parser: add support for mutable if guards (#13132)

vlib/v/ast/ast.v

@@ -1418,6 +1418,7 @@ pub mut:
 pub struct IfGuardExpr {
 pub:
 	var_name string
+	is_mut   bool
 	pos      token.Position
 pub mut:
 	expr      Expr

vlib/v/fmt/fmt.v

@@ -1893,6 +1893,9 @@ fn branch_is_single_line(b ast.IfBranch) bool {
 }
 
 pub fn (mut f Fmt) if_guard_expr(node ast.IfGuardExpr) {
+	if node.is_mut { 
+		f.write('mut ') 
+	}
 	f.write(node.var_name + ' := ')
 	f.expr(node.expr)
 }

vlib/v/parser/if_match.v

@@ -81,10 +81,17 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 		comments << p.eat_comments()
 		mut cond := ast.empty_expr()
 		mut is_guard := false
+
 		// `if x := opt() {`
-		if !is_comptime && p.peek_tok.kind == .decl_assign {
+		if !is_comptime && (p.peek_tok.kind == .decl_assign
+			|| (p.tok.kind == .key_mut && p.peek_token(2).kind == .decl_assign)) {
 			p.open_scope()
 			is_guard = true
+			mut is_mut := false
+			if p.tok.kind == .key_mut {
+				is_mut = true
+				p.check(.key_mut)
+			}
 			var_pos := p.tok.position()
 			var_name := p.check_name()
 			if p.scope.known_var(var_name) {
@@ -96,10 +103,12 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			expr := p.expr(0)
 			cond = ast.IfGuardExpr{
 				var_name: var_name
+				is_mut: is_mut
 				expr: expr
 			}
 			p.scope.register(ast.Var{
 				name: var_name
+				is_mut: is_mut
 				expr: cond
 				pos: var_pos
 			})

vlib/v/tests/if_guard_test.v

@@ -106,21 +106,28 @@ fn test_chan_pop_empty() {
 }
 
 struct Thing {
-	name string
+mut:
+	value int
 }
 
-fn test_return_if_guard() {
-	ret := option_check('zoo')
-	println(ret)
-	assert ret == 'zs'
-}
-
-fn option_check(name string) string {
-	return if thing := find_thing_by_name(name) { thing.name } else { 'safename' }
-}
-
-fn find_thing_by_name(name string) ?&Thing {
-	return &Thing{
-		name: 'zs'
+fn maybe_a_thing() ?Thing {
+	return Thing{
+		value: 1
+	}
+}
+
+fn test_if_guard() {
+	if val := maybe_a_thing() {
+		assert val.value == 1
+	} else {
+		assert false
+	}
+
+	if mut val := maybe_a_thing() {
+		assert val.value == 1
+		val.value = 2
+		assert val.value == 2
+	} else {
+		assert false
 	}
 }