fmt: keep trailing comments after fn header decl (#13596)

vlib/v/ast/ast.v

@@ -530,7 +530,8 @@ pub mut:
 	has_await         bool // 'true' if this function uses JS.await
 	//
 	comments      []Comment // comments *after* the header, but *before* `{`; used for InterfaceDecl
-	next_comments []Comment // coments that are one line after the decl; used for InterfaceDecl
+	end_comments  []Comment // comments *after* header declarations. E.g.: `fn C.C_func(x int) int // Comment`
+	next_comments []Comment // comments that are one line after the decl; used for InterfaceDecl
 	//
 	source_file &File = 0
 	scope       &Scope

vlib/v/fmt/fmt.v

@@ -914,6 +914,26 @@ pub fn (mut f Fmt) enum_decl(node ast.EnumDecl) {
 pub fn (mut f Fmt) fn_decl(node ast.FnDecl) {
 	f.attrs(node.attrs)
 	f.write(node.stringify(f.table, f.cur_mod, f.mod2alias)) // `Expr` instead of `ast.Expr` in mod ast
+	// Handle trailing comments after fn header declarations
+	if node.end_comments.len > 0 {
+		first_comment := node.end_comments[0]
+		if first_comment.text.contains('\n') {
+			f.writeln('\n')
+		} else {
+			f.write(' ')
+		}
+		f.comment(first_comment)
+		if node.end_comments.len > 1 {
+			f.writeln('\n')
+			comments := node.end_comments[1..]
+			for i, comment in comments {
+				f.comment(comment)
+				if i != comments.len - 1 {
+					f.writeln('\n')
+				}
+			}
+		}
+	}
 	f.fn_body(node)
 }
 

vlib/v/fmt/tests/fn_headers_with_comments_expected.vv

@@ -0,0 +1,16 @@
+fn C.Mix_LoadMUS1(file byteptr) voidptr // *Mix_Music
+
+fn C.Mix_LoadMUS2(file byteptr) voidptr //*Mix_Music
+
+fn C.Mix_LoadMUS3(file byteptr) voidptr // 1
+
+// 2
+
+// 3
+
+// Loads music
+fn C.Mix_LoadMUS4(file byteptr) voidptr
+
+/*
+Test
+*/

vlib/v/fmt/tests/fn_headers_with_comments_input.vv

@@ -0,0 +1,9 @@
+fn C.Mix_LoadMUS1(file byteptr) voidptr // *Mix_Music
+
+fn C.Mix_LoadMUS2(file byteptr) voidptr /* *Mix_Music */
+
+fn C.Mix_LoadMUS3(file byteptr) voidptr /* 1 */ /* 2 */ /* 3 */
+
+// Loads music
+fn C.Mix_LoadMUS4(file byteptr) voidptr /* Test
+*/

vlib/v/fmt/tests/fn_headers_with_comments_keep.vv

@@ -0,0 +1,5 @@
+fn C.Mix_LoadMUS1(file byteptr) voidptr // Mix_Music
+
+fn C.Mix_LoadMUS2(file byteptr) voidptr // Mix_Music 2
+
+// Comment

vlib/v/parser/fn.v

@@ -527,6 +527,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		is_builtin: p.builtin_mod || p.mod in util.builtin_module_parts
 		scope: p.scope
 		label_names: p.label_names
+		end_comments: p.eat_comments(same_line: true)
 	}
 	if generic_names.len > 0 {
 		p.table.register_fn_generic_types(fn_decl.fkey())