fmt: don't remove `mut` from `if mut` smart cast (#6188)

vlib/v/ast/ast.v

@@ -472,9 +472,10 @@ pub:
 	pos          token.Position
 	body_pos     token.Position
 	comments     []Comment
+	left_as_name string // `name` in `if cond is SumType as name`
+	mut_name     bool // `if mut name is`
 pub mut:
-	smartcast    bool // should only be true if cond is `x is sumtype`, it will be set in checker - if_expr
-	left_as_name string // only used in x is SumType check
+	smartcast    bool // true when cond is `x is SumType`, set in checker.if_expr
 }
 
 pub struct UnsafeExpr {

vlib/v/fmt/fmt.v

@@ -1357,35 +1357,30 @@ pub fn (mut f Fmt) if_expr(it ast.IfExpr) {
 			}
 		}
 		if i == 0 {
+			// first `if`
 			f.comments(branch.comments, {})
-			f.write('if ')
-			f.expr(branch.cond)
-			if smartcast_as {
-				f.write(' as $branch.left_as_name')
-			}
-			f.write(' {')
-		} else if i < it.branches.len - 1 || !it.has_else {
+		} else {
+			// `else`, close previous branch
 			if branch.comments.len > 0 {
 				f.writeln('}')
 				f.comments(branch.comments, {})
 			} else {
 				f.write('} ')
 			}
-			f.write('else if ')
-			f.expr(branch.cond)
-			if smartcast_as {
-				f.write(' as $branch.left_as_name')
-			}
-			f.write(' {')
-		} else if i == it.branches.len - 1 && it.has_else {
-			if branch.comments.len > 0 {
-				f.writeln('}')
-				f.comments(branch.comments, {})
-			} else {
-				f.write('} ')
-			}
-			f.write('else {')
+			f.write('else ')
 		}
+		if i < it.branches.len - 1 || !it.has_else {
+			f.write('if ')
+			if branch.mut_name {
+				f.write('mut ')
+			}
+			f.expr(branch.cond)
+			if smartcast_as {
+				f.write(' as $branch.left_as_name')
+			}
+			f.write(' ')
+		}
+		f.write('{')
 		if single_line {
 			f.write(' ')
 		} else {

vlib/v/fmt/tests/sum_smartcast_keep.vv

@@ -1,4 +1,6 @@
 struct S1 {
+mut:
+	i int
 }
 
 struct S2 {
@@ -7,7 +9,8 @@ struct S2 {
 type Sum = S1 | S2
 
 fn f(sum Sum) {
-	if sum is S1 {
+	if mut sum is S1 {
+		sum.i++
 	}
 	if sum is S1 as s1 {
 	}

vlib/v/parser/if_match.v

@@ -21,15 +21,11 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 	for p.tok.kind in [.key_if, .key_else] {
 		p.inside_if = true
 		start_pos := p.tok.position()
-		if p.tok.kind == .key_if {
-			p.next()
-		} else {
+		if p.tok.kind == .key_else {
 			comments << p.eat_comments()
 			p.check(.key_else)
 			comments << p.eat_comments()
-			if p.tok.kind == .key_if {
-				p.next()
-			} else {
+			if p.tok.kind == .lcbr {
 				// else {
 				has_else = true
 				p.inside_if = false
@@ -64,9 +60,18 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 				break
 			}
 		}
+		// `if` or `else if`
+		p.check(.key_if)
+		comments << p.eat_comments()
+		// `if mut name is T`
+		mut mut_name := false
+		if p.tok.kind == .key_mut && p.peek_tok2.kind == .key_is {
+			mut_name = true
+			p.next()
+			comments << p.eat_comments()
+		}
 		mut cond := ast.Expr{}
 		mut is_guard := false
-		comments << p.eat_comments()
 		// `if x := opt() {`
 		if p.peek_tok.kind == .decl_assign {
 			p.open_scope()
@@ -121,6 +126,7 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 			body_pos: body_pos.extend(p.prev_tok.position())
 			comments: comments
 			left_as_name: left_as_name
+			mut_name: mut_name
 		}
 		comments = p.eat_comments()
 		if p.tok.kind != .key_else {