extend(end_pos)
 		is_builtin: p.builtin_mod || p.mod in util.builtin_module_parts
+		ctdefine: ctdefine
 	}
 }
 

vlib/v/parser/parser.v

@@ -31,6 +31,7 @@ mut:
 	builtin_mod       bool // are we in the `builtin` module?
 	mod               string // current module name
 	attr              string
+	attr_ctdefine     string
 	expr_mod          string
 	scope             &ast.Scope
 	global_scope      &ast.Scope
@@ -490,8 +491,10 @@ pub fn (mut p Parser) stmt() ast.Stmt {
 
 fn (mut p Parser) attribute() ast.Attr {
 	p.check(.lsbr)
+	mut is_if_attr := false
 	if p.tok.kind == .key_if {
 		p.next()
+		is_if_attr = true
 	}
 	mut name := p.check_name()
 	if p.tok.kind == .colon {
@@ -505,6 +508,9 @@ fn (mut p Parser) attribute() ast.Attr {
 	}
 	p.check(.rsbr)
 	p.attr = name
+	if is_if_attr {
+		p.attr_ctdefine = name
+	}
 	return ast.Attr{
 		name: name
 	}

vlib/v/table/table.v

@@ -27,6 +27,7 @@ pub:
 	is_generic  bool
 	is_pub      bool
 	mod         string
+	ctdefine    string // compile time define. myflag, when [if myflag] tag
 }
 
 pub struct Arg {