From 6da66226e55cbf05acd8e6164c03e781f5d6cf0b Mon Sep 17 00:00:00 2001
From: Delyan Angelov <delian66@gmail.com>
Date: Thu, 4 Mar 2021 15:30:30 +0200
Subject: [PATCH] vfmt: keep `mut:` in interface declarations (#9053)

---
 vlib/v/ast/ast.v                                   | 1 +
 vlib/v/fmt/fmt.v                                   | 5 ++++-
 vlib/v/fmt/tests/interface_with_mut_fields_keep.vv | 6 ++++++
 vlib/v/parser/struct.v                             | 3 +++
 4 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/fmt/tests/interface_with_mut_fields_keep.vv

diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index a3bb7e03ad..930998823c 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -231,6 +231,7 @@ pub:
 	field_names  []string
 	is_pub       bool
 	methods      []FnDecl
+	mut_pos      int // mut:
 	fields       []StructField
 	pos          token.Position
 	pre_comments []Comment
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index d87df10417..b3681ff55d 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -817,7 +817,10 @@ pub fn (mut f Fmt) interface_decl(node ast.InterfaceDecl) {
 		f.writeln('')
 	}
 	f.comments_after_last_field(node.pre_comments)
-	for field in node.fields {
+	for i, field in node.fields {
+		if i == node.mut_pos {
+			f.writeln('mut:')
+		}
 		// TODO: alignment, comments, etc.
 		mut ft := f.no_cur_mod(f.table.type_to_str(field.typ))
 		if !ft.contains('C.') && !ft.contains('JS.') && !ft.contains('fn (') {
diff --git a/vlib/v/fmt/tests/interface_with_mut_fields_keep.vv b/vlib/v/fmt/tests/interface_with_mut_fields_keep.vv
new file mode 100644
index 0000000000..4b8595b45c
--- /dev/null
+++ b/vlib/v/fmt/tests/interface_with_mut_fields_keep.vv
@@ -0,0 +1,6 @@
+interface Toto {
+	a int
+mut:
+	b int
+	f()
+}
diff --git a/vlib/v/parser/struct.v b/vlib/v/parser/struct.v
index bf781148ce..9d0c612868 100644
--- a/vlib/v/parser/struct.v
+++ b/vlib/v/parser/struct.v
@@ -461,6 +461,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	mut fields := []ast.StructField{cap: 20}
 	mut methods := []ast.FnDecl{cap: 20}
 	mut is_mut := false
+	mut mut_pos := -1
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
 		if p.tok.kind == .key_mut {
 			if is_mut {
@@ -470,6 +471,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			p.next()
 			p.check(.colon)
 			is_mut = true
+			mut_pos = fields.len
 		}
 		if p.peek_tok.kind == .lpar {
 			method_start_pos := p.tok.position()
@@ -566,5 +568,6 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		is_pub: is_pub
 		pos: pos
 		pre_comments: pre_comments
+		mut_pos: mut_pos
 	}
 }