v.markused: support `./v -skip-unused -freestanding run vlib/os/bare/bare_example_linux.v` too

vlib/v/ast/ast.v

@@ -367,6 +367,7 @@ pub:
 	is_main         bool           // true for `fn main()`
 	is_test         bool           // true for `fn test_abcde`
 	is_conditional  bool           // true for `[if abc] fn abc(){}`
+	is_exported     bool           // true for `[export: 'exact_C_name']`
 	is_keep_alive   bool           // passed memory must not be freed (by GC) before function returns
 	receiver        StructField    // TODO this is not a struct field
 	receiver_pos    token.Position // `(u User)` in `fn (u User) name()` position

vlib/v/markused/markused.v

@@ -103,6 +103,18 @@ pub fn mark_used(mut table ast.Table, pref &pref.Preferences, ast_files []&ast.F
 		'os.init_os_args',
 		'os.init_os_args_wide',
 	]
+
+	if pref.is_bare {
+		all_fn_root_names << [
+			'strlen',
+			'memcmp',
+			'memcpy',
+			'realloc',
+			'vsnprintf',
+			'vsprintf',
+		]
+	}
+
 	if pref.gc_mode in [.boehm_full_opt, .boehm_incr_opt] {
 		all_fn_root_names << [
 			'__new_array_noscan',
@@ -232,6 +244,7 @@ pub fn mark_used(mut table ast.Table, pref &pref.Preferences, ast_files []&ast.F
 		all_consts: all_consts
 	}
 	// println( all_fns.keys() )
+	walker.mark_exported_fns()
 	walker.mark_root_fns(all_fn_root_names)
 
 	if walker.n_asserts > 0 {

vlib/v/markused/walker.v

@@ -46,6 +46,14 @@ pub fn (mut w Walker) mark_root_fns(all_fn_root_names []string) {
 	}
 }
 
+pub fn (mut w Walker) mark_exported_fns() {
+	for _, mut func in w.all_fns {
+		if func.is_exported {
+			w.fn_decl(mut func)
+		}
+	}
+}
+
 pub fn (mut w Walker) stmt(node ast.Stmt) {
 	match mut node {
 		ast.EmptyStmt {}

vlib/v/parser/fn.v

@@ -180,6 +180,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	conditional_ctdefine := p.attrs.find_comptime_define() or { '' }
 	mut is_unsafe := p.attrs.contains('unsafe')
 	is_keep_alive := p.attrs.contains('keep_args_alive')
+	is_exported := p.attrs.contains('export')
 	is_pub := p.tok.kind == .key_pub
 	if is_pub {
 		p.next()
@@ -423,6 +424,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		params: params
 		is_manualfree: is_manualfree
 		is_deprecated: is_deprecated
+		is_exported: is_exported
 		is_direct_arr: is_direct_arr
 		is_pub: is_pub
 		is_variadic: is_variadic